Check Lock Hydraulics
General information
The hydraulics comprises the high-pressure pump, solenoid valve, pressure reservoir and supply tank. The components are located on on a joint bracket in the rear-left wheel housing.
The brake fluid for operating the transverse lock is obtained from the piston reservoir mounted on the high pressure pump. The operating pressure is between 140 and 180 bar. The pump is activated via a pressure warning switch.
The solenoid valve familiar from the ABS system controls the functions pressure increase, pressure maintenance and pressure reduction, and applies pressure to the slave cylinder of the transverse lock.
Important note
Check hydraulics of the lock (energy production) if "PSD off" appears in the display fields of the instrument cluster even though no fault is stored.
Check hydraulics of the lock
This comprises: check of the lock hydraulics without pressure gauge (special Tool 9509)
Precondition
Read out DTC memory with System Tester 9288. No fault stored.
1. Jack up the vehicle on one side until a rear wheel can be turned freely.
2. Switch on System Tester 9288. Select lock test (system PSD/Drive Links menu).
3. Pulse transverse lock (pressing function key 1 on tester once = 1 pulse) and simultaneously turn the rear wheel.
4. The wheel must be completely fixed (locked) after several pulses. Caution: If the vehicle is fully raised, hold the opposite rear to keep it from turning.
5. If the wheel does not lock, continue with troubleshooting "System depressurized". If the wheel does lock, continue with troubleshooting: System pressure present.
Troubleshooting
If the system is depressurized, the following faults are possible:
- Brake fluid reservoir (No. 12) empty (system leaks).
- Voltage supply for high-pressure pump (No. 1) not OK (check pump relay No.38 and fuse in the luggage compartment).
- Pressure warning switch (No. 2) not OK, Checking Locking Torque
- High-pressure pump (No. 1) defective. Check pump operation by bridging terminals 30 and 87 on the pump relay (in the luggage compartment).
Important: Pump operating time when bridging the terminals max. 3 seconds. (Perform test only once.)
- Mechanical defect of lock solenoid valve (No. 9).
The following faults are possible if system pressure is present:
- Pressure warning switch (No. 2) not OK.
- Electrical wire for activating the fault display "PSD off' not OK (check in combination with the wiring diagram).
- High-pressure pump no longer operates (defect, or voltage supply not OK), but there is still a residual pressure of less than 107 bar in the system. This pressure is reduced each time the pump is operated.
Important note
If the pressure warning switch (No. 2) and 1 or the pressure reservoir (No. 5) is detached with the high-pressure pump (No. 1) removed, do not set down the pump upside down (with the pressure reservoir connection facing down). If this is not observed the pump might not function after installation (air in the pump). Fast turning or shaking motions from the normal position might make the pump operational again.