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The tach was working until I replaced the light socket that was incorrect. I don't believe I relocated any of the wires
Looking at the back of my 1968 GTO in dash tach wiring. I would like confirmation or correction on my assumptions about their function.
Three terminals.
One upper right post, I believe is the switched 12 volt for the tach and the light.
Below the 12 volt post is what I believe is the ground post. This post has a small woven flat cable running to a dash structure ground.
To the left of the ground post is the post for the tachometer wire. White wire running from this post to the negative post on coil.
Checked the tach wire continuity from the negative post on the coil to the post location above. The test was with ignition on and battery ground cable disconnected. In hind sight I guess all I was checking was the ground circuit.
How do I check that the signal from the coil to the tach?

Thanks for your help.
 

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Doesn't the 1968 Service Manual show the tach and its wiring? If the continuity of the coil wire-to-tach checks out, then really no need to test the signal from the coil if the car runs.
 

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sounds like an aftermarket tach
none of my originals have a braided wire for a ground as the cases are metal and are tied together with factory flat metal ground straps... 2 connections gray for lights ....brown for coil ...
sounds like maybe the inline fuse took a pooh ?....
 

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It's a reproduction, 68 Gto's dash tach only had one connection point for neg coil on the back of the tach and grounded through the housing. The orig lamp sockets had one wire and ground through the housing. The continuity check from neg side of coil connection to tach is sufficient. The repro dash tach I have seen that had 3 connections, was configured upper LEFT is ground, lower left 12V, lower right neg side of coil.... but if your tach isnt laid out this way I wouldnt be moving wires around to match this. If you swap 12V n ground you will likely damage the circuit board. Might look at repro catalogs for a tach that matches your configuration, reach out to then for wiring connection points ..
 
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