Checking Locking Torque
CHECKING LOCKING TORQUE (FRICTION COEFFICIENT) OF TRANSVERSE LOCK
Important notes
Check with cold transmission. Parking brake released, no gear engaged.
When checking the locking torque, it is not the rotating torque (starting torque) but rather the breakaway torque that is measured. For this reason, self-disengaging torque wrenches must be used.
Procedure
1. Carry out general preparations.
- Remove rear wheels.
- Fit torque adapter plate (Special Tool 9510/1) with original wheel bolts to left-hand rear wheel hub.
- Remove rear left-hand inner wheel arch panel.
- Prepare self-disengaging torque wrenches (click wrenches) covering a range of approx. 5 Nm (4 ftlb) to 500 Nm (369 ftlb). To fit them to the torque adapter plate, suitable reduction pieces or adapters are required.
2. Prepare vehicle for transverse lock measurement.
- Remove rear left-hand brake caliper (do not open hydraulic brake system) and place support block (arrow) - Special Tool 9509/4 - into the brake caliper, replacing the brake disc. Place brake caliper on rear axle trailing arm or suspend in a suitable place.
- Connect ring flange (No. 3) to measuring line (No. 2). Proceed by fitting this assembly, complete with banjo bolt, seals and fitting (Nos. 4, 5 and 6), to the transverse lock slave cylinder, replacing the bleeder valve.
Important note: Tighten Nos. 4 and 6 carefully and use a second wrench to lock when tightening, making sure the parts do not bind. If this is not observed, the fitting (No. 6) may shear off. Pilot pressure is normally present at the bleeding valve (No. 6). Open valve slowly before removing it (to allow the pressure level to be dumped slowly).
- Connect pressure gauge to transverse lock cylinder:
Connect high-pressure measuring line (No. 2) to pressure gauge (arrow) and suspend pressure gauge (No. 1) in rear left-hand wheel housing area so that it may be read off easily.
- Connect System Tester 9288 along with adapter line 9288/1 to diagnosis socket (located next to the passenger's seat under the booster cover).
- Bleed pressure gauge by connecting the bleeding device to the PSD reservoir. Clamp venting line. Build up bleeding pressure (approx. 1.5 to 2.0 bar). To keep the reservoir level from dropping too far (low volume), connect a bleeder unit to the reservoir. Bleeding pressure approx. 1.5 to 2.0 bar. The venting line remains clamped.
- Clock lock solenoid valve complete with System Tester 9288 in 'Drive links Bleed position' menu and allow brake fluid to escape at the bleeder valve (No.1 a) until it is free from air bubbles. (Use collector bottle).
- Close bleeder valve and press stop button. Select pressure reduction position.
- Switch off bleeding device.
3. Perform transverse lock measurement (locking torque to pressure ratio).
Testing cond. / important information.
- Transmission cold / parking brake released, no gear (driving position) selected.
- Before starting measurements, close lock once for a short time (activate full pressure).
- To measure the locking torque, rotate only in forward direction! Always measure across adapter plate at left-hand rear hub.
- Lock right-hand rear wheel fully using the service brake.
- Measure breakaway torque / locking torque at the specified test pressures using the torque wrench. Do not jerk when rotating. Advance carefully from bottom (low value) to top until the breakaway torque / locking torque is reached.
- Establish test pressures using System Tester 9288, clocking the transverse lock solenoid valve. (Drive links / Check transv. lock menu).
Test pressures / measuring process
Measurem. 1 = press. between 0...5 bar
Measurem. 2 = press. between 10...20 bar
Measurem. 3 = press. between 20...40 bar
Measurem. 4 = break away torque 500 Nm (369 ftlb) minimum, with pressure held at 60 bar or above.
Measurem. 5 = check pilot pressure transverse lock. Specified value approx. 3.0...4.5 bar.
Execution
- Observe testing conditions.
- Switch on System Tester. Select "Check transv. lock".
- Fully close transverse lock by applying approx. 20 pulses (gauge pressure =140...180 bar). Pressing operating button at tester once = 1 pulse.
- Switch to pressure release position and start with measurement no.1.
- Measurement 1 = gauge pressure 0...5 bar.
Set torque wrench to approx. 5 Nm and use adapter plate to connect to left-hand rear wheel hub (No. 7). Observe clicking direction of torque wrench.
While performing measurements, actuate service brake to ensure that the right-hand rear wheel is fully locked. Turn torque wrench carefully (i.e. avoiding jerks) in forward direction.
If the wheel hub does not yet rotate when the torque wrench clicks the actual locking torque has not yet been reached.
Adjust torque wrench to higher value and repeat operation.
The actual locking torque (actual value) is achieved when the wheel hub turns and the moment when the torque wrench starts to click is just about reached.
If the wheel hub turns but the wrench does not yet click, the torque set at the wrench is too high (actual locking torque is lower).
When measuring the breakaway torque, measure at several locations (angular positions) within one wheel revolution.
After the actual locking torque has been reached, immediately read off the pressure at the pressure gauge. Record both actual values (torque + pressure).
Locking torque-to-pressure limits:
- Measurement 2 = Gauge pressure 1O...20 bar.
Use the System Tester to build up test pressure, applying a corresponding number of pulses. Set torque wrench to bottom most value (depending on pressure) according to boundary value window.
Example:
Measurement 2 = Gauge pressure 10...20 bar.
Actual values: Gauge pressure = 13 bar, locking torque measured = 240 Nm (177 ftlb).
Specified value according to table at 13 bar: 120...300 Nm (88...221 ftlb).
Measurement 2 o.k. since results are within the tolerance band (boundary value window).
Repeat process as described for measurem. no.1.
- Measurement 3 = Gauge pressure 20...40 bar.
Repeat process as described for measurements 1 and 2, respectively.
Compare actual values obtained in measurements 1/2/3 with values specified in pressure graph.
- Measurement 4 = Minimum locking torque of 500 Nm (369 ftlb) must be ensured. Equivalent pressure 60 bar or higher.
- Measurement 5 = pilot pressure valve.
Proceed to pressure release position and read off pressure at gauge. Specified value: approx. 3.0...4.5 bar. If measurements are out of limits, repeat process, starting from low pressure max. 20 bar. In this case, release pressure by switching off ignition.
Possible causes for deviation from specification.
- Test requirements not met.
Example: RH rear wheel not fully locked, pressure gauge and lock hydraulics not properly bled.
- Faulty discs.
- Hydraulic pressure does not become effective to the degree indicated at the discs, e.g. due to faulty setting of the mating bearing screw of the engaging lever (engaging lever rides).
4. With the System Tester switched off (to ensure that only pilot pressure is present at the measuring line), remove pressure gauge complete with connecting lines.
- Open measuring line and transverse lock cylinder carefully (observing pilot pressure). Remove measuring line and fit bleeder valve.
- Bleed transverse lock using System Tester 9288, with bleeding device connected to reservoir and switched on (vent line clamped off). Follow by correcting reservoir level (to a level between MIN and MAX mark) with accumulator fully charged.
Correct reservoir fluid level.
- Screw on reservoir lid, open vent line at reservoir again.
5. Check lock hydraulics for leaks. Disconnect System Tester. Install rear brake caliper.
- Check hydraulic lines for outside leaks, controlling full system pressure at lock cylinder using System Tester 9288.
- Then disconnect System Tester.
- Tighten brake caliper mounting bolts to 85 Nm (63 ftlb).
6. Fit rear left-hand wheel housing inner panel. Fit rear wheels. Take vehicle for a short test drive (to check lock and brake operation).