Shudder from the gearbox?

[The_Laird]

My observations are as follows:

In lower gears, at around 1500-2000 rpm, often with the engine under load (going up an incline), I get a shudder until the revs increase to above 2K.

It only happens when the engine/box is warm and it’s not got any worse in over 4 years.

But I do have an intermittent F125 fault on the VCDS -0 could it be that?

And where can I find a new F125 switch? Perhaps one of these https://www.autodoc.co.uk/car-parts/oem ... gIpg_D_BwE

[MikkiJayne]

The torque converter lockup clutch is toast. Either the friction material is worn out, or the seal on the piston is perished. The converter locks up (or tries to) at 1800 rpm, so as it passes through that speed you get the shudder as the lockup clutch is slipping slightly, just like judder in an ordinary manual clutch when it is worn out or contaminated.

A reconditioned converter is ~£500 or so. The challenge of course is replacing it, since it's sandwiched between the engine and gearbox.

[Ronin]

F125 only tells the gearbox ECU what position the shifter is in and can be tested in vcds.

If your car wouldn't shift into any gear or the engine wouldn't start because the ECU wouldn't be able to see that shifter is in P, then f125 could be the cause.

If you are going to have the gearbox out for torque converter replacement, it would be a good idea to replace it at the same time.

[The_Laird]

The thing with this fault is that it’s got no worse for at least a couple of years, doesn’t seem to improve after a Mackie’s service and, strangely, seems to have got better after a set of new plugs!

[The_Laird]

Another update on this. I recently replaced on of the ‘bellows’ on the intake manifold and, not only has the fault code for the secondary air injection system gone away, so has this judder! The S8 is again running like a dream

[SharkGill]

Would the torque converter cause flashing PDRS lights on the dash? This happen to me this morning paired with hardly any forward drive. I've noiticed the occasional judder and a whine on acceleration similar to a supercharger but without the extra power I'm hoping the car gets me home after work.

[MikkiJayne]

No, PRNDS is limp mode - it'll be stuck in 4th gear when the dash is lit up. You need to see what codes it's stored and hope its something benign like the F125.

A supercharger whine (which is coming from the converter) usually means the ATF is low, which could cause judders, as could a bad maf. Guessing without codes though.

[HPsauce]

PRNDS that goes away; first suspect is always the F125, but codes are mandatory.