antoncole

Hi Folks,

My '95 A8 4.2 quattro started doing the ominous "thump" at around 50mph while driving steady recently and now it seems to be shifting gear a little heavily while town driving at low speeds and rpm.

The car has done 145'000 miles now and getting on a bit....

Ive read through the archives and seen some info on the thump problem being associated with the torque converter dropping in and out of lock.

Is the torque converter the root cause of this and can it be replaced without changing the whole transmission?

Could it be the gearbox ECU??

Is it likely to damage the gearbox eventually?

Are there any reasonable solutions to this problem?

Id love to fix this as i adore the car and apart from this, she's perfect.
Could probably buy another car for the same money as a gearbox re-build but this is all thats wrong with her.

Your help is, as ever, greatly appreciated.

Cheers

theraudi1

wow ..1995 the first "true" year of the A8. I thought that the 4 speeds were a lot better .... hmmmm

CalG

The function of the TC it's self is not likely the problem. The fact that it "opens" and then closes positively (with a thump) speaks to the functional operation of the friction surfaces and materials. That said, The TC could contribute to pressurizing fluid usage beyond the output capabilities of a high milage Valve body and fluid pump. There is a seal or two in the TC that "could" contribute to the undesired performance.

If a log is made of the transmission control functions, one might observe that the undesirable TC function is associated with command output from the transmission control module. (TCM) The operation may be correct and normal, but the acutation a bit off due to fluid system deficiencies. Deficiencies brought on by high milage,wear and system design.

Tozo has done much study of these ZF transmissions and has come up with some modifications to reduce or elliminate the Thump. Cheers to him!

My own experience is with a 1999 vehicle that had the "hunt" but no "thump". I o btained a later model Valve body and replaced the original unit. Now the "hunt" is mostly absent, but the "thump" appears infrequently but does from time to time. Hmmmm.

I have replaced filters and fluid as well.

Considering the effort to R&R the transdmission and TC, and the evidence of some who have replaced the TC without improvement may suggest a "complete" transmission rebuild as a wise choice.

A search of these pages will produce many different stories of repair and frustration.
Some owners have accumulated high milage even with the "problems" and without failure.
It would be so refreshing to understand the exact weakness and be able to address the problem with consideration for all trade offs.

Cheers

CalG

Always Overcharged

have a 4 speed CML trans and 176000KM's and my shifts are smooth but of late I have noticed a clunk when shifting from reverse to foward gear.
If this gets worse it can cause your symptoms.
The CML is usually pretty solid