Reverse Puts S8 Into Limp

ahd91 · #1 26th April 2015, 04:59 PM

Got a bit of an issue with my recently acquired 02 S8.

Whenever I put the car into reverse, I get a bit of a clunk, and all the transmission lights come on. The car then goes into some sort of limp mode in that power appears restricted, but will actually change through some forward gears, and does reverse without issue.

Restarting the car immediately rectifies the problem, and all forward gears are extremely smooth.

The problem was intermittent for a few days, but now appears to be happening every time I go into reverse. It's annoying, but I can live with it if it's no threat to the transmission as a whole.

Ideas?

Adrian E · #2 26th April 2015, 05:28 PM

Most likely the F125 range select switch, if there's nothing fundamentally wrong with the box. If it will set off fine in D and not play up until you next move the gear selector about then it's most likely that. It will show up with a fault code scan.

If you post where you're located somebody may be able to scan it with VCDS if you don't have it?

Part itself is about £150 and it's on the side of the gearbox so bit of a ballache to get to, but not impossible

ahd91 · #3 29th April 2015, 10:32 AM

Many thanks for the reply.

Yes, it does sound like it's that. I'm in Leeds if there's anyone around that might be able to help.

Where would I source a F125? Having trouble, and I understand there are two types?

Adrian E · #4 29th April 2015, 11:14 AM

There are 2 types (the plug is the difference) but TPS should be able to source you the right one based on VIN

http://www.thetradepartsspecialists.co.uk/

RanjS8 · #5 30th April 2015, 10:18 PM

Your symptoms definitely sound like the f125 switch is faulty. The switch lives on the passenger side of the gearbox and the connector can be accessed from below or by removing the passenger side wheel.

Have a look on audipages for how it is replaced. The writeup makes it sound easy...it really isnt. Absolute pita.

The torx bolts that hold the switch in place can strip easily unless you use the right torx bit....T27.....and to get access to the bolt next to the exhaust I undone the bolts on the flange holding the exhaust in place to get a bit more movement and used a modified torx bit to get to the bolt.

Not a nice job at all...good luck

ahd91 · #6 9th September 2015, 03:45 PM

So, things took an awful turn for the worse yesterday.

Driving about at 10mph, when I got the dreaded bang and real difficulty selecting gears 1 and 2. After driving about for a min, I can get the car to light up the PRNS on the dash and go into limp mode. When it's in limp mode it drives about in what I'm assuming is 3rd, 4th and 5th perfectly well. Restart it and there's all manner of slippage.

Now I'm almost certain that the thing is mechanically dead, but the really odd thing is that all of a sudden the tiptronic function that never worked on the dash before is doing exactly what it should, and I already knew from VCDS that I deffo had a F125 problem.

There's no way this is all down to the F125 is there?

David's8 · #7 9th September 2015, 07:33 PM

Doesnt sound good at all. Best thing - even at this stage - is to try and get it to a ZF specialist (not sure where you are but there is a list of suitable places in the section on Transmission). At least you will get an opinion in whether it's terminal or not. Best of luck.

The_Laird · #8 10th September 2015, 06:53 AM

When my F125 switch went, I had all sorts of issues. Initially, like yours, it was worse in reverse, but it eventually started not changing up and, although the PRNDS lights weren't on, it was definitely in limp mode (not good on the M8 in the rush hour!). But it often went away for a while on a restart.

As David says, get it to a ZF specialist PDQ!