Transmission issues summary

[RRPhil]

I’ve already posted this over on a Range Rover forum but, as an identical torque converter is used in the 4.4l E53, I thought it may also be of interest to a few of you on here.

I’ve now disassembled my old (108k mile) 5HP24 torque converter so that the lock-up clutch, which was giving me vibration problems, can be examined. The vibration issue disappeared when the converter was replaced with a re-manufactured unit.



Impeller vanes are radially straight in the F38 ZF 5HP24 torque converter



Torque converter stator element which houses a one-way clutch so that the stator is held stationary during the torque multiplication phase but can then freewheel once the coupling point is reached and the fluid flow direction changes



Torque converter turbine



Engine-side of turbine showing castellated slots which engage on lock-up clutch friction plate



Following removal of turbine, lock-up clutch assembly is exposed



Clutch piston is friction welded into impeller casing and part of its hub has to be machined away to release piston. Friction welding process has to be accurate to apply correct preload from six tangential springs to clamp LUC in ‘on’ position (i.e. transmission has to feed hydraulic pressure to engine side of piston to hold the converter ‘open’)



Lock-up clutch piston after removal



Lock-up clutch friction plate in impeller casing



Though I was expecting to find the friction plate completely worn it had 0.72mm (0.028”) of material left on each side (but unfortunately I don’t know what the thickness of a new plate is nor ZF’s wear limit) and looked in fine condition





The only indication that there’d been any problem with the clutch were chatter marks on metal friction face of the piston



Opposing metal track on impeller casing looked smooth & free from damage


So, although not immediately obvious from its condition, this lock-up clutch was causing vibration issues in my Range Rover at low engine speeds (the cattle grid noise) which were cured once it was replaced with a re-manufactured unit. My only theories at the moment are that :

a) the friction plate had worn sufficiently for the preload applied by the tangential spring straps to reduce enough to cause a stick-slip effect during the converter’s ‘continuous slip’ operating phase (typically only around 3% relative speed)

b) the friction plate started out with grooves in the friction material to allow fluid to flow across the surface when it was engaged and these had worn away causing the friction faces to overheat (or at least change the friction coefficient).


ZF vs GM

Okay, so maybe the friction material in the ZF 5HP24 torque converter LUC wears eventually to the point at which it causes a vibration due to lack of clamp force – a sort of early warning system. Judging by the amount of material still left on the plate though, replacement isn’t urgent and the transmission will soldier on for a good while without causing further damage.

On the other hand, the GM 5L40-E transmission in the 3.0l wears through its LUC friction material (due to low system pressures) without any warning at all - it’s fitted with a torsional vibration damper - and the first you know about it is when it dumps huge quantities of metallic debris into the fluid causing the whole transmission to grind to a halt, by which time it's too late.



I think I prefer the ZF system!

Phil