2000 S4, M-Box.

As I posted in the antilag thread, when you lose VSS reference, things go wonky fast.

Basically the speedo started failing - it would intermittently lose speed reference, until it finally stopped getting ANY speed reference.  I changed the ECU, VSS, and verified there is continuity in the harness.  I did not verify if the tone ring is intact, because that's a pain in the ass and my garage is already full of another project.  I'll verify it soon.

ECU DTC:
16885 VSS Implausible signal

Cluster DTC:
01314 Engine Control Module - No communications - intermittent

I swapped the ECU thinking maybe something went awry from using antilag all the time, but no luck.

Over the last 3-4 years I've noticed that on a cold start, occasionally the instrument cluster initializes, the needles creep a few steps, and then they sit there and jitter for a few seconds before they settle down and start reporting RPM/speed.  Sometimes I'll be driving a few blocks before the cluster starts working.

The part that bothers me and makes me think the cluster has gone apeshit is that as soon as this started, the fuel gauge started randomly flapping back and forth between the actual fuel level and full.

Now, perhaps the sending unit is coincidentally failing a second time at this exact moment of electrical gremlinity (that's a word!).... but...  I would think the signal from the tank is filtered to prevent that type of behavior.

Thoughts?

Yeah, there is zero output from the VSS when logging.

I have another car with the engine out in my garage, otherwise I'd look at the tone ring.

I don't think it's the tone ring however.  I was fiddling with it earlier, and when you get into boost, you get a VSS signal.

trombone.wav

Swapped in another cluster tonight.  Speedo worked for about a half mile, and then everything went nuts again.

I am happy to report the problem suddenly fixed itself and has been fine for 200 miles.