Method and apparatus for controlling a clutch for vehicles

In a system for controlling a clutch for vehicle having an automatic gear-transmission control system, the clutch is controlled so as to be disengaged when a gear of the transmission is in a prescribed low-speed gear position at which strong engine braking power can be expected and fuel-powered operation of an engine for powering the vehicle has stopped during the running of the vehicle, whereby it can be prevented to apply sudden engine braking power to the vehicle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for controlling a 
clutch for vehicles. 
2. Description of the Prior Art 
There is known a vehicular automatic transmission system in which 
gear-shifting operations of a gear transmission are automatically 
controlled by means of an electronic control system and the operation of 
the clutch is also controlled automatically in relation to the 
gear-shifting operations. In most conventional automatic systems, when 
fuel-powered operation of the engine stops because, for example, the 
ignition key has been turned off, control of the transmission and the like 
is discontinued while supply of electric power to the associated 
electronic control unit is maintained. 
With this conventional arrangement, when fuel-powered operation of the 
engine stops because, for example, the ignition switch has been turned 
off, rotation of the engine will halt if the transmission is in the 
neutral position or the clutch is disengaged, even if the vehicle is 
running. This results in malfunctioning of the power steering system and 
the power braking system. To overcome these disadvantages, there has been 
proposed a control system in which the clutch is engaged after the 
transmission is shifted into a prescribed gear position other than the 
neutral position when fuel-powered operation of the vehicle engine stops 
in the case where the vehicle is running (see, for example, U.S. Pat. No. 
5,005,687 and European Patent Application Public Disclosure No. 314409). 
However, the proposed system operates so as to maintain the supply of the 
electric power to the control systems for the transmission and the clutch 
so as to carry out the aforesaid clutch and transmission control 
operations even after the ignition switch is turned off and fuel-powered 
operation of the engine stops. It follows, therefore, that engine braking 
power is applied to the vehicle when the clutch is engaged. This will 
cause sudden braking of the vehicle, which may be dangerous to the 
occupant(s). 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved method and 
apparatus for controlling a vehicle clutch. 
It is another object of the present invention to provide a method and an 
apparatus for controlling a vehicle clutch which is capable of controlling 
the clutch to prevent sudden braking from being applied to the vehicle by 
the engine during the vehicle running. 
According to the present invention, in a vehicular clutch control system 
for automatically controlling a clutch installed between an internal 
combustion engine for powering the vehicle and a gear transmission whose 
gear shift operation is automatically controlled by a transmission 
controller, the clutch is coupled with an actuator for actuating the 
clutch and a control signal for controlling the engagement/disengagement 
of the clutch is produced by a control means in relation to the control 
operation of the gear transmission. The system further comprises a first 
discriminating means for discriminating whether or not the fuel-powered 
operation of the internal combustion engine has stopped, means for 
producing a position signal representing the gear position to which the 
gear transmission is shifted, and a second discriminating means responsive 
to the position signal for discriminating whether or not the transmission 
is shifted into a prescribed low-speed gear position or one of the 
prescribed low-speed gear positions. In the preferred embodiment, a first 
speed gear position, a second speed gear position and a reverse gear 
position are determined as the prescribed low speed gear positions. Other 
low-speed gear positions at which strong engine braking power can be 
expected may be additionally prescribed in view of, for example, the 
nature of the gear transmission. 
The system is further provided with means responsive to the first and 
second discriminating means for controlling the actuator so as to 
disengage the clutch when it is discriminated that fuel-powered operation 
of the internal combustion engine has stopped in a case where the gear 
transmission has been shifted into a predetermined low-speed gear 
position. 
As a result, when the vehicle is running with the gear transmission shifted 
into a prescribed low speed gear position, wherein excessive engine 
braking power may be applied to the vehicle, the clutch is disengaged to 
avoid the occurrence of sudden braking. On the other hand, the clutch is 
not disengaged when the transmission is shifted into a gear position other 
than a prescribed low-speed gear position. Therefore, prescribed hydraulic 
power can be supplied to the power steering system, the power braking 
system and the like so long as the vehicle is running. 
The invention will by better understood and other objects and advantages 
thereof will be more apparent from the following detailed description of 
preferred embodiments with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a diesel engine 1 has an output shaft 1a coupled 
through a clutch 2 and a gear transmission 3 with a vehicular wheel shaft 
4. The fuel to be supplied to the diesel engine 1 is electronically 
controlled by a fuel control system 10. The clutch 2 and gear transmission 
3 are electronically controlled by a clutch control system 20 and a 
transmission control system 30. The systems 20 and 30 are part of an 
automatic gear-shifting control system AM which electronically controls 
the operations for automatic gear shifting. 
A pulser 5 having a plurality of uniformly spaced cogs on its peripheral 
surface is mounted on the output shaft 1a and an electromagnetic pick-up 
element 50 is disposed so as to face the pulser 5, whereby a rotational 
speed sensor of well-known design is formed. The pick-up element 50 
produces an electric signal PE whose frequency is proportional to the 
rotational speed of the pulser 5. A rotation speed calculator 51 
responsive to the electric signal PE produces an engine speed signal Ne 
representing the rotational speed of the output shaft 1a, in the 
conventional manner. 
The automatic gear-shifting control system AM further comprises an 
acceleration sensor 60 for detecting the amount of operation of an 
accelerator pedal 6 and producing an acceleration signal Ac, selected 
position sensor 70 for detecting the position selected by a selector 7 and 
producing a selected position signal Se, and a vehicle speed sensor 80 for 
generating a vehicle speed signal Sp indicating the running speed of the 
vehicle. 
As illustrated in detail in FIG. 2, the fuel control system 10 has a fuel 
injection pump 12 coupled by pipes 11 with a fuel injection valve (not 
shown) mounted on the diesel engine 1. The fuel injection pump 12 has a 
control rack (not shown) for regulating the amount of fuel injected, and 
the position of the control rack is controlled by a governor 13. A driving 
circuit 14 for an actuator (not shown) incorporated into the governor 13 
receives a governor control signal Gvc produced by a governor control unit 
15. A PID calculation is carried out in the governor control unit 15 to 
produce the governor control signal Gvc on the basis of the difference 
between a target rack position signal Lt produced by a target rack 
position calculation unit 16 and an actual rack position signal Li 
produced by a position sensor 17 coupled with the control rack by a link 
mechanism (not shown). 
As shown in FIG. 5, the target rack position calculation unit 16 has a 
first calculation unit 16a and a second calculation unit 16b. The first 
calculation unit 16a calculates an optimum amount of fuel injection on the 
basis of the acceleration signal Ac and the engine speed signal Ne, and 
further calculates the target rack position necessary for obtaining the 
optimum amount of fuel injection. The second calculation unit 16b carries 
out a PID calculation for determining the target rack position on the 
basis of the difference between the engine speed signal Ne and a target 
engine speed signal Nref generated by a target engine speed calculation 
unit 16c. 
The target engine speed signal Nref is produced by the target engine speed 
calculation unit 16c on the basis of an actual clutch position signal CLp 
and a target gear position signal Gt, which will be described later, in 
addition to the vehicle speed signal Sp. 
A switch 16d is connected with the output lines of the first calculation 
unit 16a and the second calculation unit 16b. The switch 16d is controlled 
by a gear-change signal Gch produced by the transmission control system 30 
as described later, and the result of the calculation obtained by the 
first calculating unit 16a is selected by the switch 16c as the target 
rack position signal Lt when the gear-change signal Gch does not indicate 
that a gear change operation is being carried out. As a result, the 
position of the control rack is controlled by the governor control unit 15 
in order to realize the target rack position determined by the first 
calculation unit 16a, and the optimum amount of fuel injected for the 
amount of operation of the accelerator pedal 6 and the engine speed is 
determined. 
On the other hand, when the gear-change signal Gch indicates that a 
gear-change operation is being carried out, the switch 16c is switched 
over to select the output signal from the second calculation unit 16b and 
the output signal of the second calculation unit 16b is selected as the 
target rack position signal Lt. Thus, the position of the control rack is 
controlled irrespective of the amount of operation of the accelerator 
pedal 6, whereby a temporary excessive increase in engine speed can be 
prevented. 
Returning to FIG. 1, the ON/OFF control operation of the power supply to 
the fuel control system 10 and other control systems is carried out in 
response to a power control signal ES produced by a power control unit 90. 
Specifically, the power is supplied thereto when the level of the power 
control signal ES is high, and the supply of power is stopped when the 
level of the power control signal ES is low. 
As illustrated in FIG. 6, the power control unit 90 has a relay 90a 
installed between the respective circuits of the automatic gear-shifting 
control system AM and the power supply +B. One terminal of a relay switch 
90b of the relay 90a is connected with the power supply +B and the other 
terminal thereof is connected with the respective circuits of the 
automatic gear-shifting control system AM. A relay coil 90c associated 
with the relay switch 90a is controlled by a driving circuit 90e which 
receives the output signal of an OR circuit 90d. On the input side of the 
OR circuit 90d, there are installed a running speed discriminator 90g and 
an engine speed discriminator 90i. The former detects whether or not the 
vehicle is running in response to the vehicle speed signal Sp, and has an 
output line 90f on which a high level signal appears when it detects that 
the vehicle is running. The latter is responsive to the engine speed 
signal Ne and has an output line 90h on which a high level signal appears 
when the engine speed is not zero. Each of the output lines 90f and 90h is 
connected with an input terminal of the OR circuit 90d having another 
input terminal to which an ignition switch signal KY produced by an 
ignition switch 18 (FIG. 1) is applied. 
The level of the ignition switch signal KY becomes low only when the 
ignition switch 18 is turned to its OFF position, and becomes high when 
the ignition switch 18 is in a position other than the OFF position. 
Accordingly, when the ignition switch 18 is OFF, the output level of the OR 
circuit 90d becomes low only when both the vehicle speed and the engine 
speed are zero. The relay coil 90c is energized by the driving circuit 90e 
to close the relay switch 90b only when the output level of the OR circuit 
90d has become high. 
A serial circuit consisting of a switch 90j and a diode 90k is connected in 
parallel with the relay switch 90b. The switch 90j is arranged to operate 
in response to the ignition switch 18 in such a manner that it opens when 
the ignition switch 18 is in the OFF position and closes when the ignition 
switch 18 in the ON position. The diode 90k is provided in the direction 
enabling current flow from the power supply +B to the respective circuits 
of the control system AM. The cathode electrode of the diode 90k is 
connected with one input terminal of an AND circuit 90p having another 
input terminal to which the gear-change signal Gch is applied. The output 
line 90m of the AND circuit 90p is connected with one input terminal of an 
OR circuit 90n having another input terminal to which the voltage 
appearing at the connecting point of the diode 90k and the switch 90j is 
applied. Thus, the engine control signal ES is derived from the OR circuit 
90n. 
Accordingly, whenever the ignition switch 18 is in the ON position, the 
level of the power control signal ES is high, and electric power is 
supplied to the fuel control system 10 and the respective portions of the 
control system AM. On the other hand, the relay switch 90b is closed so 
long as at least one of the vehicle speed and the engine speed is not 
zero, even if the ignition switch 18 is turned off, whereby the required 
electric power is supplied to the respective portions of the control 
system AM. 
In this case, the level of the gear-change signal Gch is high if a gear 
shift operation is being carried out, so that the output level of the AND 
circuit 90p is high. Consequently, even if the ignition switch 18 is 
turned off, the supply of power to the fuel control system 10 is 
maintained so long as the gear shifting operation is being carried out 
while electric power is supplied through the relay switch 90b to the 
respective portions of the control system AM. 
Referring to FIG. 3, the clutch control system 20 comprises an actuator 21, 
a clutch control unit 23 for providing a clutch control signal CT to a 
driving circuit 22 for driving the actuator 21, a clutch sensor 24 for 
detecting the position of a pressure plate (not shown) of the clutch 2 as 
a clutch position, and a clutch position detector 25 for producing an 
actual clutch position signal CLp indicating the actual clutch position in 
response to the output of the clutch sensor 24. 
As illustrated in FIG. 7, the clutch control unit 23 has a first comparator 
23a and a second comparator 23b. 
A first signal generator 23c responsive to a clutch ON/OFF signal 
CL.sub.ON/OFF produces a first signal FS representing the time-course 
pattern of the clutch position for engaging the clutch 2. A second signal 
generator 23d, also responsive to the clutch ON/OFF signal CL.sub.ON/OFF, 
produces a second signal SS representing the time-course pattern of the 
clutch position for engaging the clutch 2. The first comparator 23a 
compares the actual clutch position signal CLp with the first signal FS, 
which is produced when the start of the clutch engaging operation is 
commanded by the clutch ON/OFF signal CL.sub.ON/OFF and the first 
comparator 23a produces a first output FO for controlling the clutch 
position so as to reduce the difference therebetween, whereby the clutch 2 
is engaged in accordance with the time-course pattern indicated by the 
first signal FS. The second comparator 23b compares the actual clutch 
position signal CLp with the second signal SS, which is produced when the 
start of the clutch disengaging operation is commanded by the clutch 
ON/OFF signal CL.sub.ON/OFF, and the second comparator 23b produces a 
second output SO for controlling the clutch position so as to reduce the 
difference therebetween, whereby the clutch 2 is disengaged in accordance 
with the time-course pattern indicated by the second signal SS. 
A switch 23e is connected with the output lines of the first and the second 
comparator 23a and 23b for enabling selection of one of the first and 
second outputs FO and SO. The switch 23e is controlled in the following 
way. A control signal CON produced by a target gear position calculation 
unit 34, to be described later, is applied to one input terminal of an AND 
circuit 23f having another input terminal to which the gear-change signal 
Gch is applied. One input terminal of an OR circuit 23g receives the 
output signal of the AND circuit 23f and another input terminal of the OR 
circuit 23g receives the clutch ON/OFF signal CL.sub.ON/OFF. 
The output terminal of the OR circuit 23g is connected with one input 
terminal of an AND circuit 23k having another input terminal to which an 
output line 23j of a discriminating unit 23i is connected, and the ON/OFF 
operation of the switch 23e is controlled in accordance with the output 
from the AND circuit 23k. In the embodiment shown in FIG. 7, the switch 
23e is arranged to select the first output FO when the output level of the 
AND circuit 23k is high and to select the second output SO when it is low. 
The discriminating unit 23i comprises a first discriminator 23m and a 
second discriminator 23n. The first discriminator 23m is responsive to an 
actual gear position signal Gp produced by a gear position sensor 35, 
which will by described later, and discriminates whether or not the gear 
transmission 3 has shifted into one of the predetermined low-speed gear 
positions. While in this embodiment the first speed, second speed and 
reverse gear positions are defined as the predetermined low-speed gear 
positions, it suffices to define a minimum of one gear position as a 
predetermined low-speed gear positions. The level of the output of the 
first discriminator 23m is high when the result of the discrimination is 
that the transmission is in one of the first speed, second speed and 
reverse gear positions, and the level thereof is low when the result of 
the discrimination is that the gear position is one other than a 
predetermined gear position. The second discriminator 23n is responsive to 
the ignition switch signal KY and is arranged to produce a high level 
output when the ignition switch 18 is in the OFF position and to produce a 
low level output when the ignition switch 18 is in a position other than 
the OFF position. 
The output signals of the first and second discriminators 23m and 23n are 
applied to respective input terminals of a NOR circuit 23o. 
Therefore, the level of the output line of the NOR circuit 23o is low only 
when the ignition switch 18 is in the OFF position and the gear 
transmission 3 is shifted into one of the predetermined low speed gear 
positions, that is, one of the first speed, second speed and reverse gear 
positions. The level of the output line 23j is high in cases other than 
the foregoing. 
Accordingly, in the case where the ignition switch 18 is in the OFF 
position and the gear transmission 3 is shifted into one of the first 
speed, second speed and reverse gear positions, the clutch 2 is always in 
the disengaged state irrespective of the output level of the OR circuit 
23g. On the other hand, since the level of the output line 23j is high 
when the ignition switch 18 is in the ON or ST position, or the gear 
transmission 3 is shifted into a position other than the first speed, 
second speed and reverse gear positions, the control operation for 
engaging/disengaging the clutch 2 is carried out depending upon the output 
level of the OR circuit 23g. 
That is, in the case where the level of the control signal CON is low, the 
first output FO is applied through the switch 23e to the driving circuit 
22 to engage the clutch 2 when the level of the clutch ON/OFF signal 
CL.sub.ON/OFF is high, and the second output So is applied through the 
switch 23e to the driving circuit 22 to disengage the clutch 2 when the 
level of the clutch ON/OFF signal CL.sub.ON/OFF is low. 
Referring to FIG. 4, the transmission control system 30 has an actuator 31, 
a gear-change control unit 33 for supplying a control signal to a driving 
unit 32 associated with the actuator 31, the target gear position 
calculation unit 34 for producing the target gear position signal Gt which 
is sent to the gear-change control unit 33, and the gear position sensor 
35 for producing the actual gear position signal Gp indicating the actual 
gear position of the gear transmission 3. The gear-change control unit 33 
compares the target gear position signal Gt with the actual gear position 
signal Gp and controls the actuator 31 in accordance with the result, 
whereby the transmission 3 is shifted into the target gear position. In 
conjunction with gear shift operations, the gear-change control unit 33 
produces the clutch ON/OFF signal CL.sub.ON/OFF for commanding the 
engaging/disengaging operation of the clutch 2 and the gear-change signal 
Gch indicating that a gear-shifting operation is being carried out in the 
transmission 3. Both signals are sent to the clutch control unit 23. 
As illustrated in FIG. 8, the target gear position calculation unit 34 has 
a first calculation unit 34a to which the acceleration signal Ac, the 
vehicle speed signal Sp and the actual gear position signal Gp are 
applied. The target gear position calculation unit 24 is also provided 
with a first map unit 34c for supplying map data for a drive range 
selected by the selector 7, a second map unit 34d for supplying map data 
for a fourth range selected by the selector 7, a third map unit 34e for 
supplying map data for a third range selected by the selector 7, and a 
fourth map unit 34f for supplying map data for a second range selected by 
the selector 7, one of which is connected with the first calculation unit 
34a by a switch 34g in response to the selected position signal Se. As a 
result, appropriate map data for the selector position selected by the 
selector 7 is supplied through the switch 34g to the first calculating 
unit 34a and the optimum gear position for the acceleration signal Ac and 
the vehicle speed signal Sp is calculated in accordance with the map data 
supplied from the selected map unit. Then, the resulting optimum gear 
position is compared with the actual gear position, and the target gear 
position is determined on the basis of the result. 
The target gear position calculation unit 34 further comprises a selecting 
unit 34h having switches 34i and 34k, a neutral discrimination unit 34m, a 
second calculation unit 34n, and a fifth map unit 34p. The neutral 
discrimination unit 34m is for discriminating in response to the actual 
gear position signal Gp whether or not the transmission 3 is shifted into 
the neutral position. In the case where the neutral discrimination unit 
34m determines that the transmission 3 is shifted into the neutral 
position, the target gear position for the case where the vehicle is 
running with fuel-powered operation of the engine stopped is calculated in 
advance by the second calculation unit 34n on the basis of the vehicle 
speed signal Sp and the map data supplied from the fifth map unit 34p. The 
switch 34i is controlled by a discrimination signal DS supplied from a 
detection unit 100. 
The detection unit 100 is for detecting whether or not the vehicle is 
running with fuel-powered operation of the engine 1 stopped and the 
transmission 3 in the neutral position. Accordingly, the detection unit 
100 comprises a running detector 100b responsive to the vehicle speed 
signal Sp for discriminating whether or not the vehicle is running, and a 
neutral detector unit 100d responsive to the selected position signal Se 
for discriminating whether or not the neutral range is selected by the 
selector 7. The running detector 100b generates a high level signal on its 
output line 100a when the vehicle is running, and the neutral detector 
100d generates a high level signal on its output line 100c when the 
neutral range is selected by the selector 7. 
The output line 100a is connected with an input terminal of an AND circuit 
100f having another input terminal to which the output line 100c is 
connected through an invertor 100g. The ignition switch signal KY is 
applied through an OR circuit 100e to an inverting input terminal of the 
AND circuit 100f and one input terminal of the OR circuit 100e is 
connected to the output line 100i of an engine speed detector 100h, which 
is responsive to the engine speed signal Ne and discriminates whether or 
not the rotational speed of the engine 1 is zero. The engine speed 
detector 100h is arranged to generate a high level signal on the output 
line 100i when the rotational speed of the engine 1 is not zero. As a 
result, the OR circuit 100e produces a low level signal when the zero 
engine speed condition is detected by the engine speed detector 100h and 
the ignition switch 18 is in the OFF position. 
Accordingly, the level of the discrimination signal DS is high when the 
engine speed is zero and the ignition switch 18 is turned off in the case 
where the vehicle is running with the transmission 3 shifted into the 
neutral position. In contrast, the level of the discrimination signal DS 
is low in cases other than the foregoing. In addition, the detection unit 
100 may be arranged to have the OR circuit 100e receive only one of the 
ignition switch signal KY and the output signal from the engine speed 
detector 100b. 
The switch 34i is arranged to select the output signal from the first 
calculation unit 34a when the level of the discrimination signal DS is 
low, and this selected output signal is derived as the target gear 
position signal Gt from the switch 34i. On the other hand, the output 
signal of the switch 34k is selected as the target gear position signal Gt 
by the switch 34i when the level of the discrimination signal is high. The 
switch 34k is switched over as illustrated in FIG. 8 when a neutral output 
signal indicating that the transmission 3 is shifted into the neutral 
position is not received from the neutral discrimination unit 34m, so that 
the actual gear position signal Gp is output as the target position signal 
Gt. As a result, the gear shift position of the transmission 3 is 
maintained as it is when the vehicle is running with the ignition switch 
18 turned off. 
When the level of the discrimination signal DS becomes high in the case 
where the switch 34k receives the neutral output signal from the neutral 
discrimination unit 34m, the output of the second calculation unit 34n is 
selected as the target gear position signal Gt. Therefore, even if the 
transmission 3 is shifted into the neutral position when the ignition 
switch 18 is off, the transmission 3 is shifted into a position suitable 
for the vehicle running speed on the basis of the map data according to 
the fifth map unit 34p. 
The discrimination signal DS is applied to an input terminal of an AND 
circuit 110b having another input terminal to which the neutral output 
signal from the neutral discrimination unit 34m is applied through an 
inventor 110a. Consequently, the AND circuit 110b outputs a control signal 
CON whose level is high only when the level of the discrimination signal 
DS is high and the actual position of the gear of the transmission 3 is a 
position other than the neutral position. 
The control signal CON is applied to the AND circuit 23f in the clutch 
control unit 23 (See FIG. 7). Since the output level of the AND circuit 
23f is not changed to low so long as the gear change signal Gch is applied 
thereto, even if the control signal CON is supplied, the disengaged state 
of the clutch 2 is maintained. The output level of the AND circuit 23f is 
changed to high at the time the production of the gear change signal Gch 
stops, so that the switch 23e is changed over. As a result, the first 
output FO is sent to the driving circuit 22 as the clutch control signal 
CT and the clutch 2 is disengaged. 
As will be understood from the foregoing description, when it is 
discriminated by the detection unit 100 that the ignition switch 18 is 
turned off when the vehicle is running with the transmission shifted into 
a position other than the neutral position, the signal produced by the 
selecting unit 34h is sent as the target gear position signal Gt to the 
gear-change control unit 33, and the gear shift operation is carried out 
in response thereto. When the foregoing discrimination result is obtained 
by the detection unit 100 in the case where the transmission 3 is shifted 
into a position other than the neutral position, the level of the control 
signal CON is changed to high. Thus, the clutch control unit 23 produces a 
clutch control signal CT for engaging the clutch 2 when the level of the 
gear-change signal Gch is changed to low. 
That is, when turning-off of the ignition switch 18 causes fuel-powered 
operation of the engine 1 to stop in the case where the vehicle is running 
with the transmission 3 shifted into a position other than the neutral 
position, the transmission 3 is always shifted into an appropriate 
position other than the neutral position and the clutch 2 is engaged. As a 
result, the rotation of the wheel shaft 4 can be transmitted through the 
transmission 3 and the clutch 2 to the engine 1, so that it is possible to 
maintain the operability of the power steering system, the power braking 
system and the like, whereby appropriate braking power can be applied to 
the vehicle when necessary and the smooth steering can be ensured until 
the vehicle stops. 
Even in this case, since the clutch 2 is disengaged when the ignition 
switch 18 is changed over to the OFF position in the case where the 
transmission 3 is shifted into the first or second gear position, sudden 
application of large braking power to the vehicle is effectively prevented 
at that time. 
If the ignition switch 18 is turned off in the case where the transmission 
3 is shifted into a position higher than second speed gear, the engaged 
state of the clutch 2 is maintained to keep the diesel engine 1 rotating 
owing to the running of the vehicle, whereby appropriate engine breaking 
power can be applied to the vehicle. 
The present invention has been described by way of one embodiment, but the 
present invention is not limited to the arrangement of the foregoing 
embodiment. For example, the same constitution as that of the embodiment 
can be realized by the use of a microcomputer arranged to execute an 
appropriate control program.