Brake Fluid Pressure Sensor/Switch: Service and Repair

REMOVING AND INSTALLING PRESSURE WARNING SWITCH OF LOCK HYDRAULICS
Removal

1. With the ignition key in position 0, pull oft both plugs at the pump assembly.

2. Completely depressurize the system. To do this, slowly open the bleeder valve on the lock solenoid valve and drain the brake fluid into a collection
bottle.

Important: The system is under a pressure of up to 180 bar. Wear goggles and protective gloves.




3. Remove pressure warning switch (No.2) with special tool 9524. Prevent the pump assembly from turning while loosening the switch.

Important: First clean the area around the pressure warning switch and cover it with lint free cleaning cloths to trap the small amount of residual brake fluid that emerges.


Installing

1. Screw in the pressure warning switch (No.2) and tighten to a torque of 26 Nm (19 ft lb). Replace the O-ring if necessary. Prevent the pump assembly from turning while tightening the pressure warning switch.

Note: Wet the sealing ring with brake fluid only. Never use brake cylinder paste. Absolute cleanliness is essential. Use only lint-free cleaning cloths.




2. If there is an electrical fault at the pressure warning switch, always replace the hydraulic pump relay (in the luggage compartment) as well.




3. Attach plug to pressure warning switch (plug on the pump side remains disconnected at this point).

4. Partially bleed the lock hydraulics as follows:
- With the ignition switched on open the bleeder valve on the lock solenoid valve Attach the electrical plug to the pump. As soon as brake fluid emerges free from air bubbles pull off the electrical plug and close the bleeder valve.

Next, charge the pressure reservoir completely.
- To do this attach the electrical plug. As soon as the pump is heard to switch off, pull oft the electrical plug and reduce the pressure at the bleeder valve of the lock solenoid valve. Slowly open the bleeder valve and hold the bleed hose firmly.

Important: A pressure of up to 180 bar is present in the system. Wear goggles and protective gloves.

5. If necessary connect pressure gauge -special tool 9509 - to the pressure reservoir and check the switching points of the pressure warning switch and also for leakage in the differential lock hydraulics.

Note: If necessary, top up the brake fluid level at intervals so that the brake fluid reservoir is not drained completely. The lock hydraulics must be at zero pressure when connecting and removing the pressure gauge.

6. After the testing and assembly work has been completed, correct the brake-fluid level when the pressure reservoir is fully charged