Hi i was wondering anyone know why my speedometer goes up to like around 60-70 mph when i rev the tach up, the speedometer goes up with it. But when i unhook the speed sensor on the tailshaft the tach is fine. And the speedometer is still. This is on my 1988 lincoln lsc mark VII. None digital dash.

Sounds like you have a bad ground.

Either you need to ground the orange wire behind the speedo cluster or it's a bad alternator.

Either you need to ground the orange wire behind the speedo cluster or it's a bad alternator.

Thanks for the advice the alternator was bad, put new one on solved problem.