69 Charger electrical question

[sixt9rsx33]

I have a 69 Charger RT 440 auto with air PS and rear defroster. I restored the car and upon completion I have an electrical gremlin. When I turn my fan on for the AC/heat and at idle my tach will move up to around 2000 rpm’s. When I accelerate the tach will settle to the correct reading. The tach works otherwise when the fan is not on or while driving. Seems at a low voltage situation the tach acts up. Any ideas?

Thanks

Lawrence

[thehornworks]

Probably a bad or missing ground.

[sixt9rsx33]

Thanks will try and find that.

[njsteve]

On the old Mopars I usually have to add extra dedicated ground wires. Behind that gauge pod is a big foil printed circuit. If it is missing any of the little nuts or screws, the ground suffers and the back feed into the gauges.

Another familiar glitch is the tail light system. Sometimes you see the reverse lights dimly light up when you step on the brakes, or one of the turn signals get dim when you step on the brakes. An extra ground wire from the tail lamp stud to an existing bolt on the trunk panel cures that.