I am having a wierd problem with my gauges. I have a 91 Jetta diesel. Then I first start the car I have no tach. When I start driving, it suddenly starts working and works fine for the trip. If I shut off the car and start it again, I get the same problem. Is this a problem with the alternator?
The alternator has to reach a certain rpm before it starts charging. After it starts charging, it can drop below that rpm and still charge. It is normal on a lot of alternators. I imagine that is your problem.
Betcha the red alternator light comes on as well when you first start the car.... if you blip the throttle the red light goes off and the tach starts to work?
Tracked it down to the D+ wire. It was broken. It preloads the alternator. Reattached, and everything works great now.