1978 Corvette Tach Failed

[Brian F.]

I have a 1978 pace car with the L-82 engine. Anyone know a way to troubleshoot a failed tach? The needle is frozen, but I can get some movement usually during engine start, but it just freezes to a new value.

Should I replace the tach circuit board? Or, could it be the tach filter that is mounted on the intake manifold?

[Duke W.]

Re: 1978 Corvette Tach Failed

I think the tach filter cap is in series, so try bypassing it. If it's in parallel disconnect it.

Check to see whether a test tach works off the test tach pin on the HEI.

If the above actions don't alter tach behavior, remove it and take it to an AC Speedometer Service Station that has experience with this vintage GM electric tachs.

Duke

[Stuart F.]

Re: 1978 Corvette Tach Failed

Brian;

If you want to trouble shoot your problem, instead of throw new parts at it, I believe I have all the necessary equipment to do as Duke suggests.

Stu

p.s. Did taking one of those dashes apart again scare you off?

[Tom R.]

Re: 1978 Corvette Tach Failed

Brian:

I'm doing some trouble shooting on my tach for the Pace Car...mine simple fails to register at all and concluded its tach board. I've already thrown the condenser at it. Here's some background info done by CM

http://www.corvettemagazine.com/content/view/52/

[Ken K.]

Re: 1978 Corvette Tach Failed

This is my very first visit/post on the forum. I also have a 78 (Red). I've owned it since new and it currently has 16,700 miles on it. My tach quit working several years ago. What would be the first thing I could check before taking it to a GM Corvette mechanic?

[Jim T.]

Re: 1978 Corvette Tach Failed

Ken the first place I would check is the connection of the tack to the distributor cap. Don't know if you work on your car or you take it to a shop for everthing encluding oil/filter changes. Someone could of forgot to connect the tach wire to the distributor cap. On the drivers side of the cap there are two connections, one closest to firewall is for 12 volts of power to the coil and the other connection is for tachometer.

[Stuart F.]

Re: 1978 Corvette Tach Failed

Ken;

It was my son Brian that began this thread when he had a tach problem on his 78 P.C..

He just did a turn to P.R. this morning (pilot) and is probably catching some ZZZZZ's. I will talk to him later and bring your post to his attention. I'd try to tell you what he did, but it is better if he explains. I know he put in a new circuit board, but there was something to do about their calibration. He tried new ones w/o success, then got a clibrated one off E-Bay - but again, it's best he repost to explain.

Stu Fox

[Joe L.]

Re: 1978 Corvette Tach Failed

All-----


The most common problem with inoperative electronic tachs in Corvettes (1975-82) is the tach filter. This is a "condenser-like" device which is mounted with a small screw to the intake manifold in front of the distributor. Check it for a broken lead which is the usual failure mode.

The tach filter is long-since GM-discontinued. However, it's available in reproduction.

[Brian F.]

Re: 1978 Corvette Tach Failed

Ken,

If you decide on replacing your circuit board, I suggest avoiding mail order places (Chinese boards). The ones I received from there did not work because they are not callibrated correctly. You don't have to be a mechanic to replace the circuit board.

I purchased a tach board from a dealer on eBay for $55.95. He gauranteed his to be callibrated for the 1978 Corvette.

Here's his description and contact information:

Corvette, 1975 to 1982 New Tachometer Circuit Boards,These boards are calibrated to read correctly for the year of your Corvette,it comes with instructions and 1 year warranty.Must specify the year of your Corvette if the winning bidder. ****Note,This is my product, and my words ,all the information below is from my own knowledge, testing, experiences, a 35 year love affair with Corvettes., and not copied or taken from any one else.**** NOTE, I STAND BEHIND MY PRODUCT, THEY WORK GREAT, AND, NO FINE PRINT, LIKE NO RETURN ON ELECTRONIC PARTS, IF YOU HAVE A PROBLEM, I'LL TAKE CARE OF IT. JTG

*** TACHOMETER NOT WORKING ON YOUR 1975 TO 1982 CORVETTE ?.HERE ARE SOME HELPFULL INSTRUCTIONS ON HOW TO CHECK AND FIX YOUR TACH PROBLEMS
..
1. The tachometer circuit board is the most likely problem why the tach is not working, or working correctly, but not always. On 1975-76 Corvette tach the printed circuit board usually show a burned spot, actually melts the thin copper wire and the problem is obvious. 1977 to 1982 circuit boards don't , and you can't tell by looking at it.
2. Checks to do to see if the tachometer is working for 1975 to 1982 Corvettes, or if 12 volts or ground are missing, or the signal from the distributor is missing. The tach needle usually stays about 1000 to 1500 RPMs ,when the vehicle is off, this is normal .Turn the key to the on position engine not running, the needle should go to zero. If the needle goes to zero this indicates the tach is working and 12 volts and ground are present. If the needle does not move,either the circuit board is bad or 12 volts or ground are missing to the tach. Remove the tach from the dash and apply 12 volts and ground, tach needle should go to zero if working. On 1978 to 1982 Tachs, the Speedo and Tach housing's main wire connection causes problems and should be checked. Now , start the engine, if the tach does not move off zero, the signal from the distributor is missing, the likely problem is the tach filter, follow the wire from the distributor cap, you can't miss it, about the size of your thumb. Use a wire and bypass the tach filter all together.(Remove , unclip both ends completely) If the tach works now ,that your problem.The Tach filter also, if not working correctly makes the tach work erratically, jumps around, etc. bypass it if you think it's bad. On a lot of 1978 to 82 Corvette the problem is the tach is reading too high, 1000 to 2000 RPM's or more too high, Circuit board is bad, you need a new board. Also on 1978-82, sometimes the needle pegs past 7000 RPMs and stays there. It stay there because it can't get back to zero because it's bottoming out .You need to move it back to zero by hand, take the lens off and move it back. If the needle pegs to the bottom again either on the zero side or 7000 side, then the board is bad.
3.All these instructions assume we are dealing with a factory tach and wiring etc, that has not been messed with or replaced, etc. NOTE, if the needle is removed for any reason, you can NOT just put it back on at any place and expect it to read right.You must rezero the tach needle. To rezero the needle 12 volts and ground must be applied to the tach, then the needle must be set on so it reads zero. Also REZEROING is NOT CALIBRATION . Also, switching circuit boards, 1978-79 only, 1980-82 only. 1980 to 82 tachs have a hole for the choke light, 1978-79 don't have this hole.1978-79 tachs Calibrate differently from 1980 to 82. Another problem in warmer climates, the needle has a dampening effect, there is oil inside the meter head, if this dry's up, the needle swings freely and causes it not to settle down , it fluctuate a lot .A new circuit board will not fix this problem. Also,on 1975 to 1982 Corvette the tachometer needle is different from the speedometer needle . You CAN NOT switch them .

4. . *** Note, the original G.M., Delco circuit board sold over the counter were not calibrated, so buying a new old stock (N.O.S. )board will not make your tach work. Also make sure the circuit board you plan to buy is Calibrated to read correctly for the year of your corvette. Or don't buy it.

J.GARDNER ,1975-82 CORVETTE TACHS, BOTTOM LINE MY CIRCUIT BOARDS WORK AND READ CORRECTLY. HAVE TACH QUESTIONS,WANT ANSWER ?? I HAVE THE CORRECT ANSWERS . Email, Jtandjudyg@aol.com 1- 610-797-4048

[Stuart F.]

Re: 1978 Corvette Tach Failed

Brian;

Don't know how you added the picture, but it can't be blown up larger in order to view.

Stu Fox