Pressure Tests on the Lock Hydraulics
1. General
The following points can be checked by means of the pressure test on the differential lock hydraulics (lock control):
- Freedom from leaks of the lock hydraulics (any internal leak can thus be localized).
- Gas filling pressure of the pressure reservoir.
- Switching points for the lock hydraulics (display "PSD off�' and operating pressure). This is controlled by the pressure warning switch of the pump assembly.
2. Pressure gauge connection
- Depressurize the lock hydraulics. To do this, disconnect the electrical plug at the pressure pump (pump assembly) and then relieve the pressure at the pressure relief valve of the lock solenoid valve (use a collection bottle).
Important: Wear goggles and protective gloves, as the pressure may be as high as 180 bar.
- Connect pressure gauge 9509 - in combination with the high-pressure measuring line 9509/1 (No. 2), ring connection (No. 3), banjo bolt (No. 4) and two seal rings (No. 5) - at the lock solenoid valve instead of the bleeder valve.
Important: Machine ring connection (No. 3) beforehand (apply face/arrow).
- Bleed pressure gauge (No. 1). To do this, connect a collection bottle to the pressure gauge bleeder valve (No. 1a) and open the valve.
Turn ignition key to position 2 (necessary for pump operation).
Plug the electrical plug onto the pump. Disconnect the electrical plug and close the bleeder valve as soon as no air bubbles are visible anymore in the transparent bleeder line of the collection bottle. Make sure that the brake fluid reservoir is not sucked empty. Fill in brake fluid from time to time if necessary.
Note: Depressurize the lock hydraulics before removing the pressure gauge after the test.
3. Tests
Note: Operation of the pump is regulated by means of the ignition key for testing purposes.
Position 0 = Pump off.
Position 2 = Pump on until switched off by the pressure warning switch.
- In ignition key position 0, plug the electrical plug onto the pump. Locate the pressure gauge in a position where it can be seen.
For the tests, it is expedient to observe the following sequence:
a. Gas filling pressure of the pressure reservoir.
Turn the ignition key to position 0 at approx. 100 bar. Briefly open the bleeder valve on the pressure gauge (use collection bottle) and close it again immediately. The pressure drop on the pressure gauge should be approx. 5 bar. Continue this process in intervals. The pressure reservoir gas filling pressure has been reached at the instant the pointer of the pressure gauge abruptly falls to 0 bar. Refer to step 4 for nominal values.
Important: Always wear goggles and protective gloves when decreasing the pressure via the bleeder valve.
b. Checking the switching points for the warning display in the instrument cluster.
During pressure increase: Start the engine and observe the warning display. Immediately turn the ignition key to 0 position at the instant the warning display (PSD off) goes out. Read off the pressure on the pressure gauge.
During pressure reduction: Produce a system pressure of approx. 140 bar. Disconnect the plug at the pressure pump. Start the engine. Reduce the pressure - in a similar manner to the procedure described in Gas filling pressure of the pressure reservoir - until the warning display (PSD off) lights up. Read off the pressure on the pressure gauge. Refer to step 4 for nominal values.
c. Checking the switching points of the pressure pump.
Cut-out pressure: Turn the ignition key to position 2. The electrical plug must be plugged onto the pressure pump for this. Read off the pressure on the pressure gauge immediately after the pump cuts out automatically.
Cut-in pressure: Turn ignition key to position 2. Wait until the pump cuts out automatically if necessary. Carefully reduce the system pressure at the pressure gauge bleeder valve (use collection bottle) until the pump cuts in again. Read off the pressure on the pressure gauge at this moment. Refer to step 4 for nominal values.
d. Checking the tightness (pressure loss) of the lock hydraulics.
Turn the ignition key to position 2. Wait until the pump cuts out automatically. Carefully lower the system pressure at the pressure gauge bleeder valve (use collection bottle) until the pump cuts in again. After the pump cuts out automatically, turn the ignition key to position 0 and disconnect the electrical plug at the pump. Measure the pressure decrease over time.
The permissible values are given in step 4.
Note: Depressurize the lock hydraulics before removing the pressure gauge.
4. Nominal values
Pressure reservoir - Gas filling pressure
new 80 ± 5 bar
wear limit 40 bar
Note: Replace pressure reservoir if the gas filling pressure has reached or fallen below the wear limit
Switching points of the pressure warning switch
PSD off Warning point
for pressure increase up to approx. 115 bar
max. up to 133 bar
for pressure reduction as of 105 + 2 / - 5 bar
Note: If the actual values deviate from the nominal values, replace the pressure warning switch.
Cut-out point of the pump:
approx. 180 bar at the latest
approx. 160 bar at the earliest
Cut-in point of the pump:
140 + 5/ - 6 bar (after pressure reduction)
Note: Read off the pressure immediately after the pump cuts out automatically.
Freedom from leaks of the lock hydraulics
Pressure reduction, starting from the cut-out point of the pump (precondition: the nominal value has been achieved).
not below 140 bar in 30 minutes
not below 100 bar in 3 hours
Note: Fill the pressure reservoir completely 2x previously (start the pump running again after the 1st cut-out operation by opening the bleeder valve on the pressure gauge). Use a collection bottle. Then turn the ignition key to position 0 and disconnect the electrical plug at the pump.
External leak: Retighten lines or replace the corresponding pans.
Possible internal leaks:
- Lock solenoid valve
- Pump assembly
First check the lock solenoid valve. Then remove the pressure line from the pump assembly (with the system depressurized).
Connect high-pressure measuring line 9509/1 (No.2) - without ring connector - to the pump assembly.
Repeat the test. If the nominal values are now achieved, replace the lock solenoid valve. If the nominal values are not achieved, the fault lies in the pump assembly.
Replace faulty parts and bleed the system.
Caution: Wear goggles and protective gloves when reducing pressure via the bleeder valve.