Speedometer?

[ray12]

I have an issue with my speedometer/odometer that I haven't been able to resolve. Any suggestions and/or advice will be appreciated! Here's the details:

- Speedometer and Odometer BOTH stop working when I switch on the car headlights.
- Speedometer and Odometer BOTH work perfectly fine as long as headlights aren't turned on.
- As soon as headlight is switched on the speedometer drops immediately to 0 mph and the odometer also stops moving.
- All the rest of the gauges in the car work properly both when headlight is on and off.
- The speedo gauge lights and the green headlight light all work properly when headlight is switched on.
- There is an aftermarket stereo/amplifier installed in car.
- I took out speedo and sent out to Hollywood speedometer and they were very nice and took a look and said the speedo unit itself was functioning properly. They had suggested and thought maybe it was the ground connection spade accidentally touching the metal backing of the speedo unit. When I plugged in the connections on the back of speedo i made sure the spade connectors were not touching the anything but the connection terminal.

I don't even know where to begin to troubleshoot this and not familiar with there the wiring would be for the speedo unit in the car.

[targa80]

First, you will need to post the year and model. Is the car original or modified. Sounds like you have an issue between two separate circuits.

[ClickClickBoom]

Wiring on the back of the speedo and tach. The speedo and tach internal lighting is pulling ground through the internal lighting. Check your internal instrument lighting wiring.
Deduction, since the problem is instrument electrical its a SC or Carerra, both use electronic signals for the tach and speedo, and both are separate systems, the only common system to both instruments is the internal lighting. You need a schematic specific to your year car. With basic electrical theory, a multimeter and lots of time you can deduce the wiring, but a schematic is quicker.