TROOPER88

Hi guys
I have just acquired a 1998 A8 D2 4.2.

It has a few issues which I am slowly sorting. The main issue is that the oil pressure light flashes on the dash BUT this is brought on for some reason by the rev counter working very intermittently.

You can be driving along and all of a sudden the rev counter drops to zero and instantly the oil light will flash.

Anyway...... yesterday I undo the two screws for the cluster and pulled it forward. I then turned the car on and the rev counter worked perfectly. I guessed that it must have been a loose connection on the back.
I ensure the plugs on the back were tight before putting it back together.

I drove the car but after 5 minutes it failed again. At this point I was not too concerned as I knew I had found the problem.

I once again pulled the cluster forward but this time I can not get it to work again at all....

I have removed the cluster completely and can not see a fault.
There is 100% definite a fault with the cluster or wiring just behind as the first time the rev counter immediately worked.

There are 4 plugs on the back, one small white plug and 3 coloured rectangular plugs.

Does anyone know which one controls the rev counter??

I am really unsure what the problem is unless there is a tiny break inside one of the wires?

Any help would be much appreciated.

I may try and buy a second hand cluster but with the way it suddenly worked when I pulled it forward leads me to think it must be the wiring?

If I know what plug powers it I can have a really good look at it and maybe wire a new section in!

Cheers

silverd2

I believe you are on the right track. If there was a problem before the cluster or it's wiring, the loss of tach signal would shut the engine off. G28, the engine speed sensor ("crank position sensor") sends signal to the ECM first, which in turn, sends signal to the fuel pump relay (any interupt cuts ground to relay and shuts off fuel) and to the instrument cluster...2 separate wires from the ECM, though.

Both tach signal and oil pressure signal go through the red 26 pin plug on the cluster:

#7 on that plug, gray w/black stripe .35mm wire = Oil pressure

#18 on that plug, yellow .35mm wire = Tach signal

TROOPER88

Hi
Many, many thanks for taking the time to reply!

I called by my garage on the way home from work tonight (before I had read your post) and started the car.
The rev counter worked!! I was very surprised and stopped and started the car 5 times and each time it worked.

I thought; lets take it for a spin and see if it continues to work.

I opened the garage door, started the car and guess what it did not work!!

I spent another hour looking for a fault but could not find one. How could it possible not work yesterday at all, then work today?? The car literally had not been touched or moved, it makes no sense.

I will look into the wires that you mention tomorrow afternoon but there is a part of me now that is thinking that perhaps the fault lies within the cluster unit itself.

I will keep you updated..

Hans.sfj

I'm having the same problem now. What was the solution?