Hydraulic Modulator
The hydraulic modulator is installed in the left side of the engine compartment, near the spring strut dome.
| The hydraulic modulator' task is, independent of the pressure in the brake master cylinder, to maintain r reduce the pressure to the wheel brake cylinders during regulation. To do this, the solenoid valves hich are responsible for the front left, front right and rear wheel brake cylinders are actuated by the control unit. | |||
|---|---|---|---|
| .P. E4102.tif | 12 | 7 | tiff;; |
Hydraulics diagram
1 Wheel brake, front right
2 Solenoid valve, front right
3 Brake master cylinder
4 Pressure reservoir
5 Wheel brake, rear right
6 Wheel brake, rear left
7 Solenoid valve, rear
8 Return pump
9 Pressure dampener
10 Solenoid valve, front left
| 11 Wheel brake, front left | |||
|---|---|---|---|
| .P. E7315.tif | 12 | 11 | tiff;; |
Pressure build - up phase
(one brake circuit shown)
1 Control unit
2 Brake master cylinder
3 Return pump
4 Pressure reservoir
5 Solenoid valve
6 Wheel brake cylinder
During the pressure build - up phase, the solenoid valve (5) is not actuated by the control unit (1), t is in the off position.
There is an unobstructed connection between the brake master cylinder (2) and the wheel brake cylinder (6). The braking pressure can build up during
- normal braking without ABS participation
- ABS regulation.
The pressure reservoir (4) is depressurized in this case.
The return pump (3) is not operating.
| .P. E4097.tif | 18 | 10 | tiff;; |
Pressure retention phase
(one brake circuit shown)
1 Control unit
2 Brake master cylinder
3 Return pump
4 Pressure reservoir
5 Solenoid valve3
6 Wheel brake cylinder
During the pressure retention phase, a retaining current (approx. 2 A) is applied to the solenoid valve (5) by the control unit (1), morning the valve against the spring force into the pressure retention position. There is no connection between the brake master cylinder (2) and the wheel brake cylinder (6). The brake pressure is retained
- during ABS regulation
to prevent the wheel from locking.
The pressure reservoir (4) is depressurized in this case. The return pump (3) is not operating.
| .P. E4098.tif | 18 | 10 | tiff;; |
Pressure reduction phase
(one brake circuit shown)
1 Control unit
2 Brake master cylinder
3 Return pump
4 Pressure reservoir
5 Solenoid valve
6 Wheel brake cylinder
During the pressure reduction phase, current (approx. 5 A) is applied to the solenoid valve (5) by the control unit (1), moving the valve further against the spring force into the pressure retention position. There is a connection between the wheel brake cylinder (6) and pressure reservoir (4) and via the return pump (3) to the brake master cylinder (2). The return pump (3) is now operating. The brake pressure is suddenly reduced
- during ABS regulation.
The pressure reservoir (4) now serves to recover the pressure peak resulting from the pressure reduction. The brake pedal pulsates in operation.
| .P. E4099.tif | 18 | 10 | tiff;; |
· Service
The components of the hydraulic modulator are monitored by the self - diagnosis. If a component or the feed line to the component is defective, the ABS telltale illuminates and a trouble code is stored in the control unit.
Important test instructions:
- The hydraulic modulator itself must not be repaired.
- The solenoid valves must never be subjected to battery voltage (risk of destruction).
- After installing a new hydraulic modulator, ensure that the brake lines are correctly connected; carry out brake check (high and low pressure check), self - diagnosis and actuator test with TECH 1.
- There is no special bleeding procedure to be followed.