Vehicle height adjust control apparatus and method

A vehicle height adjust control apparatus and method adjusts an actual vehicle height to a target vehicle height using a microcomputer that controls an electric motor, leveling valves and an accumulator valve on the basis of the actual vehicle height detected by vehicle height sensors, so as to eliminate any deviation of the actual vehicle height from the target vehicle height. If a hydraulic fluid temperature detected by a fluid temperature sensor is very low or very high, the microcomputer suspends the vehicle height adjusting control and the supply of hydraulic fluid to an accumulator by stopping the operation of the electric motor and a hydraulic pump and/or switching the valves to a closed state. The suspending control thereby prevents very high and low viscosities of the hydraulic fluid, or very low and high fluidities thereof, which prevent an undesirable increase of the load on the hydraulic pump and an undesirable decrease of the ejecting performance thereof.

INCORPORATION BY REFERENCE
 The disclosure of Japanese Patent Application No. HEI 10-6946 filed on Jan.
 16, 1998 including the specification, drawings and abstract is
 incorporated herein by reference in its entirety.
 BACKGROUND OF THE INVENTION
 1. Field of Invention
 The present invention relates to a vehicle height adjust control apparatus
 and method for setting a vehicle height to a target vehicle height by
 supplying hydraulic fluid to and discharging it from hydraulic actuators
 provided between a vehicle body and wheels.
 2. Description of Related Art
 A vehicle height adjust control apparatus described in Japanese Patent
 Application Laid-Open No. Hei 2-3515, for example, includes a hydraulic
 actuator for changing a vehicle height by hydraulic fluid supplied and
 discharged, a supply-discharge device made up of a hydraulic pump, a
 control valve for supplying hydraulic fluid into and discharging it from
 the hydraulic actuator, a vehicle height detection device for detecting an
 actual vehicle height, and a supply-discharge control device for
 controlling the operation of the supply-discharge device so as to
 eliminate deviation of the detected actual vehicle height from a
 predetermined target vehicle height. The apparatus controls and sets the
 actual vehicle height to a target vehicle height by supplying hydraulic
 fluid to and discharging it from the hydraulic actuator using the
 supply-discharge device.
 In this vehicle height adjust control apparatus, various problems occur
 depending on various factors, such as the construction of the hydraulic
 pump or the control valve, the control of the supply-discharge device by
 the supply-discharge control device, the condition of the hydraulic fluid,
 the condition of a battery, and the like. Specifically, if the hydraulic
 pump is operated at a very low hydraulic fluid temperature, an excessively
 great load is caused on the hydraulic pump. This adversely affects the
 durability of the hydraulic pump because at a very low temperature the
 fluidity of hydraulic fluid is very low and the viscosity thereof is very
 high. At a very high hydraulic fluid temperature, the viscosity of
 hydraulic fluid becomes very low, so that the ejecting performance of the
 hydraulic pump decreases. Therefore, it becomes necessary to operate the
 hydraulic pump for longer periods. Long-time operation of the hydraulic
 pump further increases the hydraulic fluid temperature, thereby adversely
 affecting the durability of the hydraulic pump. This temperature-dependent
 problem is significant, particularly if the hydraulic pump is a gear pump.
 If stopping of the hydraulic pump and switching of the control valve from a
 open state to an close state are simultaneously performed to stop the
 supply of hydraulic fluid from the hydraulic pump to the hydraulic
 actuator, hydraulic fluid may impact the control valve because the
 hydraulic pump will not immediately stop ejecting hydraulic fluid, due to
 the inertia of an electrical motor or the like. Such impact on the control
 valve produces impact noise and degrades the durability of the hydraulic
 system, including the hydraulic pump, the control valve and the like. This
 problem becomes more significant if the hydraulic fluid temperature is
 lower and the hydraulic fluid viscosity is higher, and if the hydraulic
 fluid ejecting pressure produced by the hydraulic pump is higher.
 If the control valve is an electromagnetic on-off valve, a size reduction
 of the control valve results in a reduction in the number of turns of the
 coil, so that it may become difficult for the coil to provide a
 sufficiently great attraction force for drawing a plunger. Furthermore,
 since a temperature increase correspondingly increases the resistance of
 the coil, the coil may not be provided with a current sufficient to
 attract the plunger if the hydraulic fluid temperature is high. In
 addition, an insufficient current through the coil for attracting the
 plunger also results from a voltage reduction of the battery that
 energizes the coil.
 SUMMARY OF THE INVENTION
 Accordingly, it is an object of the invention to improve the durability of
 a vehicle height adjust control apparatus and ensure reliable operation of
 the apparatus by solving the aforementioned various problems of the
 conventional art.
 According to a first aspect of the invention, there is provided a vehicle
 height adjust control apparatus including a hydraulic actuator capable of
 increasing and reducing a vehicle height using hydraulic fluid, and a
 hydraulic pump for ejecting hydraulic fluid. The vehicle height adjust
 control apparatus further includes a supply/discharge device for enabling
 the supplying of the hydraulic fluid to the hydraulic actuator and the
 discharging of the hydraulic fluid from the hydraulic actuator, a vehicle
 height detection device for detecting a vehicle height, a supply/discharge
 control device for controlling operation of the supply/discharge device so
 as to eliminate a deviation of the vehicle height detected by the vehicle
 height detection device from a predetermined target vehicle height, a
 fluid temperature detection device for detecting a temperature of the
 hydraulic fluid, and a suspending control device for suspending control of
 the operation of the supply/discharge device by the supply/discharge
 control device if the temperature of the hydraulic fluid detected by the
 fluid temperature detection device is equal to or lower than a first
 predetermined temperature or equal to or higher than a second
 predetermined temperature.
 If the temperature of the hydraulic fluid is equal to or lower than the
 first predetermined temperature or equal to or higher than the second
 predetermined temperature, the operation of the supply/discharge device,
 including the hydraulic pump, is stopped by the suspending control device.
 Therefore, by suitably setting the first and second predetermined
 temperatures, the operation of the hydraulic pump provided in the
 supply/discharge device will be stopped if the temperature of the
 hydraulic fluid becomes very low or very high (or if the viscosity of the
 hydraulic fluid becomes very high or very low so that the fluidity thereof
 is very low or very high). Therefore, the durability or service life of
 the supply/discharge device, including the hydraulic pump, is increased.
 According to another aspect of the invention, there is provided a vehicle
 height adjust control apparatus including a hydraulic pump for ejecting
 hydraulic fluid into a supply/discharge device, an accumulator for
 accumulating hydraulic fluid ejected by the hydraulic pump, a fluid
 temperature detection device for detecting a temperature of the hydraulic
 fluid, and a suspending control device for suspending the supplying of the
 hydraulic fluid from the hydraulic pump to the accumulator if the
 temperature of the hydraulic fluid detected by the fluid temperature
 detection device is equal to or lower than a first predetermined
 temperature or equal to or higher than a second predetermined temperature.
 Therefore, if the temperature of the hydraulic fluid is equal to or lower
 than the first predetermined temperature or equal to or higher than the
 second predetermined temperature, the operation of the hydraulic pump for
 supplying the hydraulic fluid to the accumulator is stopped by the
 suspending control device. Consequently, by suitably setting the first and
 second predetermined temperatures, the durability or service life of the
 supply/discharge device, including the hydraulic pump and the accumulator,
 is increased, as in the construction described above.
 According to still another aspect of the invention, there is provided a
 vehicle height adjust control apparatus including a hydraulic actuator
 capable of increasing and reducing a vehicle height using hydraulic fluid,
 and a hydraulic pump for ejecting the hydraulic fluid. The apparatus
 further includes a control valve provided in a fluid passage between the
 hydraulic pump and the hydraulic actuator, for opening and closing the
 fluid passage, a vehicle height detection device for detecting a vehicle
 height, a supply/discharge control device for controlling operation of the
 hydraulic pump and the opening and closing of the control valve so as to
 eliminate a deviation of the vehicle height detected by the vehicle height
 detection device from a predetermined target vehicle height, and a delay
 control device provided in the supply/discharge control device for
 outputting an instruction to switch the control valve from the open state
 to the closed state, at the elapse of a predetermined delay time following
 output of an instruction to switch the hydraulic pump from the operating
 state to the stopped state. The delay control device operates when the
 hydraulic pump is switched from an operating state to a stopped state and
 the control valve is to be switched from an open state to a closed state.
 Therefore, if the hydraulic pump does not stop ejecting the hydraulic
 fluid immediately after the instruction to stop the hydraulic pump, due to
 the inertia of the electric motor or the like, the vehicle height adjust
 control apparatus is able to switch the control valve from the open state
 to the closed state after the ejection of hydraulic fluid from the
 hydraulic pump has substantially stopped. Therefore, impact on the control
 valve by the hydraulic fluid is reduced, so that impact noise caused
 thereby will be considerably reduced, and so that degradation of the
 durability of a hydraulic system, including the hydraulic pump, the
 control valve and the like, can be substantially prevented.
 The vehicle height adjust control apparatus according to this aspect may
 also include a fluid temperature detection device for detecting a
 temperature of the hydraulic fluid, and a delay time correction device
 provided in the supply/discharge control device, for increasing the
 predetermined delay time with a decrease in the temperature of the
 hydraulic fluid detected by the fluid temperature detection device. With
 this construction, if the temperature of the hydraulic fluid is low, so
 that the viscosity thereof is high and the impact of the hydraulic fluid
 on the control valve will be great, the time from the output of the
 instruction to stop the hydraulic pump to the output of the instruction to
 switch the control valve from the open state to the closed state is
 increased so that the impact of the hydraulic fluid on the control valve
 is favorably reduced. Therefore, the vehicle height adjust control
 apparatus precisely reduces impact noise without unnecessarily delaying
 the switching of the control valve from the open state to the closed state
 if the temperature of the hydraulic fluid changes. As a result, it becomes
 possible to precisely prevent or minimize deterioration of the durability
 of a hydraulic system, including the hydraulic pump, the control valve and
 the like, that is caused by impact of hydraulic fluid thereon.
 The vehicle height adjust control apparatus may also include a hydraulic
 pressure detection device for detecting a pressure of the hydraulic fluid
 supplied from the hydraulic pump to the hydraulic actuator, and a delay
 time correction device provided in the supply/discharge control device,
 for increasing the predetermined delay time with an increase in the
 pressure of the hydraulic fluid detected by the hydraulic pressure
 detection device. With this construction, if the pressure of the hydraulic
 fluid ejected by the hydraulic pump is high, so that the impact of the
 hydraulic fluid on the control valve will be great, the time from the
 output of the instruction to stop the hydraulic pump to the output of the
 instruction to switch the control valve from the open state to the closed
 state is increased so that the impact of the hydraulic fluid on the
 control valve is favorably reduced. Therefore, the vehicle height adjust
 control apparatus precisely reduces impact noise without unnecessarily
 delaying the switching of the control valve from the open state to the
 closed state if the ejecting pressure of the hydraulic pump changes. As a
 result, it becomes possible to precisely prevent or minimize deterioration
 of the durability of a hydraulic system, including the hydraulic pump, the
 control valve and the like, that is caused by impact of hydraulic fluid
 thereon.
 According to a further aspect of the invention, there is provided a vehicle
 height adjust control apparatus including a hydraulic actuator capable of
 increasing and reducing a vehicle height using hydraulic fluid, and a
 supply/discharge device for enabling the supplying of the hydraulic fluid
 to the hydraulic actuator and the discharging of the hydraulic fluid from
 the hydraulic actuator. The supply/discharge device has an electromagnetic
 on-off valve for controlling passage of the hydraulic fluid. The vehicle
 height adjust control apparatus further includes a vehicle height
 detection device for detecting a vehicle height, a supply/discharge
 control device for controlling operation of the supply/discharge device so
 as to eliminate a deviation of the vehicle height detected by the vehicle
 height detection device from a predetermined target vehicle height, and a
 duty ratio control device provided in the supply/discharge control device
 for controlling a duty ratio. The duty ratio control device may set the
 duty ratio of voltage applied to the electromagnetic on-off valve,
 immediately after voltage application thereto is started, to a ratio that
 is greater than the duty ratio of voltage applied afterwards. Therefore, a
 great current flows through the coil of the electromagnetic on-off valve
 during a period immediately after the start of application of voltage
 thereto, during which a large attraction force is needed to move the
 plunger of the electromagnetic on-off valve. During a subsequent period
 when only a small attraction force is needed to retain the plunger at a
 predetermined position, a small current flows through the coil.
 Consequently, it becomes possible to ensure precise operation of the
 electromagnetic on-off valve while minimizing power consumption.
 The vehicle height adjust control apparatus according to this aspect of the
 invention may further include a fluid temperature detection device for
 detecting a temperature of the hydraulic fluid, wherein the duty ratio
 control device increases the duty ratio of voltage applied to the
 electromagnetic on-off valve with an increase in the temperature of the
 hydraulic fluid detected by the fluid temperature detection device. With
 this construction, it becomes possible to ensure sufficient current
 through the coil of the electromagnetic on-off valve needed to attract the
 plunger even if the temperature of the coil increases so that the
 resistance of the coil increases. Therefore, the vehicle height adjust
 control apparatus is able to prevent malfunction of the electromagnetic
 on-off valve due to temperature changes and therefore ensure precise
 operation of the electromagnetic on-off valve.
 The duty ratio control device may also increase the duty ratio of voltage
 applied to the electromagnetic on-off valve with a decrease in output
 voltage of a battery provided for applying voltage to the electromagnetic
 on-off valve. With this construction, it becomes possible to ensure
 sufficient current through the coil of the electromagnetic on-off valve
 needed to attract the plunger even if the output voltage of the battery
 decreases. Therefore, the vehicle height adjust control apparatus is able
 to prevent malfunction of the electromagnetic on-off valve due to changes
 in the battery voltage and therefore ensure precise operation of the
 electromagnetic on-off valve.
 According to a further aspect of the invention, there is provided a method
 of adjusting vehicle height, comprising: providing a hydraulic actuator
 capable of increasing and reducing a vehicle height using hydraulic fluid;
 supplying and discharging the hydraulic fluid to and from the hydraulic
 actuator; detecting a vehicle height; detecting a temperature of the
 hydraulic fluid; controlling operation of the supplying and discharging of
 hydraulic fluid to and from the hydraulic actuator so as to eliminate a
 deviation of the detected vehicle height from a predetermined target
 vehicle height; and suspending operation of the supplying and discharging
 of hydraulic fluid if the detected temperature of the hydraulic fluid is
 either equal to or lower than a first predetermined temperature or equal
 to or higher than a second predetermined temperature.
 According to yet another aspect of the invention, there is provided a
 method of adjusting vehicle height, comprising: providing a hydraulic
 actuator that increases and decreases a vehicle height using hydraulic
 fluid; providing a hydraulic pump that ejects the hydraulic fluid into the
 hydraulic actuator; providing a control valve in a fluid passage between
 the hydraulic pump and the hydraulic actuator that opens and closes the
 fluid passage; detecting a vehicle height; controlling operation of the
 hydraulic pump and the opening and closing of the control valve so as to
 eliminate a deviation of the detected vehicle height from a predetermined
 target vehicle height; and delaying the switching of the control valve
 from an open state to a closed state until after lapse of a predetermined
 delay time following an instruction to switch the hydraulic pump from an
 operating state to a stopped state.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 A preferred embodiment of the present invention will be described in detail
 hereinafter with reference to the accompanying drawings.
 FIG. 1 is a schematic illustration of the entire vehicle height adjust
 control apparatus according to an embodiment of the invention.
 The vehicle height adjust control apparatus has hydraulic cylinders 11a-11d
 that form hydraulic actuators for setting vehicle heights, near left and
 right front wheels W1, W2 and left and right rear wheels W3, W4,
 respectively. Each of the hydraulic cylinders 11a-11d is connected at its
 lower end to a lower arm 12a-12d connecting to the corresponding one of
 the wheels W1-W4. A piston rod 13a-13d of each hydraulic cylinder 11a-11d
 protrudes from an upper surface thereof. Upper end portions of the piston
 rods 13a-13d are fixed to a vehicle body 10. Hydraulic fluid is supplied
 to and discharged from the hydraulic cylinders 11a-11d through fluid
 passages P1-P4, respectively. In accordance with supply and discharge of
 hydraulic fluid, the hydraulic cylinders 11a-11d change the vehicle
 heights at the respective wheel positions.
 Coil springs 14a-14d are disposed between the vehicle body 10 and the
 hydraulic cylinders 11a-11d, respectively. The fluid passages P1-P4 are
 provided with variable orifices 15a-15d, respectively. Accumulators
 16a-16d are connected to the fluid passages P1-P4, respectively. In
 cooperation with the coil springs 14a-14d, the variable orifices 15a-15d
 and the accumulators 16a-16d, the hydraulic cylinders 11a-11d elastically
 support the vehicle body 10 relative to the wheels W1-W4, and also
 function as shock absorbers for damping oscillations of the vehicle body
 10. The variable orifices 15a-15d are electrically controlled so as to
 vary their orifice openings. The control of the orifice openings of the
 variable orifices 15a-15d is not directly related to the invention, and
 will not be described.
 The fluid passages P1, P2 and the fluid passages P3, P4 are connected, at
 their ends opposite the hydraulic cylinders 11a, 11b and 11c, 11d, to
 common fluid passages, respectively. Therefore, hydraulic fluid is
 collectively supplied to and discharged from the hydraulic cylinders 11a,
 11b through the fluid passages P1, P2, and hydraulic fluid is collectively
 supplied to and discharged from the hydraulic cylinders 11c, 11d through
 the fluid passages P3, P4. The fluid passages P2, P4 are provided with
 gate valves 17b, 17d formed by electromagnetic on-off valves,
 respectively. The gate valves 17b, 17d are open as indicated in FIG. 1
 when not energized, and are switched to a closed state when energized. The
 gate valves 17b, 17d are energized when the vehicle body 10 rolls, for
 example, at the time of cornering or turning, so as to prevent
 communication between the hydraulic cylinders 11a and 11b and
 communication between the hydraulic cylinders 11c and 11d, respectively.
 The operation of the gate valves 17b, 17d is not directly relevant to the
 invention, and the description below will be made on the assumption that
 the gate valves 17b, 17d are always in the open state as indicated in FIG.
 1. The fluid passages P1, P3 are provided with invariable orifices 17a,
 17c, respectively, for providing the fluid passages P1, P3 with a passage
 resistance equivalent to that provided by orifice openings that are formed
 by the gate valves 17b, 17d when in the open state.
 A hydraulic pressure supply-discharge device for supplying hydraulic fluid
 to and discharging fluid from the hydraulic cylinders 11a-11d has a
 hydraulic pump 22 that is driven by an electric motor 21. The hydraulic
 pump 22 draws hydraulic fluid from a reservoir tank 23, and ejects the
 fluid into a fluid passage P5 through a check valve 22a. In this
 embodiment, the hydraulic pump 22 is formed by a gear pump. The fluid
 passage P5 divides into fluid passages P6, P7. The branch fluid passage P6
 is connected to the connecting point of the fluid passages P1, P2. The
 branch fluid passage P7 is connected to the connecting point of the fluid
 passages P3, P4. The fluid passages P6, P7 are provided with leveling
 valves 24a, 24b that are formed by electromagnetic on-off valves, each
 made up of a plunger, a coil and the like. The leveling valves 24a, 24b
 remain closed as indicated in FIG. 1 when not energized, and are switched
 to an open state when energized. If the hydraulic pressure in the fluid
 passages P1-P4 becomes abnormally high, the leveling valves 24a, 24b allow
 discharge of hydraulic fluid from the fluid passages P1-P4 into the fluid
 passage P5 for protection of the apparatus even while the valves are in
 the closed state.
 An accumulator 25 that accumulates high-pressure hydraulic fluid is
 connected to the fluid passage P5, via an accumulator valve 26. The
 hydraulic fluid accumulated in the accumulator 25 is used to increase the
 vehicle height. The accumulator valve 26 is formed by an electromagnetic
 on-off valve made up of a plunger, a coil and the like. The accumulator
 valve 26 remains in a state indicated in FIG. 1 when not energized. When
 energized, the accumulator valve 26 is switched from the state indicated
 in FIG. 1 to an open state. The accumulator valve 26 allows hydraulic
 fluid to flow from the fluid passage P5 into the accumulator 25 only when
 the hydraulic pressure in the fluid passage P5 is a predetermined amount
 higher than the hydraulic pressure in the accumulator 25.
 A discharge valve 27 and a relief valve 28 are disposed between the fluid
 passage P5 and the reservoir tank 23. The discharge valve 27 is normally
 kept in an open state, and mechanically switched to a closed state when
 the ejecting pressure of the hydraulic pump 22 increases. The passage area
 of the discharge valve 27 when the discharge valve 27 is kept in the open
 state is at least twice as large as the passage area of the leveling
 valves 24a, 24b when they are in the open state. The relief valve 28 is
 normally kept in a closed state. Only when the hydraulic pressure in the
 fluid passage P5 becomes very high is the relief valve 28 switched to an
 open state to let hydraulic fluid out of the fluid passage P5 into the
 reservoir tank 23 for protection of the apparatus.
 The electric motor 21, the leveling valves 24a, 24b and the accumulator
 valve 26 are connected to a microcomputer 30 that forms an electric
 control device. The microcomputer 30 receives a voltage BV from a battery
 31, via an ignition switch (not shown). When the ignition switch is turned
 on, the microcomputer 30 repeatedly executes a main program illustrated in
 FIG. 2 (including the subroutines illustrated in FIGS. 3 through 10) and a
 drive control program illustrated in FIG. 11 (including first to third
 duty ratio control routines illustrated in FIG. 12) at predetermined short
 intervals of time, thereby controlling the supply of hydraulic fluid to
 and discharge thereof from the hydraulic cylinders 11a-11d. The
 microcomputer 30 is connected to an A/D converter 30a, a target vehicle
 height changing switch 32, vehicle height sensors 33a-33c, a fluid
 temperature sensor 34 and a hydraulic pressure sensor 35.
 The A/D converter 30a converts the output voltage BV of the battery 31, and
 outputs the converted voltage. The A/D converter 30a serves as a device
 for detecting the output voltage BV of the battery 31. The target vehicle
 height changing switch 32 is provided for an occupant to operate to change
 the vehicle height. The target vehicle height changing switch 32 includes
 an up-setting element 32a for increasing the vehicle height from a present
 level, and a down-setting element 32b for reducing the vehicle height from
 a present level. The vehicle height sensors 33a, 33b are disposed between
 the vehicle body 10 and the lower arms 12a, 12b at the left and right
 front wheels W1, W2, respectively. Each of the vehicle height sensors 33a,
 33b detects the height of the vehicle body 10 at the left or right front
 wheel W1, W2 relative to a road surface (or an under-spring member), and
 outputs a detection signal indicating the actual vehicle height Hf1, Hf2.
 The vehicle height sensor 33c is disposed at a transversely middle
 position in a rear portion of the vehicle body 10, between the vehicle
 body 10 and a frame (corresponding to an under-spring member not shown)
 connecting the lower arms 12c and 12d. The vehicle height sensor 33c
 detects the height of the vehicle body 10 at the transversely middle
 position in the rear portion of the vehicle, relative to the road surface
 (or the under-spring member), and outputs a detection signal indicating
 the actual vehicle height Hr.
 The fluid temperature sensor 34 is provided in the fluid passage P5, and
 detects a temperature T of hydraulic fluid ejected into the fluid passage
 P5 from the hydraulic pump 22, and outputs a detection signal indicating
 the temperature T. The temperature T of hydraulic fluid thus detected is
 substantially equal to the temperature of hydraulic fluid in the fluid
 passages P1-P7 and the temperature of various component parts of the
 hydraulic system, for example, the temperature of the hydraulic pump 22.
 Therefore, the fluid temperature sensor 34 may also be provided in any of
 the fluid passages P1-P7 or a component part such as the hydraulic pump
 22, so as to detect a temperature of hydraulic fluid in that fluid passage
 or a temperature of that component part of the hydraulic system. The
 hydraulic pressure sensor 35 detects a hydraulic pressure P of hydraulic
 fluid ejected from the hydraulic pump 22, and outputs a detection signal
 indicating the hydraulic pressure P.
 A duty ratio control circuit 36 is connected between the microcomputer 30
 and the leveling valves 24a, 24b and between the microcomputer 30 and the
 accumulator valve 26. The duty ratio control circuit 36 is supplied with
 power from the battery 31, and applies rectangular waveform voltages to
 the valves 24a, 24b and 26 having duty ratios determined by control
 signals from the microcomputer 30.
 The operation of the thus-constructed embodiment will be described. When an
 ignition switch (not shown) is turned on to start the engine, the
 microcomputer 30 executes a program (not illustrated) to initially set "0"
 in various flags used in programs described below, and then starts to
 repeatedly execute the main program illustrated in FIG. 2 and the drive
 control program illustrated in FIG. 11 at predetermined short time
 intervals.
 When the main program is started in step 100, the microcomputer 30 receives
 inputs of detection signals from the vehicle height sensors 33a-33c, the
 fluid temperature sensor 34 and the hydraulic pressure sensor 35
 indicating the actual vehicle heights Hf1, Hf2, Hr, the temperature T and
 the hydraulic pressure P in step 102. If the actual vehicle heights Hf1,
 Hf2, Hr, the temperature T and the hydraulic pressure P from the sensors
 33a-33c, 34, 35 have instantaneous changes and therefore are not suitable
 for direct use in the operations by the microcomputer 30, the signals of
 the actual vehicle heights Hf1, Hf2, Hr, the temperature T and the
 hydraulic pressure P are subjected to low-pass filter processing. After
 performing step 102, the microcomputer 30 calculates an actual vehicle
 height Hf (=(Hf1+Hf2)/2) of a front portion of the vehicle body 10 by
 averaging the actual vehicle heights Hf1, Hf2 in step 104.
 Subsequently in step 106, the microcomputer 30 determines whether a second
 suspension flag STP2 is "0". If the second suspension flag STP2 is "0",
 the execution of an accumulator control routine in step 108 is allowed. If
 the second suspension flag STP2 is "1", the execution thereof is
 prohibited. The second suspension flag STP2 is initially set to "0", and
 then set to "1" or "0" by the execution of a suspending control routine in
 step 124 in accordance with the condition of the fluid temperature T
 determined in a fluid temperature determining routine in step 122. After
 steps 106, 108, it is determined in step 110 whether a first suspension
 flag STP1 is "0". If the first suspension flag STP1 is "0", the execution
 of a routine in steps 112 through 120 is allowed. If the first suspension
 flag STP1 is "1", the execution of the routine is prohibited. The first
 suspension flag STP1 is initially set to "0", and then set to "1" or "0"
 by the execution of the suspending control routine in step 124 in
 accordance with the condition of the fluid temperature T determined in the
 fluid temperature routine in step 122.
 The accumulator control routine in step 108 controls the outflow and inflow
 of hydraulic fluid with respect to the accumulator 25. A front raising
 control routine in step 112 raises a front portion of the vehicle body 10
 when the actual vehicle height Hf of the front portion of the vehicle
 deviates at least a predetermined amount downward from a target front
 vehicle height Hf*, so as to automatically return the actual vehicle
 height Hf of the front portion of the vehicle body 10 to the target
 vehicle height Hf*. A rear raising control routine in step 114 raises a
 rear portion of the vehicle body 10 when the actual vehicle height Hr of
 the rear portion of the vehicle deviates at least a predetermined amount
 downward from a target rear vehicle height Hr*, so as to automatically
 return the actual vehicle height Hr of the rear portion of the vehicle
 body 10 to the target vehicle height Hr*. A front lowering control routine
 in step 116 lowers the front portion of the vehicle body 10 when the
 actual vehicle height Hf of the front portion of the vehicle deviates at
 least a predetermined amount upward from the target front vehicle height
 Hf*, so as to automatically return the actual vehicle height Hf of the
 front portion of the vehicle body 10 to the target vehicle height Hf*. A
 rear lowering control routine in step 118 lowers the rear portion of the
 vehicle body 10 when the actual vehicle height Hr of the rear portion of
 the vehicle deviates at least a predetermined amount upward from the
 target rear vehicle height Hr*, so as to automatically return the actual
 vehicle height Hr of the rear portion of the vehicle body 10 to the target
 vehicle height Hr*. A target vehicle height changing routine in step 120,
 when the target vehicle height changing switch 32 is operated, changes the
 target vehicle heights Hf*, Hr* in accordance with the operation on the
 target vehicle height changing switch 32, and raises or lowers the front
 and rear portions of the vehicle body 10 so that the actual vehicle
 heights Hf, Hr become equal to the target vehicle heights Hf*, Hr*.
 The drive control program illustrated in FIG. 11, including steps 800
 through 816, controls the hydraulic pump 22, the leveling valves 24a, 24b
 and the accumulator valve 26. Specifically, the process of steps 801
 through 803 controls the operation and non-operation of the hydraulic pump
 22 in accordance with a pump flag PM that indicates the non-operation of
 the hydraulic pump 22 by "0" and the operation of the hydraulic pump 22 by
 "1". The process of steps 804 through 815 controls the energization and
 non-energization of the valves 24a, 24b, 26 in accordance with valve flags
 LV1, LV2, ACV that indicate the non-energization of the valves 24a, 24b,
 26, respectively, by "0", and the energization thereof by "1". Since these
 flags PM, LV1, LV2, ACV are initially set to "0", the hydraulic pump 22 is
 kept in a non-operated state by the process of steps 801, 802, and the
 valves 24a, 24b, 26 are kept in a non-energized state by the process of
 steps 804, 805, 808, 809, 812, 813. Therefore, the hydraulic fluid in the
 hydraulic cylinders 11a, 11b is retained, and the hydraulic fluid in the
 hydraulic cylinders 11c, 11d is also retained, so that the actual vehicle
 heights Hf, Hr of the front and rear portions of the vehicle are
 maintained at levels where they have been. The operation of the vehicle
 height adjust control apparatus will be described below separately for
 each of the aforementioned routines. For the convenience in description,
 the accumulator control routine will be described after the target vehicle
 height changing routine.
 a. Front Raising Control Routine
 FIG. 3 illustrates the front raising control routine in step 112 in detail.
 When the front raising control routine is started in step 200, the
 microcomputer 30 determines in step 201 whether a front raising flag FU is
 "0". The front raising flag FU indicates by "1" that the control of
 raising the front portion of the vehicle body 10 is being executed. The
 front raising flag FU is initially set to "0". Therefore, during an
 initial period, the determination in step 201 becomes affirmative (YES),
 so that the operation of the program proceeds to step 202. In step 202,
 the microcomputer 30 calculates a vehicle height deviation .DELTA.Hf
 (=Hf-Hf*) by subtracting the target front vehicle height Hf* from the
 actual vehicle height Hf of the front portion of the vehicle. Subsequently
 in step 203, it is determined whether the vehicle height deviation
 .DELTA.Hf is equal to or less than a predetermined negative threshold
 -.DELTA.H, that is, whether the actual vehicle height Hf of the front
 portion of the vehicle deviates at least the threshold .DELTA.H downward
 from the target vehicle height Hf*. If the actual vehicle height Hf of the
 front portion of the vehicle does not deviate downward from the target
 vehicle height Hf* by at least the threshold .DELTA.H, the determination
 in step 203 becomes negative(NO), and the execution of the front raising
 control routine ends in step 226. In this case, the actual vehicle height
 Hf is maintained at a level where it has been.
 Next described will be the operation performed in a case where the actual
 vehicle height Hf of the front portion of the vehicle is reduced by, for
 example, an occupant or a load added. If the actual vehicle height Hf of
 the front portion of the vehicle decreases and the vehicle height
 deviation .DELTA.Hf becomes equal to or less than the threshold -.DELTA.H,
 the microcomputer 30 makes an affirmative determination (YES) in step 203.
 Then in step 204, the microcomputer 30 calculates an accumulated value
 .DELTA.Hfa of vehicle height deviations .DELTA.Hf by performing an
 arithmetic operation represented by expression 1.
EQU .DELTA.Hfa=.DELTA.Hfa+.DELTA.Hf (1)
 Until the accumulated value .DELTA.Hfa becomes equal to or less than a
 predetermined negative value -.DELTA.Ha, the microcomputer 30 repeatedly
 makes a negative determination (NO) in step 205. Because the accumulated
 value .DELTA.Hfa is initially cleared to zero and because step 204 is
 executed at predetermined time intervals, the accumulated value .DELTA.Hfa
 is substantially equivalent to the integral of the vehicle height
 deviation .DELTA.Hf (the amount of deviation of the actual vehicle height
 Hf from the target vehicle height Hf*). If the accumulated value
 (integral) .DELTA.Hfa becomes equal to or less than the predetermined
 value -.DELTA.Ha, the microcomputer 30 makes an affirmative
 determination(YES) in step 205. Then in step 206, the microcomputer 30
 sets the pump flag PM and the valve flag LV1 to "1". After clearing the
 accumulated value .DELTA.Hfa to zero in step 207, the microcomputer 30
 sets the front raising flag FU to "1" in step 208. Subsequently in step
 209, the microcomputer 30 clears a timer count value TMd1 to zero, and
 sets a delay flag DLF1 to "0". The timer count value TMd1 indicates
 elapsed time from the start of energization of the leveling valve 24a, and
 is used to change the duty ratio of the voltage applied to the leveling
 valve 24a during the energization thereof, in accordance with the elapse
 of time. The delay flag DLF1 is used in an operation for discontinuing the
 energization of the leveling valve 24a at a predetermined time following a
 stop of operation of the hydraulic pump 22.
 When the pump flag PM and the valve flag LV1 are set to "1" as described
 above, the microcomputer 30 makes an affirmative determination (YES) in
 steps 801, 804 in the drive control program in FIG. 11. Therefore, by the
 process of steps 803, 807, the instruction to operate the electric motor
 21 is outputted, and the instruction to energize the leveling valve 24a is
 outputted via the duty ratio control circuit 36. As a result, the
 hydraulic pump 22 is driven so that the hydraulic pump 22 draws hydraulic
 fluid from the reservoir tank 23 and ejects it into the fluid passage P5.
 In response to the ejection of hydraulic fluid, the discharge valve 27 is
 switched to the closed state, so that hydraulic fluid is supplied to the
 hydraulic cylinders 11a, 11b through the leveling valve 24a and the fluid
 passages P6, P1, P2. Consequently, the hydraulic cylinders 11a, 11b start
 to raise the positions of the vehicle body 10 at the left and right front
 wheels W1, W2.
 For the energization of the leveling valve 24a, the duty ratio of the
 voltage applied to the leveling valve 24a is determined by the execution
 of the first duty ratio control routine in step 806, and the duty ratio
 control data DC1 representing the duty ratio determined in step 806 is
 outputted to the duty ratio control circuit 36 in step 807. Therefore,
 controlled by the data DC1, the duty ratio control circuit 36 applies to
 the leveling valve 24a a rectangular waveform voltage signal having the
 duty ratio. The first duty ratio control routine is illustrated in detail
 in FIG. 12. When the first duty ratio control routine is started in step
 820, the microcomputer 30 receives inputs of the fluid temperature T from
 the fluid temperature sensor 34 and the output voltage BV of the battery
 31 through the A/D converter 30a in step 821.
 After the execution of step 821, it is determined in step 822 whether the
 timer count value TMd1 becomes equal to or greater than a predetermined
 value TMd0. The timer count value TMd1 has been cleared to zero in step
 209, so that the timer count value TMd1 is less than the predetermined
 value TMd0 immediately after the energization is started. Therefore, the
 determination in step 822 becomes negative (NO), and the operation of the
 program proceeds to step 823. In step 823, the microcomputer 30 sets the
 duty ratio control data DC1 to a large predetermined value I1 (for
 example, a value representing a duty ratio of 100%). Subsequently in step
 824, the microcomputer 30 adds 1 to the timer count TMd1. In step 828, the
 execution of the first duty ratio control routine is ended. The process of
 steps 821 through 824 in the first duty ratio control routine is executed
 every time the drive control program is executed, until the timer count
 TMd1 becomes equal to or greater than the predetermined value TMd0.
 Therefore, the voltage having the duty ratio represented by the large
 predetermined value I1 is continuously applied to the leveling valve 24a
 as indicated in FIG. 13A.
 When the timer count TMd1 becomes equal to or greater than the
 predetermined value TMd0 as time elapses, an affirmative determination
 (YES) is made in step 822, so that the duty ratio control data DC1 is
 determined by the process of steps 825 through 827. The microcomputer 30
 determines correction coefficients It, Ib corresponding to the fluid
 temperature T and the output voltage BV of the battery 31 inputted in step
 821, with reference to the first and second duty ratio correction
 coefficient tables provided in the microcomputer 30, in steps 825, 826,
 respectively. The correction coefficient It gradually changes from a value
 less than "1.0" to a value greater than "1.0" as the fluid temperature T
 increases, as indicated in FIG. 13B. The correction coefficient Ib
 gradually changes from a value greater than "1.0" to a value less than
 "1.0" as the output voltage BV of the battery 31 increases, as indicated
 in FIG. 13C. In step 827, a predetermined value 12 (for example, a value
 indicating a duty ratio of 50%) that is less than the predetermined value
 I1 is multiplied by the correction coefficients It, Ib, and the result of
 the multiplication It.times.Ib.times.I2 is set as the duty ratio control
 data DC1. From this time on, the voltage having the duty ratio
 It.times.Ib.times.I2 calculated in step 827 is continuously applied to the
 leveling valve 24a as indicated in FIG. 13A.
 Through this control of the duty ratio of the voltage applied to the
 leveling valve 24a, the duty ratio of the voltage applied to the leveling
 valve 24a during a period immediately after the start of energization
 thereof is greater than the duty ratio of the voltage applied thereto
 afterwards. Therefore, a large current flows through the coil of the
 leveling valve 24a during the period immediately after the start of
 application of the voltage thereto, during which a large attraction force
 is needed to move the plunger of the leveling valve 24a. During the
 following period when only a small attraction force is needed to retain
 the plunger at a predetermined position, a small current flows through the
 coil. The power consumption is thus reduced while precise operation of the
 leveling valve 24a is ensured. Furthermore, while the voltage having the
 small duty ratio is being applied to the leveling valve 24a, the small
 duty ratio increases as the fluid temperature T increases and as the
 output voltage BV of the battery 31 decreases. Therefore, a coil current
 needed to attract the plunger can be secured even if the temperature of
 the coil in the leveling valve 24a increases so that the resistance of the
 coil increases, or if the output voltage BV of the battery 31 decreases.
 Consequently, the malfunction of the leveling valve 24a due to changes in
 the fluid temperature or the battery voltage can be prevented, and precise
 operation of the leveling valve 24a can be ensured.
 Although this embodiment performs control such that the duty ratio during
 the energization of the leveling valve 24a is changed in accordance with
 the elapsed time, the fluid temperature T and the output voltage BV of the
 battery 31, it is also possible to change the duty ratio in accordance
 with only one or two of the elapsed time, the fluid temperature T and the
 output voltage BV of the battery 31, depending on the operating conditions
 or vehicle types.
 During the rise of the front portion of the vehicle body 10, the front
 raising flag FU is set to "1" in step 208, and the delay flag DLF1 is set
 to "0" in step 209, as described above, so that the microcomputer 30
 continually makes a negative determination (NO) in step 201 and an
 affirmative determination (YES) in step 210. Therefore, the operation of
 the program proceeds to step 211. In step 211, the microcomputer 30
 determines whether the difference Hf-Hf* between the actual vehicle height
 Hf of the front portion of the vehicle and the target vehicle height Hf*
 is equal to or greater than a predetermined negative value -.DELTA.Hu
 whose absolute value is relatively small. As long as the amount of rise of
 the front portion of the vehicle body 10 is small so that the different
 Hf-Hf* is less than the predetermined value -.DELTA.Hu, the microcomputer
 30 continually makes negative determination (NO) in step 211, and ends the
 execution of the front raising control routine in step 266.
 When the difference Hf-Hf* becomes equal to or greater than the
 predetermined value -.DELTA.Hu through the control of raising the front
 portion of the vehicle body 10, the microcomputer 30 makes an affirmative
 determination (YES) in step 211, and determines in step 212 whether an
 accumulator flag AF is "1", and, if it is not "1", determines in step 213
 whether a rear raising flag RU is "1". The accumulator flag AF indicates
 by "1" that the accumulator 25 is under a pressure accumulating control.
 The rear raising flag RU indicates by "1" that the control of raising the
 rear portion of the vehicle body 10 is being executed. If both flags AF,
 RU are "0", the microcomputer 30 makes a negative determination (NO) in
 steps 212, 213, and proceeds to the process of steps 214 through 216. The
 microcomputer 30 sets the pump flag PM to "0" in step 214, clears a timer
 count TMv1 for delaying the switching of the leveling valve 24a to the
 closed state to zero in step 215, and sets the delay flag DLF1 to "1" in
 step 216. Then, the execution of the front raising control routine is
 ended in step 226. Therefore, when the drive control program of FIG. 11 is
 executed, the instruction to stop the electric motor 21 is outputted
 through the process of steps 801, 802. Then, the operation of the electric
 motor 21 is stopped. The next time the front raising control routine is
 executed, the determination in step 210 becomes negative (NO), so that the
 microcomputer 30 executes the process of steps 219 through 225 for
 switching the leveling valve 24a to the closed state after a delay
 following the output of the instruction to stop the hydraulic pump 22.
 After the affirmative determination (YES) in step 211, if either the
 accumulator flag AF or the front raising flag FU is "1", the microcomputer
 30 makes an affirmative determination (YES) in either step 212 or 213, and
 proceeds to steps 217, 218. The microcomputer 30 sets the valve flag LV1
 and the front raising flag FU back to "0" in steps 217, 218, respectively.
 Subsequently in step 226, the execution of the front raising control
 routine is ended. Therefore, when the drive control program of FIG. 11 is
 executed afterwards, the instruction to discontinue the energization of
 the leveling valve 24a is outputted through the process of steps 804, 805.
 Then the leveling valve 24a is switched to the closed state, so that the
 control of raising the front portion of the vehicle body 10 is ended.
 Further, since the front raising flag FU is set back to "0" in step 218,
 the microcomputer 30 will execute the process of steps 202 through 209
 during the next execution of the front raising control program. In steps
 202 through 209, the microcomputer 30 executes an operation for outputting
 an instruction to start increasing the front portion of the vehicle body
 10. The reason why the front raising control routine is ended without
 outputting the instruction to stop the electric motor 21 and the hydraulic
 pump 22 if either the accumulator flag AF or the rear raising flag RU is
 "1" is that the instruction to stop the electric motor 21 and the
 hydraulic pump 22 is outputted in the rear raising control routine or the
 accumulator control routine described below. In addition, the reason why
 the leveling valve 24a is switched to the closed state immediately after
 the detection of completion of raise of the front portion of the vehicle
 body 10 in step 211 is that impact of hydraulic fluid on the leveling
 valve 24a is avoided because the accumulator valve 26 or the leveling
 valve 24b has been set to the open state by the control of accumulating
 pressure in the accumulator or the control of raising the rear portion of
 the vehicle body 10.
 In the process of steps 219 through 225 after the instruction to stop the
 electric motor 21 and the hydraulic pump 22, the microcomputer 30 first
 determines correction coefficients Kt, Kp corresponding to the fluid
 temperature T and the hydraulic pressure P inputted in step 102 of the
 main program of FIG. 2, with reference to first and second delay time
 correction coefficient tables provided in the microcomputer 30, in steps
 219, 220, respectively. The correction coefficient Kt gradually changes
 from a value greater than "1.0" to a value less than "1.0" as the fluid
 temperature T increases, as indicated in FIG. 14A. The correction
 coefficient Kp gradually changes from a value less than "1.0" to a value
 greater than "1.0" as the hydraulic pressure P increases, as indicated in
 FIG. 14B. In step 221, a predetermined value TM0 indicating a delay time
 is multiplied by the correction coefficients Kt, Kp, and the result of the
 multiplication Kt.times.Kp.times.TM0 is set as the delay time value TMx.
 After the execution of step 221, it is determined in step 222 whether the
 timer count value TMv1 has become equal to or greater than the delay time
 value TMx calculated in the previous step. Because the timer count TMv1
 was cleared to zero in step 215, the timer count TMv1 is less than the
 delay time value TMx for a period immediately after the instruction to
 stop the electric motor 21 and the hydraulic pump 22. During such a
 period, therefore, the microcomputer 30 repeatedly makes a negative
 determination (NO) in step 222, adds 1 to TMv1 in step 223, and ends the
 execution of the front raising control routine in step 226. When the timer
 count TMv1 becomes equal to or greater than the delay time value TMx, the
 microcomputer 30 makes an affirmative determination (YES) in step 222, and
 proceeds to steps 224, 225. The microcomputer 30 sets the valve flag LV1
 and the front raising flag FU back to "0" in steps 224, 225, respectively,
 and then ends the execution of the front raising control routine in step
 226. Therefore, when the drive control program of FIG. 11 is performed
 afterwards, the instruction to stop energizing the leveling valve 24a is
 outputted by the process of steps 804, 805. Then, the leveling valve 24a
 is switched to the closed state, and the control of raising the front
 portion of the vehicle body 10 is ended. In addition, since the front
 raising flag FU has been back to "0" in step 225, the microcomputer 30
 will execute the process of steps 202-209 described above the next time
 the front raising control routine is executed. In steps 202 through 209,
 the microcomputer 30 executes an operation for outputting an instruction
 to start raising the front portion of the vehicle body 10.
 By the process of steps 210 and 219-225, the leveling valve 24a is switched
 from the open state to the closed state at the elapse of the delay time
 value TMx following the stop instruction to the electric motor 21 and the
 hydraulic pump 22 has been given. As a result, the leveling valve 24a is
 switched to the closed state after the electric motor 21 and the hydraulic
 pump 22 have substantially stopped, even though the ejection of hydraulic
 fluid from the hydraulic pump 22 may continue due to the inertia of the
 electric motor 21 and the like for a certain time after the instruction to
 stop the electric motor 21 and the hydraulic pump 22. Thus, impact of
 hydraulic pump ejected from the hydraulic pump 22 on the leveling valve
 24a can be prevented or minimized. Consequently, impact noise caused by
 the aforementioned impact can be considerably reduced, and the durability
 of the component parts of the hydraulic system, such as the hydraulic pump
 22, the leveling valve 24a and the like, can be considerably improved.
 Furthermore, the process of steps 219-221 sets the delay time value TMx so
 that as the fluid temperature T decreases and as the hydraulic pressure P
 increases, the value TMx increases. Therefore, in a condition that the
 fluid temperature T is low and the viscosity of the hydraulic fluid is
 high and, as a result, an impact of hydraulic fluid on the leveling valve
 24a will be great, or in a condition that the pressure of hydraulic fluid
 ejected from the hydraulic pump 22 is high and, as a result, an impact of
 hydraulic fluid on the leveling valve 24a will be great, this embodiment
 increases the delay time between the instruction to stop the hydraulic
 pump 22 and the instruction to switch the leveling valve 24a from the open
 state to the closed, thereby effectively preventing or minimizing impact
 of hydraulic fluid on the leveling valve 24a. Consequently, the embodiment
 is able to precisely reduce impact noise without unnecessarily delaying
 the switching of the leveling valve 24a from the open state to the closed
 state if the hydraulic fluid temperature or the ejecting pressure of the
 hydraulic pump 22 changes. As a result, the embodiment precisely prevents
 or minimizes deterioration of the durability of the hydraulic system,
 including the hydraulic pump 22, the leveling valve 24a and the like, that
 is caused by impact of hydraulic fluid thereon.
 Although in this embodiment the length of time between the instruction to
 stop the hydraulic pump 22 and the switching of the leveling valve 24a to
 the closed state is variable in accordance with the fluid temperature T
 and the hydraulic pressure P, it is also possible to omit the variable
 control in accordance with either the fluid temperature T or the hydraulic
 pressure P or both, depending on the operating conditions or vehicle
 types.
 b. Rear Raising Control Routine
 The rear raising control routine of step 114 in the main control
 illustrated in FIG. 2 has steps 250 through 276 as illustrated in detail
 in FIG. 4. The process of steps 251 through 259 controls the start of
 raise of the rear portion of the vehicle body 10. By this operation, the
 microcomputer 30 outputs an instruction to operate the electric motor 21
 and the hydraulic pump 22, and an instruction to switch the leveling valve
 24b to the open state. The process of steps 260 through 275 controls the
 end of raise of the rear portion of the vehicle body 10. By this
 operation, the microcomputer 30 outputs an instruction to stop the
 operation of the electric motor 21 and the hydraulic pump 22 and an
 instruction to switch the leveling valve 24b to the closed state. In this
 operation, the output of the valve switching instruction is delayed a
 predetermined time from the instruction to stop the operation of the
 electric motor 21 and the hydraulic pump 22.
 The rear raising control routine in FIG. 4 is substantially the same as the
 front raising control routine in FIG. 3, except that the various variables
 related to the front portion of the vehicle are replaced by various
 variables related to the rear portion of the vehicle. Therefore, the rear
 raising control routine will not be described in detail. For the rear
 raising control routine, the microcomputer 30 executes in step 810 in FIG.
 11 the second duty ratio control routine, which is illustrated in FIG. 12
 together with the first duty ratio control routine. Therefore, if a rear
 vehicle height reduction occurs, the actual vehicle height Hr of the rear
 portion of the vehicle body 10 is automatically increased to a target
 vehicle height Hr*, thereby achieving substantially the same advantages as
 achieved by the front raising control routine.
 c. Front Lowering Control Routine
 The front lowering control routine of step 116 in the main program of FIG.
 2 is illustrated in detail in FIG. 5. When the routine is started in step
 300, the microcomputer 30 determines in step 301 whether a front lowering
 flag FD is "0". The front lowering flag FD indicates by "1" that the
 control of lowering the front portion of the vehicle body 10 being
 executed. The front lowering flag FD is initially set to "0". Therefore,
 during an initial period, the determination in step 301 becomes
 affirmative (YES), so that the operation of the program proceeds to step
 302. In step 302, the microcomputer 30 calculates a vehicle height
 deviation .DELTA.Hf (=Hf-Hf*) by subtracting the target front vehicle
 height Hf* from the actual vehicle height Hf of the front portion of the
 vehicle, as in step 202 in the front raising control routine. Subsequently
 in step 303, it is determined whether the vehicle height deviation
 .DELTA.Hf is equal to or greater than a predetermined positive threshold
 .DELTA.H, that is, whether the actual vehicle height Hf of the front
 portion of the vehicle deviates at least the threshold .DELTA.H upward
 from the target vehicle height Hf*. If the actual vehicle height Hf of the
 front portion of the vehicle does not deviate upward from the target
 vehicle height Hf* by at least the threshold .DELTA.H, the determination
 in step 303 becomes negative(NO), and the execution of the front lowering
 control routine ends in step 313. In this case, the actual vehicle height
 Hf is maintained at a level where it has been.
 Next described will be the operation performed in a case where the actual
 vehicle height Hf of the front portion of the vehicle is increased by, for
 example, an occupant or a load removed. If the actual vehicle height Hf of
 the front portion of the vehicle increases and the vehicle height
 deviation .DELTA.Hf becomes equal to or greater than the threshold
 .DELTA.H, the microcomputer 30 makes an affirmative determination (YES) in
 step 303. Then in step 304, the microcomputer 30 calculates an accumulated
 value .DELTA.Hfa (=.DELTA.Hfa+.DELTA.Hf) of the vehicle height deviation
 .DELTA.Hf by performing an arithmetic operation of expression (1). Until
 the accumulated value .DELTA.Hfa becomes equal to or greater than a
 predetermined positive value .DELTA.Ha, the microcomputer 30 repeatedly
 makes a negative determination (NO) in step 305. Because the accumulated
 value .DELTA.Hfa is initially cleared to zero and because step 304 is
 executed every predetermined time, the accumulated value .DELTA.Hfa is
 substantially equivalent to the integral of the vehicle height deviation
 .DELTA.Hf (the amount of deviation of the actual vehicle height Hf from
 the target vehicle height Hf*). If the accumulated value (integral)
 .DELTA.Hfa becomes equal to or greater than the predetermined value
 .DELTA.Ha, the microcomputer 30 makes an affirmative determination (YES)
 in step 305. Then in step 306, the microcomputer 30 sets the valve flag
 LV1 to "1". After clearing the accumulated value .DELTA.Hfa to zero in
 step 307, the microcomputer 30 sets the front lowering flag FD to "1" in
 step 308. Subsequently in step 309, the microcomputer 30 clears the timer
 count value TMd1 to zero. The timer count value TMd1 is used to control
 the duty ratio of the voltage applied to the leveling valve 24a during the
 energization thereof.
 When the valve flag LV1 is set to "1" as described above, the microcomputer
 30 makes an affirmative determination (YES) in step 804 in the drive
 control program in FIG. 11. Therefore, by the process of steps 806, 807,
 the leveling valve 24a is energized under control by the duty ratio
 control circuit 36. The leveling valve 24a is thereby switched to the open
 state, so that the hydraulic pump is discharged from the hydraulic
 cylinders 11a, 11b into the reservoir tank 23, through the fluid passages
 P1, P2, P6, the leveling valve 24a, the fluid passage P5 and the discharge
 valve 27. Therefore, the hydraulic cylinders 11a, 11b start lowering the
 positions of the vehicle body 10 at the left and right front wheels W1,
 W2. As in the case of the front raising control routine, the duty ratio
 DC1 for energization of the leveling valve 24a is variably controlled in
 accordance with the elapsed time, the fluid temperature T and the output
 voltage BV of the battery 31, by the execution of the first duty control
 ratio routine of step 806, so that the current needed to attract the
 plunger of the leveling valve 24a can be secured and precise operation of
 the leveling valve 24a can be ensured.
 While the front portion of the vehicle body 10 is being lowered, the front
 lowering flag FD remains at "1" as set in step 308, so that the
 microcomputer 30 repeatedly makes a negative determination (NO) in step
 301, and proceeds to step 310. In step 310, it is determined whether the
 difference Hf-Hf* between the target vehicle height Hf* and the actual
 vehicle height Hf of the front portion of the vehicle is equal to or less
 than a relatively small predetermined value .DELTA.Hd. As long as the
 amount of descent of the front portion of the vehicle body 10 is small so
 that the difference Hf-Hf* is greater than the predetermined value
 .DELTA.Hd, the microcomputer 30 repeatedly makes a negative determination
 (NO) in step 310, and ends the execution of the front lowering control
 routine in step 313.
 When the difference Hf-Hf* becomes equal to or less than the predetermined
 value .DELTA.Hd through the control of lowering the front portion of the
 vehicle body 10, the microcomputer 30 makes an affirmative determination
 (YES) in step 310, and sets the valve flag LV1 and the front lowering flag
 FD back to "0" in steps 311, 312, respectively, and ends the execution of
 the front lowering control routine in step 313. Therefore, when the drive
 control program of FIG. 11 is executed afterwards, the instruction to
 discontinue the energization of the leveling valve 24a is outputted by the
 process of steps 804, 805. Then, the leveling valve 24a is switched to the
 closed state, and the control of lowering the front portion of the vehicle
 body 10 is ended. In addition, since the front lowering flag FD is set
 back to "0" in step 312, the operation of outputting the instruction to
 start lowering the front portion of the vehicle body 10 in steps 302
 through 309 will be executed, the next time the front lowering control
 program is executed.
 d. Rear Lowering Control Routine
 The rear lowering control routine of step 118 in the main program of FIG. 2
 has steps 350 through 363 as illustrated in detail in FIG. 6. The process
 of steps 351 through 359 controls the start of descent of the rear portion
 of the vehicle body 10. By this operation, an instruction to switch the
 leveling valve 24b to the open state is outputted. The process of steps
 360 through 362 controls end of the descent of the rear portion of the
 vehicle body 10. By this control, an instruction to switch the leveling
 valve 24b to the closed state is outputted.
 The rear lowering control routine in FIG. 6 is substantially the same as
 the front lowering control routine in FIG. 5, except that the various
 variables related to the front portion of the vehicle are replaced by
 various variables related to the rear portion of the vehicle. Therefore,
 the rear lowering control routine will not be described in detail. For the
 rear lowering control routine, the microcomputer 30 executes in step 810
 in FIG. 11 the second duty ratio control routine, which is illustrated in
 FIG. 12 together with the first duty ratio control routine. Therefore, if
 a rear vehicle height increase occurs, the actual vehicle height Hr of the
 rear portion of the vehicle body 10 is automatically decreased to the
 target vehicle height Hr*, thereby achieving substantially the same
 advantages as achieved by the front lowering control routine.
 e. Target Vehicle Height Changing Routine
 The target vehicle height changing routine of step 120 in the main program
 of FIG. 2 is illustrated in detail in FIG. 7. When the target vehicle
 height changing routine is started in step 400, the microcomputer 30
 determines in step 401 whether the up-setting element 32a of the target
 vehicle height changing switch 32 is turned on and, if it is not on,
 determines in step 402 whether the down-setting element 32b is turned on.
 If neither the up-setting element 32a nor the down-setting element 32b are
 turned on, the microcomputer 30 makes a negative determination (NO) in
 steps 401, 402, and ends the execution of the target vehicle height
 changing routine in step 416.
 If the up-setting element 32a is turned on, the microcomputer 30 makes
 affirmative determination (YES) in step 401, and determines in step 403
 whether level data LEV is 2. The level data LEV indicates LOW,
 INTERMEDIATE and HIGH target vehicle heights by 0, 1 and 2, respectively.
 If the target vehicle height has been set to HIGH and, therefore, the
 level data LEV is 2, the determination in step 404 becomes affirmative. In
 this case, the target vehicle height cannot be further increased.
 Therefore, the execution of the target vehicle height changing routine is
 ended in step 416. Conversely if the level data LEV is not 2, the
 microcomputer 30 makes a negative determination (NO) in step 403, and
 proceeds to steps 404-406. The microcomputer 30 increases the level data
 LEV by 1 in step 404, and sets the target vehicle heights Hf*, Hr* to
 values corresponding to the increased level data LEV in step 405. The
 level data LEV and the target vehicle heights Hr*, Hr* are stored in the
 non-volatile memory provided in the microcomputer 30, and retained even
 after the ignition switch has been turned off.
 Subsequently in step 406, the pump flag PM and the valve flags LV1, LV2 are
 set to "1". Therefore, when the drive control program of FIG. 11 is
 executed afterwards, the instruction to start the electric motor 21 and
 the instruction to energize the leveling valves 24a, 24b are outputted by
 the process of steps 801, 803, 804, 806, 807, 808, 810 and 811. Thus, the
 hydraulic pump 22 starts to eject hydraulic fluid so that hydraulic fluid
 is supplied to the hydraulic cylinders 11a-11d through the leveling valves
 24a, 24b, thereby simultaneously starting to raise the front and rear
 portions of the vehicle body 10.
 After executing step 406, the microcomputer 30 sets the front raising flag
 FU and the rear raising flag RU to "1" in step 407, and clears the timer
 counts TMd1, TMd2 to zero in step 408, and sets the delay flags DLF1, DLF2
 to "1". Therefore, the process of steps 210 and 219 through 225 is
 executed in the front raising control routine of FIG. 3, and the process
 of steps 260 and 269 through 275 is executed in the r ear raising control
 routine of FIG. 4. By the process of steps 210 and 219 through 225 and
 steps 260 and 269 through 275, the raise of the front and rear portions of
 the vehicle body 10 is ended when the actual vehicle heights Hf, Hr of the
 front and rear portions of the vehicle body 10 become substantially equal
 to the target vehicle heights Hf*, Hr* changed in the target vehicle
 height changing routine. The actual vehicle heights Hf, Hr are thus set to
 the changed target vehicle heights Hf*, Hr*.
 If the down-setting element 32b is turned on, the microcomputer 30 makes an
 affirmative determination (YES) in step 402, and determines in step 410
 whether the level data LEV is 0. If the target vehicle height has already
 been set to LOW and therefore the level data LEV is 0, the microcomputer
 30 makes an affirmative determination (YES) in step 410. In this case, the
 target vehicle height cannot be further reduced. Therefore, the execution
 of the target vehicle height changing routine is ended in step 416.
 Conversely if the level data LEV is not 0, the microcomputer 30 makes a
 negative determination (NO) in step 410, and proceeds to steps 411 through
 415. The microcomputer 30 reduces the level data LEV by 1 in step 411, and
 sets the target vehicle heights Hf*, Hr* to values corresponding to the
 reduced level data LEV in step 412.
 Subsequently in step 413, the valve flags LV1, LV2 are set to "1".
 Therefore, when the drive control program of FIG. 11 is executed
 afterwards, the instruction to energize the leveling valves 24a, 24b is
 outputted by the process of steps 804, 806, 807, 808, 810 and 811. Thus,
 hydraulic fluid is discharged simultaneously from the hydraulic cylinders
 11a-11d through the leveling valves 24a, 24b, thereby simultaneously
 starting to lower the front and rear portions of the vehicle body 10.
 After executing step 413, the microcomputer 30 sets the front lowering flag
 FD and the rear lowering flag RD to "1" in step 414, and clears the timer
 counts TMd1, TMd2 to zero in step 415. Therefore, the process of step 310
 through 312 is executed in the front lowering control routine of FIG. 5,
 and the process of steps 360 through 362 is executed in the rear lowering
 control routine of FIG. 6. By the process of steps 310 through 312 and
 steps 360 through 362, the lowering of the front and rear portions of the
 vehicle body 10 is ended when the actual vehicle heights Hf, Hr of the
 front and rear portions of the vehicle body 10 become substantially equal
 to the target vehicle heights Hf*, Hr* changed in the target vehicle
 height changing routine. The actual vehicle heights Hf, Hr are thus set to
 the changed target vehicle heights Hf*, Hr*.
 f. Accumulator Control Routine
 The accumulator control routine of step 108 in the main program of FIG. 2
 is illustrated in detail in FIG. 8. When the accumulator control routine
 is started in step 500, the microcomputer 30 determines in step 501
 whether the accumulator flag AF is "1". The accumulator flag AF indicates
 by "0" that the hydraulic fluid supply/discharge operation is not being
 performed on the accumulator 25, and indicates by "1" that the hydraulic
 fluid supply/discharge operation is being performed on the accumulator 25.
 The accumulator 25 is initially filled with high-pressure hydraulic fluid
 by execution of an initial program (not shown). The accumulator flag AF is
 initially set to "0" by the initial setting.
 Therefore, the microcomputer 30 initially makes an affirmative
 determination (YES) in step 501, and determines in step 502 whether both
 the front raising flag FU and the rear raising flag RU are "1". If not
 both the flags FU and RU are "1", the microcomputer 30 makes negative
 determination (NO) in step 502, and ends the execution of the accumulator
 control routine in step 519. When, with the setting described above, the
 drive control program of FIG. 11 is subsequently executed, the
 non-energization of the accumulator valve 26 is maintained by the process
 of steps 812, 813 since the valve flag ACV for controlling the
 energization of the accumulator valve 26 is initially set to "0" (NO in
 step 812). Therefore, the closed state of the accumulator valve 26 is
 maintained, so that the high-pressure hydraulic fluid in the accumulator
 25 is maintained.
 If the front raising flag FU and the rear raising flag RU are set to "1" by
 operation of the target vehicle height changing routine, or if the front
 raising flag FU and the rear raising flag RU are set to "1" by the front
 raising control routine and the rear raising control routine, the
 microcomputer 30 makes an affirmative determination (YES) in step 502, and
 executes steps 503 through 506. In step 503, the valve flag ACV is set to
 "1". In step 504, the accumulator flag AF is set to "1". In step 505, a
 timer count TMd3 that is used to determine a duty ratio of the voltage
 applied to the accumulator valve 26 is cleared to zero. In step 506, a
 delay flag ADLF that is used to delay an instruction to switch the
 accumulator valve 26 to the closed state, from an instruction to stop the
 hydraulic pump 22, is set to "0". Subsequently in step 519, the execution
 of the accumulator control routine is ended.
 When the drive control program of FIG. 11 is executed with the settings
 described above, the accumulator valve 26 is energized under control by
 the process of steps 812, 814, 815. Therefore, the high-pressure hydraulic
 fluid accumulated in the accumulator 25 is supplied together with
 hydraulic fluid ejected by the hydraulic pump 22, to the hydraulic
 cylinders 11a-11d, thereby simultaneously raising the vehicle body 10
 relative to all the wheels W1-W4. Therefore, if there is a need to supply
 hydraulic fluid simultaneously to the four hydraulic cylinders 11a-11d,
 hydraulic fluid accumulated in the accumulator 25 is used together with
 hydraulic fluid ejected by the hydraulic pump 22 so as to quickly raise
 the vehicle body 10 relative to all the wheels W1-W4. Therefore, the
 capacity of the hydraulic pump 22 does not need to be very large.
 For energization of the accumulator valve 26, the duty ratio of the voltage
 applied to the accumulator valve 26 is controlled by execution of the
 third duty ratio control routine of step 814. The third duty ratio control
 routine is similar to the first duty ratio control routine illustrated in
 FIG. 12. That is, in the third duty ratio control routine, the duty ratio
 DC3 is also variably controlled in accordance with the elapsed time, the
 fluid temperature T and the output voltage BV of the battery 31, thereby
 ensuring precise operation of the accumulator valve 26.
 When the accumulator valve 26 is switched to the open state as described
 above, the accumulator flag AF has been set to "1" by step 504, and the
 delay flag ADLF has been set to "0" by step 506. Therefore, in the next
 and later executions of the accumulator control routine, the microcomputer
 30 repeatedly makes a negative determination (NO) in step 501, and
 repeatedly makes an affirmative determination (YES) in step 507, and
 proceeds to step 508. In step 508, it is determined whether the hydraulic
 pressure P inputted in step 102, that is, the hydraulic pressure in the
 fluid passage P5, is equal to or greater than a predetermined hydraulic
 pressure P0. The predetermined hydraulic pressure P0 represents a
 hydraulic pressure accumulated in the accumulator 25, and has been set to
 a value that is higher than the hydraulic pressure accumulated in the
 hydraulic cylinders 11a-11d. Therefore, while the leveling valves 24a, 24b
 are open during the control of raising the front or rear portion of the
 vehicle body 10, the hydraulic pressure P remains lower than the
 predetermined hydraulic pressure P0, so that the microcomputer 30
 repeatedly makes a negative determination (NO) in step 508, and ends the
 execution of the accumulator control routine in step 519.
 When the control of raising the front and rear portion of the vehicle body
 10 ends and the leveling valves 24a, 24b are switched to the closed state,
 hydraulic fluid ejected from the hydraulic pump 22 starts to flow through
 the accumulator valve 26 into and accumulate in the accumulator 25 and, at
 the same time, the hydraulic pressure P in the fluid passage P5 also
 starts to increase. This is achieved by the process of steps 212, 217 in
 the front raising control routine of FIG. 3 and by the process of steps
 262, 267 in the rear raising control routine of FIG. 4. That is, if the
 accumulator flag AF is "1", the leveling valves 24a, 24b are switched to
 the closed state while the operation of the electric motor 21 and the
 hydraulic pump 22 is continued. Subsequently, when the hydraulic pressure
 P becomes equal to or greater than the predetermined hydraulic pressure
 P0, the microcomputer 30 makes an affirmative determination (YES) in step
 508, and proceeds to the process of steps 509 through 511.
 In step 509, the pump flag PM is set to "0". In step 510, a timer count ATM
 for delaying the switching of the accumulator valve 26 to the closed state
 is cleared to zero. In step 511, the delay flag ADLF is set to "1".
 Subsequently in step 519, the execution of the accumulator control routine
 is ended. Therefore, when the drive control program of FIG. 11 is executed
 afterwards, the instruction to stop the electric motor 21 is outputted by
 the process of steps 801, 802. The operation of the electric motor 21 is
 subsequently stopped. The next time the accumulator control routine is
 executed, the microcomputer 30 makes a negative determination (NO) in step
 507, and proceeds to the process of steps 512 through 518.
 The process of steps 512 through 516, similar to the process of steps 219
 through 223 in the front raising control routine of FIG. 3, switches the
 accumulator valve 26 to the closed state after the delay time value TMx
 following the output of the instruction to stop the hydraulic pump 22, and
 changes the delay time value TMx in accordance with the fluid temperature
 T and the hydraulic pressure P. In this process, the timer count value
 ATM, cleared to zero in step 510, is increased by 1 in step 516. When the
 timer count ATM becomes equal to or greater than the delay time value TMx,
 the microcomputer 30 makes an affirmative determination (YES) in step 515,
 and proceeds to steps 517, 518. In step 517, the valve flag ACV is set
 back to "0". In step 518, the accumulator flag AF is set back to "0".
 Subsequently in step 519, the execution of the accumulator control routine
 is ended. When the drive control program of FIG. 11 is next executed, the
 energization of the accumulator valve 26 is discontinued by the process of
 steps 812, 813. The accumulator valve 26 is thereby switched to the closed
 state. Therefore, the accumulator 25 retains therein hydraulic fluid
 accumulated to a high pressure that is equal to or higher than the
 predetermined hydraulic pump P0.
 By this operation, the accumulator valve 26 is also switched to the closed
 state after the delay time value TMx following the output of the
 instruction to stop the hydraulic pump 22, as in the operation for the
 leveling valves 24a, 24b. Therefore, this embodiment avoids impacts of
 hydraulic fluid on the accumulator valve 26 even if ejection of hydraulic
 fluid by the hydraulic pump 22 continues for a certain time due to the
 inertia of the electric motor 21 and the like after the instruction to
 stop the electric motor 21 and the hydraulic pump 22 has been outputted.
 g. Fluid Temperature Determining Routine
 The fluid temperature determining routine of step 122 in the main program
 of FIG. 2 is illustrated in detail in FIG. 9. After starting to execute
 the fluid temperature determining routine in step 600, the microcomputer
 30 sets first and second fluid temperature condition flag TMP1, TMP2 to
 "1" or "0" on the basis of the fluid temperature T inputted in step 102 in
 the main program of FIG. 2.
 After the fluid temperature determining routine is started in step 600, the
 microcomputer 30 determines in step 610 whether the fluid temperature T is
 equal to or higher than a predetermined temperature T2. Before the
 description of the subsequent processing, the predetermined temperature T2
 and predetermined temperatures T1, T3, T4 (mentioned below) will be
 described. The magnitude relationship among the predetermined temperatures
 T1 to T4 is T1&lt;T2&lt;T3&lt;T4 as indicated in FIG. 15. The predetermined
 temperatures T1, T2 are relatively close to each other (for example,
 -30.degree. C. and -25.degree. C.). If the fluid temperature T is lower
 than these predetermined temperatures, the viscosity of the hydraulic
 fluid becomes very high and the fluidity thereof becomes very low. If the
 hydraulic pump 22 is operated in such a hydraulic fluid condition, the
 load on the electric motor 21 and the hydraulic pump 22 becomes very
 large. The predetermined temperatures T3, T4 are relatively close to each
 other (for example, 95.degree. C. and 100.degree. C.). If the fluid
 temperature T is higher than these predetermined temperatures, the
 viscosity of the hydraulic fluid becomes very low. If the hydraulic pump
 22 is operated in such a hydraulic fluid condition, the ejecting
 performance of the hydraulic pump 22 is very low and, therefore, the
 electric motor 21 and the hydraulic pump 22 must be operated for an
 inconveniently long time in order to raise the vehicle body 10 to a
 predetermined height.
 If the fluid temperature T is equal to or higher than the predetermined
 temperature T2, the microcomputer 30 makes an affirmative determination
 (YES) in step 610, and sets a first low temperature flag TL1 to "0" in
 step 611. Subsequently in step 612, it is determined whether the fluid
 temperature T is equal to or lower than the predetermined temperature T3.
 If the fluid temperature T is equal to or lower than the predetermined
 temperature T3, the microcomputer 30 makes an affirmative determination
 (YES) in step 612, and sets a first high temperature flag TH1 to "0" in
 step 613. Subsequently in step 614, a first temperature condition flag
 TMP1 is set to "0".
 Conversely if the fluid temperature T is less than the predetermined
 temperature T2, the microcomputer 30 makes a negative determination (NO)
 in step 610, and determines in step 615 whether the fluid temperature T is
 equal to or lower than the predetermined temperature T1. If the fluid
 temperature T is equal to or lower than the predetermined temperature T1,
 the microcomputer 30 makes an affirmative determination (YES) in step 615,
 and sets the first low temperature flag TL1 to "1" in step 616, and sets
 the first temperature condition flag TMP1 to "1" in step 617. If the fluid
 temperature T is higher than the predetermined temperature T1 and less
 than the predetermined temperature T2, the microcomputer 30 makes a
 negative determination (NO) in steps 610, 615, and then determines in step
 618 whether the first low temperature flag TL1 is "0". If the first low
 temperature flag TL1 is "0", the microcomputer 30 makes an affirmative
 determination (YES) in step 618, and proceeds to step 614 while holding
 the first low temperature flag TL1 at "0". Conversely, if the first low
 temperature flag TL1 is "1", the microcomputer 30 makes a negative
 determination (NO) in step 618, and proceeds to step 617 while holding the
 first low temperature flag TL1 at "1".
 If the fluid temperature T is higher than the predetermined temperature T3,
 the microcomputer 30 makes a negative determination (NO) in step 612, and
 then determines in step 619 whether the fluid temperature T is equal to or
 higher than predetermined temperature T4. If the fluid temperature T is
 equal to or higher than the predetermined temperature T4, the
 microcomputer 30 makes an affirmative determination (YES) in step 619, and
 then sets the first high temperature flag TH1 to "1" in step 620, and sets
 the first temperature condition flag TMP1 to "1" in step 621. If the fluid
 temperature T is higher than the predetermined temperature T3 but lower
 than the predetermined temperature T4, the microcomputer 30 makes a
 negative determination (NO) in steps 612, 619, and then determines in step
 622 whether the first high temperature flag TH1 is "0". If the first high
 temperature flag TH1 is "0", the microcomputer 30 makes an affirmative
 determination (YES) in step 622, and proceeds to step 614 while holding
 the first high temperature flag TH1 at "0". If the first high temperature
 flag TH1 is "1", the microcomputer 30 makes a negative determination (NO)
 in step 622, and proceeds to step 621 while holding the first high
 temperature flag TH1 at "1".
 By the process of steps 610 through 621, the first temperature condition
 flag TMP1 is always set to "0" if the fluid temperature T is equal to or
 higher than the predetermined temperature T2 and equal to or lower than
 the predetermined temperature T3, and the first temperature condition flag
 TMP1 is always set to "1" if the fluid temperature T is equal to or lower
 than the predetermined temperature T1 or equal to or higher than the
 predetermined temperature T4. If the fluid temperature T is higher than
 the predetermined temperature T1 but lower than the predetermined
 temperature T2, or if the fluid temperature T is higher than the
 predetermined temperature T3 but lower than the predetermined temperature
 T4, the first temperature condition flag TMP1 is kept at "0" or "1" as it
 has been set, by the hysteresis process of steps 618, 622 and the like.
 Therefore, fluctuation of the fluid temperature T in the neighborhood of
 the predetermined temperatures T1, T2 or in the neighborhood of the
 predetermined temperatures T3, T4 will not cause frequent switching of the
 first temperature condition flag TMP1 between "0" and "1". However, if
 fluctuation of the fluid temperature T does not appear as a problem due
 to, for example, a relatively low sensitivity of the fluid temperature
 sensor 34 or low-pass filter processing of the fluid temperature T
 detected by the fluid temperature sensor 34, it is also possible to omit
 the hysteresis process. That is, it is possible to design a process
 wherein the first temperature condition flag TMP1 is always set to "0" if
 the fluid temperature T is higher than the predetermined temperature T1
 and lower than the predetermined temperature T4, and the first temperature
 condition flag TMP1 is always set to "1" if the fluid temperature T is
 equal to or lower than the predetermined temperature T1 or equal to or
 higher than the predetermined temperature T4.
 After executing the process as described above, the microcomputer 30
 executes the process of steps 630 through 641, and ends the execution of
 the fluid temperature determining program in step 650. The process of
 steps 630 through 641 sets a second temperature condition flag TMP2 by
 comparing the fluid temperature T with predetermined temperatures T5, T6,
 T7, T8 wherein T5&lt;T6&lt;T7&lt;T8 as indicated in FIG. 15. This process is
 substantially the same as the process of steps 610 through 621, except
 that the predetermined temperatures T1 through T4, the first low
 temperature flag TL1, the first high temperature flag TH1 and the first
 temperature condition flag TMP1 are replaced by the predetermined
 temperatures T5 through T8, a second low temperature flag TL2, a second
 high temperature flag TH2 and the second temperature condition flag TMP2.
 Therefore, the process of steps 630 through 641 will not be described in
 detail. The predetermined temperatures T5, T6 are relatively close to each
 other (for example, -15.degree. C. and -10.degree. C.). If the fluid
 temperature T is lower than these predetermined temperatures, the
 viscosity of the hydraulic fluid becomes quite high and the fluidity
 thereof becomes quite low. If the hydraulic pump 22 is operated in such a
 hydraulic fluid condition, the load on the electric motor 21 and the
 hydraulic pump 22 becomes quite large. The predetermined temperatures T7,
 T8 are relatively close to each other (for example, 85.degree. C. and
 90.degree. C.). If the fluid temperature T is higher than these
 predetermined temperatures, the viscosity of the hydraulic fluid becomes
 quite low. If the hydraulic pump 22 is operated in such a hydraulic fluid
 condition, the ejecting performance of the hydraulic pump 22 is quite low
 and, therefore, the electric motor 21 and the hydraulic pump 22 must be
 operated for a long time in order to raise the vehicle body 10 to a
 predetermined height.
 By the process of steps 630 through 641, the second temperature condition
 flag TMP2 is always set to "0" if the fluid temperature T is equal to or
 higher than the predetermined temperature T6 and equal to or lower than
 the predetermined temperature T7, and the second temperature condition
 flag TMP2 is always set to "1" if the fluid temperature T is equal to or
 lower than the predetermined temperature T5 or equal to or higher than the
 predetermined temperature T8. If the fluid temperature T is higher than
 the predetermined temperature T5 but lower than the predetermined
 temperature T6, or if the fluid temperature T is higher than the
 predetermined temperature T7 but lower than the predetermined temperature
 T8, the second temperature condition flag TMP2 is kept at "0" or "1" as it
 has been set, by the hysteresis process of steps 638, 642 and the like.
 Therefore, fluctuation of the fluid temperature T in the neighborhood of
 the predetermined temperatures T5, T6 or in the neighborhood of the
 predetermined temperatures T7, T8 will not cause frequent switching of the
 second temperature condition flag TMP2 between "0" and "1". However, if
 fluctuation of the fluid temperature T does not appear as a problem due
 to, for example, a relatively low sensitivity of the fluid temperature
 sensor 34 or low-pass filter processing of the fluid temperature T
 detected by the fluid temperature sensor 34, it is also possible to omit
 the hysteresis process. That is, it is possible to design a process
 wherein the second temperature condition flag TMP2 is always set to "0" if
 the fluid temperature T is higher than the predetermined temperature T5
 and lower than the predetermined temperature T8, and the second
 temperature condition flag TMP2 is always set to "1" if the fluid
 temperature T is equal to or lower than the predetermined temperature T5
 or equal to or higher than the predetermined temperature T8.
 h. Suspending Control
 The suspending control routine of step 124 in the main program of FIG. 2 is
 illustrated in detail in FIG. 10. When the suspending control routine is
 started in step 700, the microcomputer 30 determines in step 701 whether
 the first temperature condition flag TMP1 is "0". If the first temperature
 condition flag TMP1 is "0", the microcomputer 30 makes an affirmative
 determination (YES) in step 701, and then determines in step 702 whether
 the first suspension flag STP1 is "0". The first suspension flag STP1
 indicates by "1" that the vehicle height adjustment is suspended. The
 first suspension flag STP1 is initially set to "0". In this case,
 therefore, the microcomputer 30 makes an affirmative determination (YES)
 in step 702, and proceeds to step 710. If the first suspension flag STP1
 is "0", the microcomputer 30 makes an affirmative determination (YES) in
 step 110 in the main program of FIG. 2, so that the process of steps 110
 through 120, that is, the vehicle height adjusting operation, is allowed.
 If the fluid temperature T becomes very low or very high so that the first
 temperature condition flag TMP1 is set to "1", the microcomputer 30 makes
 a negative determination (NO) in step 701, and proceeds to step 703. In
 step 703, it is determined whether the first suspension flag STP1 is "0".
 Since the first suspension flag STP1 is initially set to "0" as mentioned
 above, the microcomputer 30 initially makes an affirmative determination
 (YES) in step 703, and then determines in step 704 whether the level data
 LEV is 2. If the level data LEV is 2, the microcomputer 30 makes an
 affirmative determination (YES) in step 704, and executes step 705.
 Step 705 is a step for setting the target vehicle heights Hf*, Hr* of the
 front and rear portions of vehicle back to the INTERMEDIATE state if they
 have been set to the HIGH state. In step 705, the microcomputer 30 changes
 the level data LEV to 1, and sets each of the target vehicle heights Hf*,
 Hr* to a value corresponding to the changed level data LEV (that is, 1).
 Then, the leveling valves 24a, 24b are energized to switch them to the
 open state, so that the font and rear portions of the vehicle body 10 are
 lowered. When the vehicle heights Hf, Hr of the front and rear portions
 become substantially equal to the target vehicle heights Hf*, Hr*, the
 leveling valves 24a, 24b are switched to the closed state, thereby ending
 the vehicle height lowering control. If the electric motor 21 and the
 hydraulic pump 22 are in operation, the electric motor 21 and the
 hydraulic pump 22 are stopped. If the accumulator valve 26 is in the open
 state, the accumulator valve 26 is switched to the closed state.
 Therefore, when the vehicle height adjustment is suspended, the vehicle
 height is maintained at not the HIGH state but the INTERMEDIATE state, so
 that good driving stability of the vehicle is ensured.
 After executing steps 704, 705, the microcomputer 30 stops the electric
 motor 21 and the hydraulic pump 22 in step 706 if they are in operation.
 Furthermore, in step 706, the leveling valves 24a, 24b and the accumulator
 valve 26 are switched to the closed state if they are in the open state.
 Subsequently, the microcomputer 30 sets the first suspension flag STP1 to
 "1" in step 707, and then proceeds to step 710. Therefore, the next time
 the main program of FIG. 2 is executed, the microcomputer 30 makes a
 negative determination (NO) in step 110, and therefore skips the process
 of steps 112 through 120. The vehicle height adjustment is thus suspended.
 In step 706 of the suspending control routine, the front raising flag FU,
 the front lowering flag FD, the rear raising flag RU and the rear lowering
 flag RD are maintained as they have been set, so as to resume the vehicle
 height adjustment after the suspension.
 When, during the suspension of the vehicle height adjustment as described
 above, the fluid temperature T increases or decreases so that the first
 temperature condition flag TMP1 is set back to "0", the microcomputer 30
 makes an affirmative determination (YES) in step 701, and then proceeds to
 step 702. In this case, the first suspension flag STP1 is still at "1",
 the microcomputer 30 makes a negative determination (NO) in step 702, and
 then executes step 708. Step 708 is a step for restarting the vehicle
 height adjustment from a suspension. In step 708, the operation of the
 electric motor 21 and the hydraulic pump 22 is restarted if the front
 raising flag FU or the rear raising flag RU is "1".
 Furthermore, if the front raising flag FU or the front lowering flag FD is
 "1", the leveling valve 24a is switched to the open state. If the rear
 raising flag RU or the rear lowering flag RD is "1", the leveling valve
 24b is switched to the open state. If the flags FU, FD, RU, RD are "0",
 the microcomputer 30 refrains from operating the electric motor 21 and the
 hydraulic pump 22, and from switching the leveling valves 24a, 24b to the
 open state. Subsequently in step 709, the microcomputer 30 sets the first
 suspension flag STP1 back to "0", and proceeds to step 710. By this
 operation, the control of raising or lowering the front or rear portion of
 the vehicle body 10 is restarted. Furthermore, an affirmative
 determination (YES) is made in step 110 of the main program of FIG. 2, so
 that the vehicle height adjustment control of steps 112 through 120 is
 restarted.
 By the process of steps 701 through 709, the vehicle height adjustment
 control is suspended if the fluid temperature T becomes equal to or lower
 than the predetermined temperature T1 or equal to or higher than the
 predetermined temperature T4. The suspended vehicle height adjustment is
 restarted when the fluid temperature T becomes equal to or higher than the
 predetermined temperature T2 or equal to or lower than the predetermined
 temperature T3. Therefore, if the fluid temperature T becomes very low or
 very high so that the viscosity of the hydraulic fluid becomes very high
 or very low, or so that the fluidity of the hydraulic fluid becomes very
 low or very high, the operation of the hydraulic pump 22 is stopped. In
 this manner, the durability or service life of the various components of
 the hydraulic system, including the hydraulic pump 22, the valves 24a,
 24b, 26 and the like, is increased.
 In step 710 of the suspending control routine, the microcomputer 30
 determines whether the second temperature condition flag TMP2 is "0". If
 the second temperature condition flag TMP2 is "0", the microcomputer 30
 makes an affirmative determination (YES) in step 710, and then determines
 in step 711 whether a second suspension flag STP2 is "0". The second
 suspension flag STP2 indicates by "1" that the hydraulic fluid
 supply/discharge operation for the accumulator 25 is suspended. The second
 suspension flag STP2 is initially set to "0". Therefore, in this case, the
 microcomputer 30 makes an affirmative determination (YES) in step 711, and
 ends the execution of the suspending control routine in step 717.
 Therefore, the next time the main program of FIG. 2 is executed, the
 microcomputer 30 makes an affirmative determination (YES) in step 106, so
 that the accumulator control routine of step 108, that is, the hydraulic
 fluid supply/discharge operation for the accumulator 25, is allowed.
 When the fluid temperature T increases or decreases to an extent such that
 the second temperature condition flag TMP2 is set to "1", the
 microcomputer 30 makes a negative determination (NO) in step 710, and
 proceeds to step 712. In step 712, it is determined whether the second
 suspension flag STP2 is "0". Since the second suspension flag STP2 is
 initially set to "0" as mentioned above, the microcomputer 30 makes an
 affirmative determination (YES) in step 712, and then executes step 713.
 In step 713, the operation of the electric motor 21 and the hydraulic pump
 22 is stopped if they are in operation. At the same time, the accumulator
 valve 26 is switched to the closed state if it is in the open state.
 Subsequently, the microcomputer 30 sets the second suspension flag STP2 to
 "1" in step 714, and ends the execution of the suspending control routine
 in step 717. Therefore, the next time the main program of FIG. 2 is
 executed, the microcomputer 30 makes a negative determination (NO) in step
 106, so that accumulator control routine of step 108 is skipped. The
 hydraulic fluid supply/discharge operation for the accumulator 25 is thus
 suspended. In step 713 of the suspending control routine, the accumulator
 flag AF is maintained as it has been set, so as to resume the hydraulic
 fluid supply/discharge operation for the accumulator 25 after the
 suspension.
 When, during the suspension of the control of the accumulator 25 as
 described above, the fluid temperature T increases or decreases so that
 the second temperature condition flag TMP2 is set back to "0", the
 microcomputer 30 makes an affirmative determination (YES) in step 710, and
 then proceeds to step 711. In this case, the second suspension flag STP2
 is still at "1", the microcomputer 30 makes a negative determination (NO)
 in step 711, and then executes step 715. Step 715 is a step for restarting
 the hydraulic fluid supply/discharge operation for the accumulator 25 from
 a suspension. In step 715, the operation of the electric motor 21 and the
 hydraulic pump 22 is restarted and the accumulator valve 26 is switched to
 the open state, if the accumulator flag AF is "1". If the accumulator flag
 AF is "0", the microcomputer 30 refrains from operating the electric motor
 21 and the hydraulic pump 22, and from switching the accumulator valve 26
 to the open state. Subsequently in step 716, the microcomputer 30 sets the
 second suspension flag STP2 back to "0", and ends the execution of the
 suspending control routine in step 717. By this operation, the control of
 the hydraulic fluid supply/discharge operation for the accumulator 25 is
 restarted. Furthermore, an affirmative determination (YES) is made in step
 106 of the main program of FIG. 2, so that the accumulator control routine
 of step 108 is restarted.
 By the process of steps 710 through 716, the control of the accumulator 25
 is suspended if the fluid temperature T becomes equal to or lower than the
 predetermined temperature T5 or equal to or higher than the predetermined
 temperature T8. The suspended control of the accumulator 25 is restarted
 when the fluid temperature T becomes equal to or higher than the
 predetermined temperature T6 or equal to or lower than the predetermined
 temperature T7. Therefore, if the fluid temperature T decreases or
 increases to an extent such that the viscosity of the hydraulic fluid
 becomes quite high or quite low, or so that the fluidity of the hydraulic
 fluid becomes quite low or quite high, the operation of the hydraulic pump
 22 is stopped. In this manner, the durability or service life of the
 various components of the hydraulic system, including the hydraulic pump
 22, the accumulator 25 and the like, is increased.
 The delay control and the duty ratio control as described above may also be
 performed for the control of the operation of the hydraulic pump 22 and
 the control of the switching of the valves 24a, 24b, 26
 In the fluid temperature determining routine and the suspending control
 routine according the aforementioned embodiment, if the fluid temperature
 T becomes equal to or lower than the predetermined temperature T1 or T5 or
 becomes equal to or higher than the predetermined temperature T4 or T8,
 the first or second temperature condition flag TMP1, TMP2 is set to "1" so
 as to suspend the vehicle height adjustment control or the control of the
 accumulator 25. However, this manner of the suspending control may be
 modified. For example, in a case where a type of hydraulic fluid is used
 which does not undergo significant viscosity reduction nor significant
 fluidity increase if the fluid temperature T increases, or in a case where
 there is substantially no possibility that the fluid temperature T will
 become equal to or higher than the predetermined temperature T4 or T8, it
 is possible to simplify the suspending control program so that only when
 the fluid temperature T becomes equal to or lower than the predetermined
 temperature T1 or T5, the first or second temperature condition flag TMP1,
 TMP2 is set to "1" to suspend the vehicle height adjustment control or the
 control of the accumulator 25. Furthermore, in a case, for example, where
 a type of hydraulic fluid is used which does not undergo significant
 viscosity increase nor significant fluidity reduction if the fluid
 temperature T decreases, or where there is substantially no possibility
 that the fluid temperature T will become equal to or lower than the
 predetermined temperature T1 or T5, the suspending control program may
 also be simplified so that only when the fluid temperature T becomes equal
 to or higher than the predetermined temperature T4 or T8, the first or
 second temperature condition flag TMP1, TMP2 is set to "1" to suspend the
 vehicle height adjustment control or the control of the accumulator 25.
 Although in the foregoing embodiment, two vehicle height sensors 33a, 33b
 are provided in the front portion of the vehicle body 10, and one vehicle
 height sensor 33c is provided in the rear portion thereof, it is also
 possible to provide one vehicle height sensor in each of the front portion
 and the rear portion of the vehicle body 10 so as to detect the actual
 vehicle heights Hf, Hr of the front and rear portions thereof. It is also
 possible to provide one vehicle height sensor at each of the positions
 corresponding to the left and right rear wheels W3, W4 and detect the
 vehicle height of the rear portion of the vehicle body 10 by averaging the
 vehicle heights detected by the sensors.
 Although in the foregoing embodiment, the invention is applied to a vehicle
 height adjust control apparatus that raises or lowers the front portion
 and the rear portion of the vehicle body 10 separately or simultaneously,
 the invention may also be applied to a vehicle height adjust control
 apparatus that raises or lowers the vehicle body 10 separately for the
 positions corresponding to the wheels W1-W4, or raises or lowers the
 entire vehicle body 10 simultaneously at all the positions. It is also
 possible to apply the invention to a vehicle height adjust control
 apparatus that raises or lowers the right portion and the left portion of
 the vehicle body 10 separately or simultaneously. In such control
 apparatus, the supply and discharge of hydraulic fluid with respect to the
 hydraulic cylinders 11a-11d disposed at positions corresponding to the
 wheels W1-W4 may be controlled separately for each of the positions or
 separately for the left and right positions.
 While the present invention has been described with reference to what is
 presently considered to be preferred embodiments thereof, it is to be
 understood that the invention is not limited to the disclosed embodiments.
 To the contrary, the invention is intended to cover various modifications
 and equivalent arrangements.