Abstract:
A vehicle cooling system includes: a channel allowing a liquid medium cooling a drive device of the vehicle to circulate; a flow rate detection unit detecting a flow rate of the liquid medium flowing in the channel; a temperature sensor detecting a temperature of the liquid medium; a pump provided on the channel for circulating the liquid medium; a rotational speed sensor detecting a rotational speed of the pump; and a control device controlling drive of the pump. The control device identifies a malfunctioning part of the cooling system based on the flow rate and the temperature of the liquid medium and the rotational speed of the pump. Thus, since abnormalities of the cooling mechanism can be detected with higher precision so that the abnormalities are distinguished from each other, it is a limited part that should be checked when repairs are made and the work efficiency is improved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2011/055070, filed on Mar. 4, 2011, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a cooling system for vehicles, and particularly to a cooling system for vehicles that has a control device identifying a malfunctioning part of the cooling system. 
     BACKGROUND ART 
     As an example of the technique for making a determination that a failure has occurred to a cooling mechanism of a vehicle, an abnormality determination apparatus is disclosed in Japanese Patent Laying-Open No. 2009-46077 (PTL 1). This abnormality determination apparatus prevents an erroneous determination that the drive state of an electric water pump is abnormal in spite of the fact that the drive state is normal. 
     Specifically, when the engine is stopped, an electronic control device pressure-feeds a coolant to a heater core provided in a flow channel extending from the electric pump to the engine. When the temperature of the coolant in the heater core is lower by a predetermined value or more than the temperature of the coolant in the engine, the control device determines that the drive state of the electric pump is abnormal. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Laying-Open No. 2009-46077 
         PTL 2: Japanese Patent Laying-Open No. 2009-221874 
         PTL 3: Japanese Patent Laying-Open No. 2004-76647 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The abnormality determination apparatus disclosed in Japanese Patent Laying-Open No. 2009-46077 is adapted to the determination that an abnormality has occurred to the water pump, among abnormalities of the cooling mechanism. Thus, if an abnormality occurs to any of other parts, the apparatus is unable to detect this abnormality as being distinguished from the abnormality of the water pump. By way of example, abnormalities of the cooling mechanism include an abnormality of a control signal for the water pump, an abnormality occurring to the hardware of the water pump itself, an abnormality of the flow channel, and an abnormality of the heat dissipation system. It has therefore been troublesome to identify a failed part, in such a case where repairs were to be done. 
     An object of the present invention is to provide a cooling system for vehicles that is capable of detecting abnormalities of the cooling mechanism with higher precision in such a manner that the abnormalities are distinguished from each other. 
     Solution to Problem 
     In summary, the present invention is a cooling system for a vehicle, and includes: a channel allowing a liquid medium cooling a drive device of the vehicle to circulate; a flow rate detection unit detecting a flow rate of the liquid medium flowing in the channel; a temperature sensor detecting a temperature of the liquid medium; a pump provided on the channel for circulating the liquid medium; a rotational speed sensor detecting a rotational speed of the pump; and a control device controlling drive of the pump. The control device identifies a malfunctioning part of the cooling system based on the flow rate of the liquid medium, the temperature of the liquid medium, and the rotational speed of the pump. 
     Preferably, in a case where the temperature of the liquid medium and the rotational speed of the pump are normal and the flow rate of the liquid medium is smaller than a normal value, the control device temporarily increases the rotational speed of the pump and, when the flow rate subsequently fails to return to the normal value, the control device identifies the channel as the malfunctioning part. 
     Preferably, in a case where the temperature of the liquid medium is normal, the rotational speed of the pump is smaller than a normal value, and the flow rate of the liquid medium is smaller than a normal value, the control device identifies the pump as the malfunctioning part. 
     Preferably, the cooling system further includes: a radiator provided on the channel; and a fan for blowing air to the radiator. In a case where the temperature of the liquid medium is abnormal and the rotational speed of the pump and the flow rate of the liquid medium are normal, the control device detects a heat-generation or heat-dissipation abnormality based on an operating state of the fan and an inverter temperature. 
     Advantageous Effects of Invention 
     According to the present invention, abnormalities of the cooling mechanism can be detected with higher precision in such a manner that the abnormalities are distinguished from each other, and therefore, it is a limited part that should be checked when repairs are to be made thereon and the work efficiency is improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram showing a configuration of a vehicle  100  mounted with a cooling system for the vehicle. 
         FIG. 2  is a diagram showing a relation between the flow resistance of the cooling mechanism and the flow rate. 
         FIG. 3  is a diagram showing possible abnormalities that are considered to occur based on the coolant temperature, the rotational speed of the pump, and the flow rate, as well as how to verify them. 
         FIG. 4  is a flowchart for illustrating a diagnostic process performed by a control device  30  in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will hereinafter be described in detail with reference to the drawings. In the drawings, the same or corresponding components are denoted by the same reference characters, and a description thereof will not be repeated. 
       FIG. 1  is a circuit diagram showing a configuration of a vehicle  100  mounted with a cooling system for the vehicle. 
     [Description of Drive System] 
     Referring to  FIG. 1 , vehicle  100  includes a battery MB which is a power storage device, a voltage sensor  10 , a power control unit (PCU)  40 , a motor generator MG, and a control device  30 . PCU  40  includes a voltage converter  12 , smoothing capacitors C 1 , CH, a voltage sensor  13 , and an inverter  14 . Vehicle  100  further includes a positive bus PL 2  for feeding electric power to inverter  14  which drives motor generator MG. 
     Smoothing capacitor C 1  is connected between a positive bus PL 1  and a negative bus SL 2 . Voltage converter  12  boosts a voltage between the terminals of smoothing capacitor C 1 . Smoothing capacitor CH smoothes the voltage boosted by voltage converter  12 . Voltage sensor  13  detects a voltage VH between the terminals of smoothing capacitor CH and outputs the detected voltage to control device  30 . 
     Vehicle  100  further includes a system main relay SMRB connected between the positive terminal of battery MB and positive bus PL 1 , and a system main relay SMRG connected between the negative terminal of battery MB (negative bus SL 1 ) and a node N 2 . 
     The conduction/nonconduction state of system main relays SMRB, SMRG is controlled in response to a control signal SE provided from control device  30 . Voltage sensor  10  measures a voltage VB between the terminals of battery MB. A current sensor (not shown) detecting current IB which flows in battery MB is provided together with voltage sensor  10 , for monitoring the state of charge of battery MB. 
     As battery MB, a secondary battery such as lead-acid battery, nickel-metal hydride battery, or lithium ion battery, or a large-capacity capacitor such as electric double-layer capacitor may be used. Negative bus SL 2  extends through voltage converter  12  toward inverter  14 . 
     Voltage converter  12  is a voltage conversion device provided between battery MB and positive bus PL 2  for making a voltage conversion. Voltage converter  12  includes a reactor L 1  having one end connected to positive bus PL 1 , IGBT elements Q 1 , Q 2  connected in series between positive bus PL 2  and negative bus SL 2 , and diodes D 1 , D 2  connected in parallel with IGBT elements Q 1 , Q 2 , respectively. 
     Reactor L 1  has the other end connected to the emitter of IGBT element Q 1  and the collector of IGBT element Q 2 . Diode D 1  has its cathode connected to the collector of IGBT element Q 1 , and diode D 1  has its anode connected to the emitter of IGBT element Q 1 . Diode D 2  has its cathode connected to the collector of IGBT element Q 2 , and diode D 2  has its anode connected to the emitter of IGBT element Q 2 . 
     Inverter  14  is connected to positive bus PL 2  and negative bus SL 2 . Inverter  14  converts a DC voltage which is output from voltage converter  12  into a three-phase AC voltage and outputs it to motor generator MG which drives a wheel  2 . Further, as the vehicle is regeneratively braked, inverter  14  feeds the electric power generated by motor generator MG back to voltage converter  12 . At this time, voltage converter  12  is controlled by control device  30  so that the voltage converter operates as a voltage step-down circuit. 
     Inverter  14  includes a U phase arm  15 , a V phase arm  16 , and a W phase arm  17 . U phase arm  15 , V phase arm  16 , and W phase arm  17  are connected in parallel between positive bus PL 2  and negative bus SL 2 . 
     U phase arm  15  includes IGBT elements Q 3 , Q 4  connected in series between positive bus PL 2  and negative bus SL 2 , and diodes D 3 , D 4  connected in parallel with IGBT elements Q 3 , Q 4  respectively. Diode D 3  has its cathode connected to the collector of IGBT element Q 3 , and diode D 3  has its anode connected to the emitter of IGBT element Q 3 . Diode D 4  has its cathode connected to the collector of IGBT element Q 4 , and diode D 4  has its anode connected to the emitter of IGBT element Q 4 . 
     V phase arm  16  includes IGBT elements Q 5 , Q 6  connected in series between positive bus PL 2  and negative bus SL 2 , and diodes D 5 , D 6  connected in parallel with IGBT elements Q 5 , Q 6  respectively. Diode D 5  has its cathode connected to the collector of IGBT element Q 5 , and diode D 5  has its anode connected to the emitter of IGBT element Q 5 . Diode D 6  has its cathode connected to the collector of IGBT element Q 6 , and diode D 6  has its anode connected to the emitter of IGBT element Q 6 . 
     W phase arm  17  includes IGBT elements Q 7 , Q 8  connected in series between positive bus PL 2  and negative bus SL 2 , and diodes D 7 , D 8  connected in parallel with IGBT elements Q 7 , Q 8  respectively. Diode D 7  has its cathode connected to the collector of IGBT element Q 7 , and diode D 7  has its anode connected to the emitter of IGBT element Q 7 . Diode D 8  has its cathode connected to the collector of IGBT element Q 8 , and diode D 8  has its anode connected to the emitter of IGBT element Q 8 . 
     Motor generator MG is a three-phase permanent-magnet synchronous motor, and three stator coils of the U, V, and W phases have respective ends connected together to a neutral point. The other end of the U phase coil is connected to a line drawn from a connection node of IGBT elements Q 3 , Q 4 . The other end of the V phase coil is connected to a line drawn from a connection node of IGBT elements Q 5 , Q 6 . The other end of the W phase coil is connected to a line drawn from a connection node of IGBT elements Q 7 , Q 8 . 
     A current sensor  24  detects the current flowing in motor generator MG as a motor current value MCRT, and outputs motor current value MCRT to control device  30 . 
     Control device  30  receives each torque command value and the rotational speed of motor generator MG, respective values of current IB and voltages VB, VH, motor current value MCRT, and a start signal IGON. Control device  30  outputs, to voltage converter  12 , a control signal PWU for giving an instruction to step up the voltage, a control signal PWD for giving an instruction to step down the voltage, and a shutdown signal for giving an instruction to inhibit operation. 
     Further, control device  30  outputs, to inverter  14 , a control signal PWMI for giving a drive instruction so that a DC voltage which is output from voltage converter  12  is converted into an AC voltage for driving motor generator MG, and a control signal PWMC for giving a regenerative brake instruction so that an AC voltage generated by motor generator MG is converted into a DC voltage and the DC voltage is fed back to voltage converter  12 . 
     [Description of Cooling Mechanism] 
     Referring again to  FIG. 1 , vehicle  100  includes, as components of the cooling mechanism for cooling PCU  40  and motor generator MG, a radiator  102 , a reservoir tank  106 , and a water pump  104 . 
     Radiator  102 , PCU  40 , reservoir tank  106 , water pump  104 , and motor generator MG are annularly connected in series by a flow channel. The flow channel is provided with a flow rate sensor  114 , and a flow rate FR is transmitted to control device  30 . Instead of flow rate sensor  114 , another method for estimating the flow rate of the coolant may be used. 
     Water pump  104  is a pump for circulating the coolant such as antifreeze, and causes the coolant to circulate in the direction indicated by arrows shown in the drawing. Radiator  102  receives from the flow channel the coolant having cooled voltage converter  12  and inverter  14  in PCU  40 , and cools the received coolant by means of a radiator fan  103 . 
     In the vicinity of the coolant inlet of PCU  40 , a temperature sensor  108  that measures the coolant temperature is provided. A coolant temperature TW is transmitted from temperature sensor  108  to control device  30 . Further, in PCU  40 , a temperature sensor  110  detecting a temperature TC of voltage converter  12  and a temperature sensor  112  detecting a temperature TI of inverter  14  are provided. As temperature sensors  110 ,  112  each, a temperature detection device or the like contained in an intelligent power module is used. 
     Control device  30  generates a signal SP for driving water pump  104 , based on temperature TC from temperature sensor  110  and temperature TI from temperature sensor  112 , and outputs the generated signal SP to water pump  104 . 
     The configuration shown in  FIG. 1  is provided with flow rate sensor  114  detecting the flow rate of the coolant which has not been detected conventionally. While a failure could have conventionally been identified merely as an abnormality of the cooling mechanism, the flow rate can be detected to identify a more specific part where the failure has occurred, as will be described later herein with reference to  FIG. 2  and the following drawings. It should be noted that a similar effect can be obtained, even if flow rate sensor  114  is not provided, by estimating the flow rate by means of another method. 
       FIG. 2  is a diagram showing a relation between the flow resistance of the cooling mechanism and the flow rate. 
     Referring to  FIG. 2 , the flow resistance (kPa) of the cooling mechanism is indicated by the vertical axis, and the flow rate (L/min) of a refrigerant such as coolant is indicated by the horizontal axis. If the flow resistance of the cooling mechanism and the flow rate have a normal relation therebetween, an increase/decrease of the flow rate is accompanied by a change of the flow resistance (kPa) along a curve passing through a point P 4  and a point P 5 . If, however, the flow channel for example of the cooling mechanism is clogged with a foreign matter (such as rust), the flow resistance increases. In this case, an increase/decrease of the flow rate causes the flow resistance (kPa) to change along a line passing through a point P 1 , a point P 2 , and a point P 3 . 
     Here,  FIG. 2  also shows a relation between the rotational speed of the water pump, the flow rate, and the flow resistance. It shows that, as compared with the case where the rotational speed N is N 0 , namely N=N 0 , the flow resistance is larger when rotational speed N is a higher rotational speed N 1 , N=N 1 , and the flow resistance is still more larger when the rotational speed N is a still more higher rotational speed N 3 , N=N 3 . 
     Here, it is supposed that a failure is occurring where a foreign matter is caught in the flow channel to increase the flow resistance. In the case where rotational speed N of water pump  104  is N 1 , N=N 1 , although the normal operating point is point P 5  in  FIG. 2 , the operating point is point P 1  when a failure occurs. The rotational speed and the flow rate satisfy a certain relation therebetween. Therefore, when control device  30  detects that the flow rate becomes lower than the normal flow rate, control device  30  reduces rotational speed N of the water pump to N 0  at operating point P 1 , in order to identify where the failure occurs. At this time, if the flow rate accordingly decreases as indicated by an arrow A 1 , the flow resistance may have increased due to the foreign matter. 
     In view of the above, control device  30  in  FIG. 1  changes control signal SP for water pump  104  to increase the rotational speed to N 3 . If the foreign matter is still caught in the flow channel, the operating point moves to point P 3  as indicated by an arrow A 2 . Here, in the case where the increased flow resistance causes the foreign matter to be removed, the flow rate is regained and the operating point moves to point P 4  as indicated by an arrow A 3 . When the fact that the flow rate is regained can be detected by the flow rate sensor, the control device changes control signal SP for water pump  104  to set the rotational speed back to N 1 . 
     Thus, in some cases, the abnormal operating point P 1  can be returned to the normal operating point P 5 . 
     In the case where any failure different from the above one occurs, a part which causes the failure can be identified as well. 
       FIG. 3  is a diagram showing possible abnormalities that are considered to occur based on the coolant temperature, the rotational speed of the pump, and the flow rate, as well as how to verify them. 
     Referring to  FIG. 3 , an abnormality of the cooling mechanism has conventionally been found based on the coolant temperature and the rotational speed of the pump. In the present embodiment, the flow rate is added as an input parameter so that a more specific part where the abnormality has occurred can be identified. 
     First, as shown in the first row in  FIG. 3 , if the coolant temperature, the rotational speed, and the flow rate are all normal, there is no possible failure. Here, the criterion for judging whether normal or abnormal is determined as appropriate by an experiment for example. From a comparison between a threshold value corresponding to this criterion and each input parameter, whether normal or abnormal is determined. 
     Next, as shown in the second row in  FIG. 3 , in the case where the coolant temperature and the rotational speed are normal and the flow rate is abnormal (falls), a possible abnormality is deterioration of the flow resistance. In this case, control device  30  temporarily changes the rotations of water pump  104  and observes how the flow rate changes. Then, as a result of the observation of how the flow rate changes from operating point P 1  in  FIG. 2 , if the operating point moves along the line extending from point P 2  to point P 3 , control device  30  determines that the flow resistance has deteriorated. In this case, control device  30  causes water pump  104  to rotate at a higher speed so that the operating point moves toward point P 3  and thus tries to improve the state where a foreign matter is caught and remains in the flow channel. If the foreign matter is moved and the flow rate returns to the original state, control device  30  causes the rotational speed to return to its original speed. If the flow rate does not return to its original state, control device  30  confirms the diagnosis that the abnormality has occurred to the pipe system. 
     As shown in the third row in  FIG. 3 , in the case where the coolant temperature is normal, the rotational speed is an abnormal low rotational speed, and the flow rate is also abnormal (falls), an abnormality is considered as occurring to water pump  104  or to control of water pump  104 . In this case, control device  30  observes the current of water pump  104  and/or the temperature of water pump  104 . If control device  30  finds an abnormality such as abnormal heat generation or overcurrent for example, control device  30  determines that an abnormality has occurred to the pump itself. If the current and the temperature have no abnormality, control device  30  determines that an abnormality has occurred to a different part of the cooling mechanism. 
     Further, as shown in the fourth row in  FIG. 3 , in the case where the coolant temperature is an abnormal high temperature while the rotational speed and the flow rate are normal, an abnormality is considered as large heat generation of the inverter or converter to be cooled, or abnormal heat dissipation from the radiator, or an abnormality of the coolant temperature sensor. In this case, control device  30  may operate the radiator fan to determine whether the fan rotates or not, or determines whether or not an abnormality of the inverter or converter has already been detected.  FIG. 3  shows a case where control device  30  includes a plurality of ECUs. In this case, control device  30  operates utilizing communication between the ECUs in the following manner. Specifically, an ECU that determines whether an abnormality occurs to the cooling mechanism may give a command to an ECU that controls the radiator fan, to change the rotational speed of the fan, or the ECU which determines whether an abnormality occurs to the cooling mechanism may obtain information about inverter&#39;s abnormality from a motor ECU that directly controls the inverter and/or converter. 
       FIG. 4  is a flowchart for illustrating a diagnostic process performed by control device  30  in  FIG. 1 . The process in this flowchart is called from a main routine and executed, at certain time intervals or each time a predetermined condition is satisfied. 
     Referring to  FIGS. 1 and 4 , the start of the process is followed by step S 1  in which control device  30  reads coolant temperature TW from temperature sensor  108 , reads rotational speed Np of water pump  104  from rotation sensor  105 , and reads flow rate FR from flow rate sensor  114 . 
     In step S 2 , control device  30  determines whether or not a condition that coolant temperature TW is normal, rotational speed Np is normal, and flow rate FR is low is satisfied. “Normal” means that the numerical value falls in the range between predetermined upper and lower limits for example. “Low” means that the numerical value is smaller than a lower limit of a predetermined normal range. 
     In the case where the condition in step S 2  is satisfied, the process proceeds from step S 2  to step S 3 . In step S 3 , control device  30  changes control signal SP so that rotational speed Np of water pump  104  is temporarily reduced. 
     In step S 4 , in the case where flow rate FR obtained from flow rate sensor  114  is not accordingly decreased in response to the reduced rotational speed, the process proceeds to step S 14 . In contrast, in the case where flow rate FR obtained from flow rate sensor  114  is accordingly decreased in response to the reduced rotational speed, the process proceeds to step S 5 . 
     In the case where the process proceeds to step S 5 , the operating point in  FIG. 2  is considered as moving from point P 1  to point P 2 . At this time, the failure is assumed to be an abnormality of the pipe system (for example, the pipe is clogged with a foreign manner and accordingly has a reduced cross section). Before the diagnosis that the abnormality has occurred to the pipe system is confirmed, a try is made to allow the abnormal pipe system (clogged with a foreign matter for example) to change back to its original state by temporarily increasing rotational speed Np of water pump  104  to increase the flow rate. 
     In step S 6 , control device  30  determines whether or not flow rate FR has returned to its normal state. This determination may be made based on whether the operating point is point P 3  (abnormal) or point P 4  (normal) in  FIG. 2 . In the normal state, rotational speed Np of water pump  104  and flow rate FR satisfy the relation represented by the curve passing through points P 4  and P 5 . It is therefore easy to define in advance the normal range of flow rate FR relative to rotational speed Np. 
     In step S 6 , in the case where the flow rate has returned to the normal flow rate, the abnormal state of the pipe system is considered as being returned to the normal state. Therefore, the process proceeds in the same way as the case where the outcome of step S 2  is “NO” and thus proceeds to step S 8 . In contrast, in the case where the flow rate has not returned to its normal state in step S 6 , the process proceeds to step S 7  in which the diagnosis that the abnormality has occurred to the pipe system is confirmed. The result of the diagnosis may be conveyed at this time to an operator, or stored in a nonvolatile memory or the like and later read and analyzed in a repair shop. 
     In step S 8 , control device  30  determines whether or not a condition that coolant temperature TW is normal, rotational speed Np is low, and flow rate RF is low is satisfied. “Normal” means for example that the numerical value falls in the range between predetermined upper and lower limits. “Low” means that the numerical value is smaller than a lower limit of a predetermined normal range. 
     In the case where the condition in step S 8  is satisfied, the process proceeds from step S 8  to step S 9 . Otherwise, the process proceeds to step S 11 . 
     In step S 9 , it is determined whether or not an abnormality has occurred to the value of the current of water pump  104 , or an abnormality has occurred to the internal temperature of water pump  104 . The abnormal value of the current of water pump  104  can be detected by providing a current sensor on a power supply line of water pump  104 . The internal temperature of water pump  104  can be detected by attaching a temperature sensor in or in the vicinity of water pump  104 . 
     In step S 9 , when none of the abnormal value of the current of water pump  104  and the abnormal internal temperature of water pump  104  has occurred, the process proceeds to step S 14 . In step S 9 , when the abnormal value of the current of water pump  104  or the abnormal internal temperature of water pump  104  has occurred, the process proceeds to step S 10  in which the diagnosis that the abnormality has occurred to the performance of water pump  104  is confirmed. This result of diagnosis may be conveyed at this time to an operator, or stored in a nonvolatile memory or the like and later read and analyzed in a repair shop. 
     In step S 11 , control device  30  determines whether or not a condition that coolant temperature TW is abnormal (high), rotational speed Np is normal, and flow rate FR is normal is satisfied. “Normal” means for example that the numerical value falls in the range between predetermined upper and lower limits. “High” means that the numerical value is larger than an upper limit of a predetermined normal range. 
     In the case where the condition in step S 11  is satisfied, the process proceeds from step S 11  to step S 12 . Otherwise, the process proceeds to step S 15 . In step S 15 , a diagnosis that a failure has occurred is not made, since none of the conditions is satisfied, and the control is returned to the main routine. 
     In step S 12 , it is determined whether or not an abnormality has occurred to operation of radiator fan  103  or an abnormal heat generation of inverter  14  has occurred. The abnormal operation of radiator fan  103  can be detected by a comparison between a command value from control device  30  and the detected rotational speed of radiator fan  103 . The abnormal heat generation of inverter  14  can be detected based on whether or not temperature TI from temperature sensor  112  incorporated in inverter  14  has exceeded a predetermined threshold value. 
     In step S 12 , in the case where none of the abnormal operation of radiator fan  103  and the abnormal heat generation of inverter  14  has occurred, the process proceeds to step S 14 . In step S 12 , in the case where one of the abnormal operation of radiator fan  103  and the abnormal heat generation of inverter  14  has occurred, the process proceeds to step S 13  in which the diagnosis that the abnormal heat dissipation or abnormal heat generation has occurred is confirmed. This result of diagnosis may be conveyed to an operator at this time or stored in a nonvolatile memory or the like and read and analyzed later in a repair shop. 
     In the case where the process proceeds to step S 14 , the diagnosis that another abnormality of the cooling mechanism has occurred (abnormality of the cooling mechanism other than the abnormalities in steps S 7 , S 10 , and S 13 ) is confirmed, and the result of diagnosis may be conveyed to an operator at this time, or stored in a nonvolatile memory or the like and later read and analyzed in a repair shop. 
     As heretofore described, in accordance with the present embodiment, the existing parameters such as the pump&#39;s rotational speed and the coolant temperature can be combined with a new parameter, namely the flow rate of the coolant to thereby identify a specific malfunctioning part in the cooling system. 
     It should be construed that embodiments disclosed herein are by way of illustration in all respects, not by way of limitation. It is intended that the scope of the present invention is defined by claims, not by the description above, and encompasses all modifications and variations equivalent in meaning and scope to the claims. 
     REFERENCE SIGNS LIST 
       2  wheel;  10 ,  13  voltage sensor;  12  voltage converter;  14  inverter;  15  U phase arm;  16  V phase arm;  17  W phase arm;  24  current sensor;  30  control device;  100  vehicle;  102  radiator;  103  radiator fan;  104  water pump;  105  rotation sensor;  106  reservoir tank;  108 ,  110 ,  112  temperature sensor;  114  flow rate sensor; C 1 , CH smoothing capacitor; D 1 -D 8  diode; L 1  reactor; MB battery; MG motor generator; PL 1 , PL 2  positive bus; Q 1 -Q 8  IGBT element; SL 1 , SL 2  negative bus; SMRB, SMRG system main relay