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My tach still isn't working. My first attempt was replacing the board on the original tach. That didn't fix it. I bought a new tach and installed it. When I turn the switch on the needle moves to zero but when I start the engine it drops slightly below zero so I have some needle movement but no RPM reading. When I disconnect the power, the needle eventually moves to like 300. I pulled the tach filter and checked the resistance. It's reading 17.75 ohms, which I've read is correct. I checked the coil connector so it's in place. I just want the tach working before I install the Ididit relay box and put the new steering column in place.
I found the following post and reply from 2015:
"Ping the wire from the tach connector to the bulkhead and see if you can find it. This should be an easy fix if you can find the wire and testing it on the tone setting is the fastest way. You can run a direct wire for testing! It won't hurt a thing.
I found the problem. Pulled the bulkhead connector and did a continuity check from the tach to the receiver pin and found out the tach wire from distributor was correct but was not making contact with receiver pin on fuse panel side. Reinstalled bulkhead connector and ran continuity check again and solved problem."
My brown wire from the filter runs behind the wiper motor and appears to go into the firewall there. These quotes reference going through the bulkhead connection. That could be where my problem is. I’m trying to find a wiring diagram or pictures that show it. Any help on this will be greatly appreciated!
Tach hookup is actually pretty basic.
these pic's are from the 77 electrical trouble shooting manual.
as you can see. It's just a 3 wire hookup. Power, ground and the signal wire from the distributor.
First thing I would do is unhook the wires from the tach. Connect a ohm meter to the brown wire at the tach board and the other end to the plug that connects to the distributor. Good connection? Now test to a good ground. Should be open. If you don't have a good connection from end to end, very low ohms, or if the wire is shorted to ground, run a new wire. You can run the tach without the filter. It'll work just fine. The filter takes some of the bounce out that electric tachs are noted for.
The other thing to check is that your polarity is correct, Power and ground.
I must have gotten lucky. I replaced mine early 2 thousands. (approximately 22 years ago). I just ordered one from one of the Corvette vendors and it's working fine to this day.
This being a new tach, I really expected it to fix the problem. I've hooked up plenty of aftermarket tachs but I'm trying to keep this car as original as possible, just because it' s numbers matching.
Anyway, I was hoping to avoid pulling the tach back out but it seems inevitable. I still have the other new tach board but I'll have to find the paperwork to see where I bought it from. As soon as I get the dash disassembled again, I'll do the electrical testing as you described and get it figured out. I'll update you when I get that done. I sincerely appreciate your input!
I finally got a chance to take the dash back apart and played with the new tach. This tach has an extra post (compared to my original) labeled "S-" that my original tach didn't have. I cut that post off to add the board that I had bought from Cajun. I installed that board to the new tach, started the engine, it went to 3000 RPMs and ran backward when I hit the throttle. I wonder if it would work if I replaced the board that came on the new tach and soldered a ground wire and grounded it to the block...It's odd that the new tach shown on Top Flight has the same 4 post board as I have on my new tach but the replacement board they sell only has 3 post holes like the one I have from Cajun...I'm kornfuzed...
Last edited by JerRollin; Mar 13, 2024 at 10:09 PM.
I'm working with a new board and a new tach that I bought after the new board didn't work on the original tach. The board only mounts to the tach one way and the wires are run into a three prong, plug in connector. It's pretty basic, not like there are three separate wires to mount on the post, so not much way to screw it up. After what I read on the threads below, I'm ordering a new 3 wire tach pigtail connector.
Last edited by JerRollin; Mar 13, 2024 at 10:58 PM.
I finally got it!!! I began by testing continuity from the tach connector to the distributor connection and it had .1 ohms resistance, as it should with the radio filter. I tried the original tach with the new board again; it's completely dead. I tried the new tach drilled again, turning the board around. I turned the switch, it went to 4,000;started the engine and the needle went higher instead of backward. I wanted to know if the 4th post, "S-" terminal, needed to be grounded. I drilled a 1/16 hole in the stud that I cut off, then drilled it 5/64. I cut a length of black wire and stripped it, put it in the hole that drilled and filled the hole with solder up to the solder on the board. I connected it, turned the switch to on, the needle went to zero. I started the engine and the needle went to 1000 at idle with the choke on. I hit the throttle and it's working perfectly. I cut the extra wire, taped the end, put the speedo and tach panel back in place and started the engine again, just to be sure that it's still right.
Still say, S post should be signal. If that post needs a ground, should be labelled appropriately. Or at the very least the supplier of your tach should have notified you in some form of instruction. This is NOT normal! How in the world you came to the conclusion that a post labelled S should be grounded is beyond me.
Still say, S post should be signal. If that post needs a ground, should be labelled appropriately. Or at the very least the supplier of your tach should have notified you in some form of instruction. This is NOT normal! How in the world you came to the conclusion that a post labelled S should be grounded is beyond me.
I'm sorry. You misunderstood. The "S" terminal that I soldered was a 4th terminal that the original tach didn't have. It was labeled "S minus" whereas the terminal for the brown wire to the distributor is labeled "S Plus". That was my confusion with the new tach - the extra post. my brown wire runs to the "S Pllus" terminal and to the distributor as it did from the factory. It's all good and back in place now. Ready for the steering column. I tried connecting the Ididit column and it didit not do anything. I'm tired of messing with it and going to rebuild the original. I sincerely appreciate you all for your input on my tach situation!!!
1976 Tach issue
