Help! This Tachometer's Driving Me Crazy!

[yello80vette]

OK. Here it is. So frustrated. The tachometer on my 1980 stopped working a few years back right after I had a new speedometer cable installed.
I've done all the normal checks, repairs, tests, etc. But the thing still won't work. Yea, the circuit board is in great condition. Here's a possibility, since I've tried everything else I can. There has been a small black wire, which I'm guessing is a ground wire hanging loose under the car. I'm almost positive it's connected to or near the transmission. I'm thinking that when the guy was underneath installing the new speedometer cable, he knocked this wire loose. And I'm thinking it's a ground wire for the tachometer but I don't have any information that, if this is a ground wire for the tach, where the other end attaches. When I start the engine or accelerate, the tach needle surges downward so it's reading some kind of a charge. Anyone, anywhere, any idea's? Heeeeeelp!
:confused

 

[bill81vette]

Have you tried replacing the printed circuit board on the back of the dash?????

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[6880 Mike]

Do you have the assembly instruction manual (AIM)? If the black is a system ground, the AIM will show where it was supposeed to have been connected by workers on the assembly line.

 

[yello80vette]

The circuit board is fine.

 

[bill81vette]

The only ground I can find from looking at my AIM (for my '81) is in the engine comp below the the master cyl and steering column...there should be 2 wires as well as a ground strap from the frame in that location....this looks like the ONLY ground I can find any where near the instrument panel.....

 

[yello80vette]

I'll spend some time looking it over. Thanks!

 

[yello80vette]

I'll crawl in there tomorrow and have a look. Thanks for the suggestion.

 

[bill81vette]

I looked at mine today....the front of the verge wad up on jack stands and I did notice a black wire attached to the ground.....

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[starrfire6]

Been there

My tach stopped working, and after replacing the circuit board with no results I found the real problem. The tachometer coil itself had failed. (wire corroded away) I found a used tach with a damaged face and swaped the coils. I replaced the two rivets with small black screws I got at the local hardware store. Now it's working good as new.

The only ground for the tach should be directly to the tach head. You can check that by pulling the head out of the dash and testing the contacts that it plugs into. You should have 12v on one, ground on one, and the ignition coil pulse from the third. If you take the circuit board off the tach head you can check the tach coil with a continuity tester across the two terminals. You can also test continuity from the ignition coil to the dash printed circuit board. If you don't have continuity there first check the plug into the guage pack. It may have gotten disturbed when the spedo was repaired. The plug dosn't have the best connections on it.

I hope this helps

 

[yello80vette]

Great info. Thanks!