Join Date: Sep 2000
Location: Shrewsbury, MA, USA
Theres only three wires on a tach. Input from the coil, instrument lamp power from the light switch, and ground. Since your tach changes readings when you turn the lights on I would guess you have a bad dash ground. Some of the dash light voltage is feeding back into the tach instead of going to ground.
There is a single ground point on the back center of the gauge pod, for all the stuff in the pod requiring a ground. Make sure it is tight and has a good connection to the body. Also make sure you have all your body ground straps. two between the engine and firewall, one between the firewall and subframe, and one between the right fender and the (-) post of the battery. 69's have one additional ground strap under the battery between the subframe and the inner fender well.
1969 Indy Pace Car
350/300HP RPO Z11
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