Tach Debugging Advice IM D Convertible

So, engine performance was unchanged, just the gauge reading changed?

Yes, engine and speedometer fine.

I can only offer my condolences and wish you luck. Electrical gremlins can be very difficult -- inside of a system that is, theoretically, extremely simple.

Is the 18t the Audi/VW water-cooled engine?  Air cooled engines have the tach driven off the coil - that wire can get corroded/loose.  Water-cooled is a different animal!

Yes Audi 1.8t water cooled.

Did you check the wires going into the heater fan and the tach, the coilpack and the fan switch? That's what I would do, fwiw.

Good clean grounds are always an issue on plastic cars.  Normal metal cars can find a ground anywhere without running extra ground wires to each gauges and lights.  I'd check that too.  Many weird electrical gremlins are eliminated by adding an extra good size ground strap (mesh ones are available) from the engine/transaxle to the chassis.

The 1.8T is being run by an ECU, and that sends the signal to the tach. This problem is not going to be in the ECU.  I would look at the wiring of the tach and fan switche. I suspect that the blower and tach may be sharing a ground that's iffy. The higher amperage present when running the fan on high may be overwhelming that ground and leading to the tach issues.

Remember, advice is usually worth exactly what you paid for it.

That seems reasonable. I am out of town but will check when I return. Seems like an excellent place to begin debugging. Certainly your advice is worth more than what I paid for it.

Thanks!

Jim

This seems to be a recurring theme here about these plastic cars.  Everyone fingers poor or multiple grounds as the root of many evils.  I have to my knowledge never had a serious issue here, but one day I did inspect the ground cable from the trans to body and while I found the braided mesh strap to be looking good . I had what it took to add a really thick braided and insulated copper cable.  Can't be too careful.

.

A quick check is to run a dedicated ground wire from the tach straight to the battery, just for test purposes.

If that doesn't fix it, it's not the ground, right? And you can then move on to other sleuthing.

If that does fix it, you can then start the more time consuming process of tracking down just which ground connections are bad.

.

Good idea. Thanks!

Jim

Sacto: you're on to something.  Will wait to hear the results.