Test Point 13
DTC 34
Valve Relay
Certain faults in the ABS valve area (short circuit to plus or short circuit to ground in a control wire) may also be shown as valve relay faults by the 9288 System Tester.
A fault in the ABS valve circuit is normally stored as an ABS valve fault in the DTC memory of the control unit. In the case described above, when the DTC memory is read out the display of two faults is possible (ABS valve and valve relay).
1. Switch off the ignition. Pull off the control unit plug. Check the control circuit (wire with relay) - from PIN 20 to PIN 7 - for any discontinuity, short circuit to ground and short circuit to plus.
If no fault is found, proceed with 2.
If there is a discontinuity, short circuit to plus or short circuit to ground, pull off the valve relay. Check the wires between terminal 85 on the relay base and PIN 7 on the control unit plug and between terminal 86 and PIN 20.
If the wires are in working order, replace the valve relay.
2. Measure resistance between the following PINs at the control unit plug:
PIN 19 to PIN 17
PIN 22 to PIN 17
PIN 2 to PIN 17
Nominal value approx. 1...2 Ohm
PIN 21 to PIN 23
Nominal value approx. 2...4 Ohm
If the nominal value is correct, proceed with 3.
If incorrect values are obtained, localize the fault as appropriate.
From PINs 19, 22, 2 to PIN 17 = ABS valves + wiring.
From PIN 21 to PIN 23 = Lock valve + wiring.
ABS valves + wires
Pull off the 12-pole plug at the hydraulic unit.
Measure the internal resistance of the solenoid valves.
PIN 1 to PIN 4
PIN 3 to PIN 4
PIN 5 to PIN 4
Nominal value approx. O.7...1.7 Ohm
Nominal value not obtained: replace hydraulic unit.
Transverse lock valve + wiring:
Separate plug connection at the transverse lock valve (arrow). Measure internal resistance of the solenoid valve. Nominal value 2...4 Ohms.
If an incorrect value is obtained: replace transverse lock valve.
3. Pull off valve relay (7-pole) at hydraulic unit. Measure at control unit plug PIN 17 in order to determine whether there is a short circuit to plus or to ground.
If necessary, interrupt the circuit at the hydraulic unit plug (12-pole) and localize the fault by reference to the wiring diagram.
4. Check whether voltage is present at terminal 87 of the relay base when the valve relay is pulled off. Required display: approx. 12 volt (battery voltage).
5. Measure resistance between terminal 87 a on the relay base and the ground wire at the pump motor.
Nominal value = 2...4 ohms. The prerequisite here is that the ground wire and the ground point are in working order.
If necessary, replace the hydraulic unit (resistance in relay base has increased in value).
6. Check wires from terminal L1 on the relay base to PIN 24 on the control unit plug and from L2 to PIN 35 for discontinuity.
Important: The test from L2 to Pin 35 can be performed only if the warning contact of the pressure warning switch is closed (see wiring diagram).
This means that the pump for the lock hydraulics must have operated beforehand (system pressure greater than 140 bar).
7. Check that the valve relay is energized (voltage present at the solenoid valves). Connect all plugs and the relay.
Push back protective tube on the transverse lock valve plug (at the rear left in the wheel housing).
Switch the ignition off, then switch it on again.
Battery voltage must be present at the black wire.
If no voltage is present try replacing the valve relay, or test the relay with the ABS 2-LED tester in order to determine whether it is actually defective.
If the LED of the ABS tester fails to light up steadily in program switch position 5 with the ignition switched on, the relay is defective.
8. If the valve relay is in working order, try replacing the PSD/ABS control unit (output stage defective).