[70KGOLD]

Assuming my old hei dist hood tach had died I ordered a new Tach for my 70 GTO. Still not working with the car running I have constant light on the brown wire going to the tach with a test light. Can someone tell me what I'm missing?

[PontiacJim1959]

Your 1970 Sevice/Chassis manual should show you the hook up. Tach wire goes on the negative side of the coil to pick up the RPM signal.

[70KGOLD]

I know how to hook the tack up.

[PontiacJim1959]

Originally Posted by 70KGOLD

I know how to hook the tack up.

Then you should have stated that you have the tach wiring diagram and have it hooked up as factory correct.

Then you should have stated, "need help on tracing a power/short problem, and not a tach problem and where would be the best place to start."

Might be the real help you needed - tracing wiring?

[cressy]

I have a factory HEI distributor with Acell coil on my 70 GTO with a repro hood tach I installed 20 years ago with the factory wiring harness and I have to put my lights on for it to work.

[Skidmark]

[gto4evr]

I seem to recall an issue that the repops that use factory 3 wire connectors are pinned out differently than the early factory harness. original harness was one wire ran lights, one ran tach and one was ground. new tachs one wire is ignition power, one wire is ground and one wire is tach signal so the light function comes from internal power source inside the tach where original harness, the light wire was a separate feed from the dash.

Originally the tach light only came on when the lights were on. There was a later tech bulletin change that had the light feed wire taken off the lighting circuit and put to a 12v ignition source to keep the light on all the time in an attempt to warm up the tach internally to combat the moisture that collected on the inside of the face. I think that happened mid way through 70 and any late 70+ tach was wired that way from the factory. anyways, depending on how your car's half of that harness is wired will affect the functionality of the aftermarket tach plugged into that harness. After that didn't work either they went to a vacuum line connecting the base of the tach to the heater box to try pulling air through it to kill the humidity. still didn't work.

from OP's post I'm guessing this isn't the case if it was working before and not now without any wiring change, just throwing this info out there for future searchers, beware of repop harnessing connectors that plug into original 67-mid 70 harnesses.