Im a new member here on the forums. We own two Vette's. A 96 C4, and a 79 C3 that has the Edelbrock top kit and upgraded carb.

I am very mechanically inclined and have worked on hundreds of cars for family and friends since I was a teenager (over 20 years). But, they all have been Fuel injection motors and more upgraded electronics than this C3 we just picked up. The speedo & tack are not working. I am smart enough to realize the speedo is cable driven off a gear in the tranny. No big deal. The tach i understand is electronic driven from the distributor through a tach filter to the dash. Both are not working and I do know that the previous owner had a new distributor put on as well as plugs and wires.

My hunch is that there is some sort of grounding issue from the distributor, but would that also effect the speedo? I assume it would only effect the tach. Before I go and troubleshoot them seperately I just wanted to check with others to make sure there isn't something done in that distributor replacement that would make both the speedo and tach not function. All the other gages work fine (fuel, Oil Pressure, Temp).

The Tack is stuck at 1400 RPMs which I understand is normal for engine off. So I kind of wonder if the Tack is OK which makes me think of a grounding issue.

I did find that the wire that was plugged into the tack port on the distributor was not going through the Tach filter. So I fixed that. The tach filter wire is brown and the wire they had in the distributor's tach port was white. I plugged the white wire into the connector coming out of the tach filter and the other end of the tach filter in the the distributor, but nothing lit up on the tach

Is the wire going to the tack through the firewall indeed white? Is it possible if that wire was bypassing the tach filter that the tach board is now fried? Do you think these are two separate issues with the tach and speedo, or could a grounding issue cause both to not work?

Thanks for the assist.

 

If your 79 has cruise control there is a transducer of some type where the cable from the trans goes in and then the cable to the speedo comes out. Two separate cables so I would look there.

 

The speedo is indeed mechanical. The tach electric.
So these are most certainly 2 separate issues. Just a coincidence that they are side by side.
Tach boards are definitely known to give out. But not because of the filter. The filter is just to reduce flutter.
As per the color of the wire. The trigger wire is brown on a 77. I am not certain about a 79. Perhaps a wiring diagram for your car would be a good investment. Or better still. The genuine GM Electrical Troubleshooting Manual for your year car could be the best investment in your car period.
On my 77, yes it's a white wire at the filter, turns brown at the connector a few inches before it goes into the harness.
Speedo. unhook cable from the back of the speedo, you may need to pull that left side duct out to get to the clip.
Can you spin the speedo inner cable? If yes there is a problem.
Spinning rear wheels, does cable turn? If no. There's a problem.
Another check. leaving cable connected to speedo. Remove cable from transmission. put drill motor on cable wire. Spin inner cable with drill motor. Memory says try reverse. (but I'm old, try both ways). Does speedo move?
Possible causes. Bad plastic gear in trans. Rusted up and or broken cable or cables. broken speedo head.
Most likely issue in bad tach. Bad board.
Good news is. If you have to pull the divers side dash to repair the tach. it's easy to spin the speedo by hand then and check operation.

 

My 78 tach went completely around clockwise while idling and is now parked at the 5 o'clock position. Any idea what happened and how I can fix it? I did not over rev it. It just spun around while idling to warm up.

 

Yup. Sounds like your tach board just bit the dust.

 

Thanks for the info, I ordered a Tach Board which I hope fixes the non functional tach. So many different tach board suppliers out there.