Hi all,

My C5 A6 has been in "transmission limp mode" for a while now. It would run fine, but wouldn't go into 5th gear. Also the PRND432 on the dash were all lit up, instead of just the selected gear being highlighted, all of them were. Scanned with VAG-COM and found F125 Multi-function Switch implausible signal. Also Brake Light Switch implausible signal. The brake light switch code didn't always reappear, and the brake lights turn on when I press on the pedal. I figured it was the multi-function switch, but decided to wait a bit because they're expensive, and a pain to install. Anyways, one day I try to start it, and nothing happens. No start, no crank, just the slightest clicking sound (probably the ignition relay). I jumped the starter solenoid toggle wire to the battery+, and it turned over, so I know it's not the battery or the starter. There's no voltage going to the solenoid toggle wire when I turn it to the start position. I have an F125 switch that i haven't installed yet, but i unplugged the old one, plugged in the new one, tried about 10 different positions on the switch, and still in limp mode and no crank. I cleared the codes at every position I tried on the switch, and the PRND432 would un-highlight for 1-2 seconds, and then rehighlight. I have a replacement TCM/TCU on the way. That might take care of limp mode, but I'm not sure about the main issue, no crank, no start. If anyone has any ideas, I'd love to hear them.

Thanks!

 

replace brake light switch. brake lights can work with bad switch, but cause symptoms like yours

 

Thanks John, I'll have the brake light switch tomorrow, and post the results after testing it. In the meantime, I've got another question:
- It's a 2-pin switch. Does it just open and close the circuit? If it's as simple as that, maybe I can bypass it?

 

You can't bypass it. You might have other issues, but see what happens with new brake light switch and go from there.

 

Ok I put the new brake light switch in, and nothing changed. Anyone know where I can find a starter system wiring diagram for this model?

 

I would check voltage from ignition switch all the way to starter. My c5 is manual trans, but perhaps your TCM is the issue. Thinks it's not in Park.

here's a link to wiring diagram.

http://audi.kris-hansen.com/bentley/audi.c5.wd.25.pdf

 

So I found the no crank issue. It was a bad ground wire leading to the starter relay. I just spliced a new wire into it and attached it to the frame.

It's still in "transmission limp mode" though. I replaced the TCM/TCU and same thing. The issue is that it doesn't go into 5th gear. Ever. It's still throwing a code for the F125 Multi-function Switch even with the new one plugged in but not installed.

That being said, I'm suspicious of other bad ground wires too, that could be causing limp mode. Any suggestions would be highly appreciated.

thanks!