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Hello all! First post here, but I've been looking through the site ever since I got my 1968 Gto about three weeks ago. A big thanks to all of you for the useful information you post. It really helps us newbies. I've searched for a specific answer to this question all over the internet, and looked for videos and pics. I'm rewiring my hood tach with a new wiring harness. The hood tach didnt work (lights or otherwise) when I bought the car, but the previous owner said it had "just quit" before I bought it. Anyways...... the new harness has 3 wires, 1 black to the ground on the firewall, a black with purple stripe with a fuseable link leading to the negative side on the coil, and the third is a grey wire with a red plug, that has a spot for another plug to be inserted behind it after its plugged onto something. That's the issue. The other wires were on the car, in place when I started the rewire. But the plug from the old harness was cut off and the wire was simply taped up in the harness. It's a hidden headlight car if that matters. The wire with the plug on the new harness doesn't seem to be anywhere near long enough to go through the firewall. Can anyone assist with where this plug is supposed to go? Thanks again to all.
 

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The gray wire is for the tachometer light. It should plug into a spare grey wire behind the dash.

Original wiring was a single plug from the tach with all three wires attached. The carbody or other side of the plug wiring went to the coil, dash and ground.

fullsizeoutput_1890.jpeg
 

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Discussion Starter #3
The gray wire is for the tachometer light. It should plug into a spare grey wire behind the dash.

Original wiring was a single plug from the tach with all three wires attached. The carbody or other side of the plug wiring went to the coil, dash and ground.

View attachment 133045
Thank you. So assuming the original wire is no longer there behind the dash, where should I run the new grey wire to?
 

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it would matter if you want the light to come on with the key or if you want it to come on with the headlight switch?
My tach lights up with the ignition.
 

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I believe with the key? The reading I've done says this is to emilinate fogging, but I'm not sure. Where was it ran from the factory (or dealer)?
 

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It was run from the spare grey wire that powers the instrument lights. This wire is on all unmodified harnesses directly behind where the clock or in dash tach would be. I you have a clock it will connected to the clock lighting with a spare open terminal.
If the wire is no longer there, run it to the lighting terminal on the fuse block.

If you want it to be on all the time run it to the ignition terminal on the fuse block or the positive coil terminal. The light will be dimmer if you run it to the coil due to the resistance wire feeding the coil.
 

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If it's a repo tach or even rebuilt original, you need to see how many wires go into the base of the tach. If 2 wires go in your ok to wire it to the light circuit. IF there are 3 wires going into the base of the tach then it has updated electronics and needs the gray wire attached to 12V whenever key is in on position. If it doesn't work and you need help with getting it fixed I repair them. In my experience stock bulbs produce heat, humidity in the tach with hot bulbs results in tach fogging up. I like LED lights that produce less heat.
 

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I installed a repo unit and wired the grey to the headlight power thinking the light would only come on when the headlights were on. Problem is that the tach does not work at all unless the lights are on so I changed it back to switched ignition power. Only issue is that the tach light is always on, no big deal.
 
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