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Discussion Starter #1
The tach in my '67 RS/SS Convertible reads 1500 rpms at idle, but my idle is actually around 1000 rpms. I noticed that when I turn on my headlight switch and increase the brightness of the guages (no lights on the speedo or tach though, only on the console guages and heater switches) the tach needle jumps another 1000 rpms. Then if I turn the dimmer down all the way then the tach needle jumps back down 1000rpms. Anyone know where I can get my hands on a good wiring chart or diagram for the rear of the tach?

Steve
 

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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.
 

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Discussion Starter #3
Thanks Mark! I'll check all these areas. Yep, sounds like a bad ground alright.

Steve
 

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Discussion Starter #4
Oh. The lamp for the guage isn't working. Also for the speedo. Perhaps the wiring to the lamp is attached to the ground connector on the tach? Or would that cause the needle to peg at the redline all the time?

Steve
 

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Do a search on tach filters. Mine was reading erratic and a filter fixed mine.
 
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