CorrieHiatt

Hi all,

I'm making this thread because after much searching and quite a bit of headache because I'm out of ideas to troubleshoot my car's issues. If I've missed something, I'd love any help.

My 2006 A8 4.2 is having some issues with the electronics, as it seems. The front left and rear left wheel speed sensors seem to intermittently work, but I can't figure out why they are intermittently working. The ABS module has been sent out to be refreshed so I don't think the module is an issue. Since the front left wheel speed sensor controls the speedometer, the car goes crazy when that sensor is acting up - air suspension won't raise or lower, the steering rack stays in its most firm setting, and all sorts of other warnings come up. I've replaced the wheel speed sensor and brake light switch, so I assume it's something deeper. Additionally, the brake pressure sensor reads ~300 bar, which to me indicates that power or signal is not being supplied to the sensor. I've also replaced the sensor and have the same story. Before I go ahead and tear apart the ABS wire harness, has anyone had any issues similar to this and done some magical trick to get everything squared away?

Interestingly, the front left wheel speed sensor (and the speedometer) work perfectly when I drive in the morning after the car has sat - it's only if I am doing city driving or have left the car for a couple hours that the speedometer stops working - could heat be causing the wires to short out? It seems like a residual heat thing, because any time I do long highway driving and come into stop-and-go traffic, the speedometer stops working.

Any help or insight is appreciated, I really am lost on what is going wrong here.

audijim113

What do you mean by "ABS Module has been sent out to be refreshed"?

CorrieHiatt

I had the ABS module rebuilt by CheapABS

dvs_dave

Almost certainly crumbling wiring loom insulation causing shorts in the first couple of feet out from the ABS module. Need to splice in new sections of wire as appropriate to fix.

Common problem as the cars get older.

CorrieHiatt

Unfortunately, that's the answer I anticipated. Thanks for letting me know it's a common problem - I may have to do this on my W12 at some point soon if this is so common. Are wire diagrams available for the harness sections for the front end of the car?

dvs_dave

It’s definitely not something I’d do as a preventative measure, only an as needed repair. You don’t need a wiring diagram to do it as when you open it up, you’ll see most of the wires are likely affected. Just splice in new sections one at time.

But wiring diagrams are all available in the workshop manuals such as www.alldatadiy.com

CorrieHiatt

Thanks for the insight. I'll report back with my findings.

CorrieHiatt

Hi all, I was finally able to get around to splicing new wires into the harness. Unfortunately, it didn't seem to fix it - if the car sits at idle and warms up, the speedometer will sit at zero. Since it's temperature-related, I imagine that the ABS module is on its way out. It's odd, because the module was sent out in early 2020, so I'm not sure why it failed so soon after being rebuilt. I've yet to figure out if my pressure sensor issue is the same at this thread but I'll give it a shot next time I'm under the car. I'm now looking at replacing the module entirely.

I went down to the junkyard and got a Bosch 5.7 ABS module from an A4. Is it possible to replace the A8's module with the one from an A4 or is there a software difference that makes it impossible to have the car communicate with it? I would imagine the CAN Bus system speaks the same 'language' regardless of an A4 or A8, but maybe the ID's are different? Any information on that would be nice, it's cold here in CO, so I'm not too keen to go freeze myself changing an ABS module in the snow until I absolutely have to!

CorrieHiatt

A little bit of an update:

I was able to test ABS modules from A4's A6's and A8's and it seems that only the A8 ABS modules have the correct software for them to be coded by the A8 computers. The good news is that the errors for the left rear wheel speed sensor and ABS pressure sensors have disappeared when I put a new module in. However, the bad news is that the front left wheel speed sensor is still going out when the car reaches operating temperature and the car isn't moving at highway speeds. To me, it seems like there are further issues with the wiring that went undiscovered when I patched the wires. Does anyone have any experience on this or any guidance? I'm thinking of just running a completely new set of wires all the way to the wheel speed connector, but I'm getting very close to the ABS connector, so patching may be very difficult. If anyone has any information, I'd appreciate it!

MP4.2+6.0

Search on wiring harness issues for D3 via Google pointed at AW. There were a variety of posts on it, including pictures, when car was younger. Especially the early (2004, etc.) ones. Threads came up several times at length maybe 5-10 years ago now. Issue IIRC is right around ABS unit area on down toward the subframe in early part of harness. If you have the issue, you find it with the harness mass with spiral wound tape removed. You then will find crumbling insulation on individual conductors, semi bare wires, etc. Repair in full and retape.

If you need new wires and are close in to a plug, Audi sells pre-wired insulated conductor jumpers a couple feet long with the terminal already fitted to the end. You need to know which connector it is and thus which type of terminal, and then have a patient Audi parts person help you. Search on prior posts for how to for harness splicing on those, as well as how to open up Audi type connectors and remove individual terminals.

I did some of these splices using the Audi pre fitted jumper conductors for W12 O2's and did a post (or maybe reply?) on it. The key Tyco connectors you can use are here for the splices--you do the splice in line with stripped bare wire area, then cover with these already sliced on the wire, and then heat with a heat gun or blow dryer. Presto, nice soldered connection and also heat shrunk sealed up. For critical sensors like ABS, airbags, O2's, etc, I would definitely not use the cheap squeeze 'em with pliers type blue and red splice connectors like used to be used a lot for aftermarket gear like stereos and amps.