headlights and speedometer

[dfink]

Well here's a good one. I finally got to test drive the car today for my christmas present to myself. (six months work total drive time 3 blocks) It was kind of crappy out so I decided to turn on the head lights. As soon as I turn on the head lights the speedometer quits working. The other gauges appear to be fine.
I know it has to be a ground issue some place but would the most likely place to start looking be the speedometer or the headlight - marker lights.
My first guess is that the ground for the speedometer is faulty and it is grounding throught the lighting circuit somehow. When I turn on the lights the ground path goes away so the speedometer quits. At this point just a guess

[RSBob]

I'm not up to speed on your model, but check to be sure there is an insulating sleeve around the bulbs for all the lights, and as you mentioned you have a good ground. Try grounding the gauge to another location and see if that helps.

[dfink]

Well my first guess was correct. There are two wires that attache from the speedometer pickup one goes to 37B and the other to "-" One wire then goes to the input for the Tach and one wire goes to the input for the heater control. I didn't realize that the one marked "-" also gets a ground. I looked all over the speedometer for a place to plug on the ground but couldn't find one. I finally pulled the speedometer from my 85' and found it attached to the "-" spot along with one of the wires from the pickup.

[JeremyD]

So how does it drive?? Details man - like you look at the speedometer anyway!

[dfink]

Originally Posted by JeremyD

So how does it drive?? Details man - like you look at the speedometer anyway!

Details are here Details