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The ZF 5HP24 transmission (A5S 440Z) is very unusual in that, in order to save weight & space, ZF removed the torsional vibration damper from the lock-up clutch and instead rely on their ‘controlled slip’ system to modulate the LUC pressures and prevent the transmission of torsional vibration at lower engine speeds.
To achieve this, the transmission control system has to be extremely sophisticated and highly sensitive to load/power. As an example, the graph below shows a constant 50mph with the cruise control activated. The throttle (shown by the blue line) is opening & closing to maintain vehicle speed as the vehicle goes up & down hill etc. as you would expect. However, the magenta line shows the current being supplied to Solenoid 4 (which pressurises the torque converter lock-up clutch) as a percentage of maximum. You can see that the transmission ECU supplies pressure to the lock-up clutch so that it mimics exactly the throttle opening and therefore the torque seen at the clutch. This maintains a (close to) constant speed ratio across the torque converter i.e. with 3-4% slip to dampen out torsional vibrations.
At higher engine/vehicle speeds the lock-up clutch will ‘lock’ completely as there’s no longer a risk of vibration being fed through the driveline.
This controlled slip function of the lock-up clutch is one of the main reasons that it’s essential to use only LT71141 fluid in this transmission as it has been specially formulated with the right friction modifier pack for the clutch characteristics.
If a vibration can be felt in the lower gears or at low engine/vehicle speeds or the engine speed fluctuates by a few hundred rpm then it could be the torque converter lock-up clutch at fault.
Phil
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