Hydraulic System: Description and Operation

Hill Holder





The hill holder is a device that facilitates starting on an uphill road. With the help of this device, even an unexperienced driver can start the vehicle smoothly.

When starting the vehicle on an uphill road, the driver depresses the clutch pedal while keeping the brake pedal depressed and then releases the brake pedal. At this time the hill holder keeps the brakes still applied until the clutch pedal is released. This enables the driver to start the vehicle in a usual manner by depressing the accelerator while releasing the clutch pedal without being troubled by brake pedal operation.

PRESSURE HOLD VALVE (PHV)





^ The PHV is located on the piping that connects the master cylinder and the ABS control module/ hydraulic control unit. It has a camshaft which turns either way by rotation of the lever which is linked to the clutch pedal. The cam of the camshaft allows the pushrod in the PHV to move rightward (in the drawing below) under the force of a spring when the clutch pedal is depressed and pushes it leftward against the force of the spring when the clutch pedal is released. At the left of the pushrod, there is a ball housed in a chamber which has a seat for the ball. When the vehicle is on a level road, this ball stays at the left (front) of the chamber, but on an uphill road, it tends to move rightward by the gravity. In the latter case, the ball makes contact with the seat unless it is forced to leave the seat by a leftward movement of the pushrod (resulting from release of the clutch pedal).
^ When the vehicle is stopped on an uphill road, the driver depresses both the brake and clutch pedals. In this condition, the ball is in the rear (right) position, making contact with its seat and preventing flow of the brake fluid toward the master cylinder. The driver then release the brake pedal while keeping the clutch pedal depressed to restart the vehicle. The brakes, however, remains applied because the pressure in the line to the wheel cylinders is kept unreleased. Then, the driver can release the clutch pedal in the same way as with on a level road while depressing the accelerator pedal since the release of the clutch pedal causes the pushrod to push the ball away from its seat and the brakes are released.

Fig. 18 Hill Holder Activated:

CONDITIONS IN WHICH THE HILL HOLDER OPERATES
The hill holder operates only when the clutch and brake pedals are depressed with the vehicle stopped on an uphill road.
In this condition, the PHV's ball blocks the fluid passage back to the master cylinder and, therefore, the hydraulic pressure in the wheel cylinder circuits is maintained even after the brake pedal has been released as long as the clutch pedal is held depressed.

CONDITIONS IN WHICH THE HILL HOLDER DOES NOT OPERATE
The hill holder does not operate in the following conditions:

Fig. 19 Hill Holder Deactivated:

^ During acceleration and constant speed driving on an uphill road
Since driver does not depress the clutch pedal during acceleration or driving at a constant speed, the pushrod keeps the ball away from the seat. The hill holder cannot maintain the pressure in the wheel cylinder circuits when the brake pedal is released.





^ During deceleration on an uphill road
Even if the clutch pedal is depressed during deceleration, the ball is kept away from the seat by the inertia force. So, the hill holder cannot maintain the wheel cylinder circuit pressure.





^ When stopping on an uphill road without depressing the clutch pedal
If the driver does not depress the clutch pedal simultaneously with the brake pedal when stopping the vehicle on an uphill road, the hill holder cannot keep the wheel cylinder circuit pressure unreleased when the brake pedal is release.





^ On a level road
The ball does not move itself toward its seat except when the vehicle is braked during reversing, so the hill holder does not function during level road driving.

OPERATIONAL PRECAUTIONS
^ The hill holder is a device that facilitates starting on an uphill road. When stopping on an uphill road, therefore, the driver must keep the brake pedal depressed or set the parking brake firmly.
^ The hill holder may not function on a slope with a small angle of inclination.
^ If the brake force is insufficient to hold the vehicle after releasing the brake pedal with the clutch pedal depressed, depress the brake pedal again with a stronger force.
^ If the clutch pedal is released halfway and then depressed again (for example, when the driver abandons an attempt of start or changes gear to the low gear from another improperly selected gear), the pressure in the wheel cylinder circuit is not retained any more. The driver, therefore, must depress the brake pedal again to use the hill holder function next time.
^ Before leaving the vehicle, the driver must apply firmly the parking brake and confirm that the vehicle does not move after releasing the clutch pedal.
^ When reversing on a level road, the phenomena listed below may occur. These phenomena are caused by activation of the hill holder and do not mean an abnormality.
- Braking effect continues when the vehicle is reversed with the clutch and brake pedals depressed and then the brake pedal released.
- A slight shock occurs in the vehicle when moving the vehicle forward after a reverse movement.