Pressure-Sensor Test

Pressure Sensor Test Via Precharge Pump

Following each reset of the control module for traction control, a precharge pump test is carried out. Here, the precharge pump is briefly activated if the following conditions are met simultaneously.
- v > 30 km/h
- Engine rpm > 2000 rpm
- Engine torque > 100 Nm

Possible cause(s) of fault:
- Brake-fluid level too low
- Stoplight switch incorrectly set
- Air in the hydraulic unit or between the hydraulic unit and master brake cylinder
- Pressure sensor defective
- Incorrect electrical connection of precharge pump
- Precharge pump defective

Storage condition(s):
- If the pressure sensor does not detect a rise in pressure during activation of the precharge pump, the fault is entered.

Troubleshooting procedure:
- Check brake-fluid level
- Check setting of the stoplight switch: If the brake pressure is greater than 2 bar when the stoplight switch is activated, the stoplight switch needs setting. Brake pressure and change in status of the stoplight switch, see Control-module functions
- Ventilate DSC hydraulics (see Service functions)
- Switch off the ignition for approx. 10 seconds and then trigger a precharge pump test (perform a test drive under the above conditions)
- Read out fault memory
Fault currently not present: the system is OK.
Fault currently not present: with engine running, press the brake pedal with maximum force.

If a brake pressure of less than 80 bar is displayed (service functions), the pressure sensor has to be replaced.

If the brake pressure displayed is greater than 80 bar (service functions), the fault lies in the area of the precharge pump. There must be a check as to whether the connections of the precharge pump have been confused. If the precharge pump has been correctly connected, then the supply rate of the precharge pump is to low. It must be renewed.

NOTE
- Renewing the control module, see Service functions
- Precharge pump replacement, see Service functions