1978 tachometer trouble shoot
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Safety Car
Joined: Jun 2004
Posts: 4,371
Likes: 223
From: Beecher Illinois
First thing, check the wire under the hood. There should be a cylinder mounted to the rear of the intake manifold. That's the tach filter. See that the wires coming out of each end are connected, one to the distributer, the other to a wire from the harness at the firewall.
Then, disconnect the connectors from the filter and plug the connector that goes to the filter directly into the left recepticle at the distributer, basically bypassing the filter. If this helps, bad filter. If no change, most likely a bad tach board.
Easy fix on 78-82s. From the front of the dash, remove one screw each side of the clear window. Look at the top, you'll see three sets of two screws. Remove the 3 SHORT screws. (the long ones are the screws that are recessed a little). Remove the clear window.
6 allen screws around the black bezel come out next and then remove the black bezel. 3 screws hold the tach in place. Pull the tach straight out.
The board is mounted on the back . Most of the vendors sell the replacement board, anywhere from $55-$75. I've had good luck with Mid America boards. Others have complained about the quality of replacement boards, tho.
Member ACECO can possibly repair your board if the problem isn't the main chip. But 2 boards I had did have bad chips so it's a pretty good chance with yours, too.
Then, disconnect the connectors from the filter and plug the connector that goes to the filter directly into the left recepticle at the distributer, basically bypassing the filter. If this helps, bad filter. If no change, most likely a bad tach board.
Easy fix on 78-82s. From the front of the dash, remove one screw each side of the clear window. Look at the top, you'll see three sets of two screws. Remove the 3 SHORT screws. (the long ones are the screws that are recessed a little). Remove the clear window.
6 allen screws around the black bezel come out next and then remove the black bezel. 3 screws hold the tach in place. Pull the tach straight out.
The board is mounted on the back . Most of the vendors sell the replacement board, anywhere from $55-$75. I've had good luck with Mid America boards. Others have complained about the quality of replacement boards, tho.
Member ACECO can possibly repair your board if the problem isn't the main chip. But 2 boards I had did have bad chips so it's a pretty good chance with yours, too.
Last edited by KapsSA; Nov 30, 2008 at 02:22 AM.
Safety Car
Joined: Jun 2004
Posts: 4,371
Likes: 223
From: Beecher Illinois
I gotta type faster
Team Owner
Joined: Jul 2006
Posts: 22,182
Likes: 4,178
From: Kannapolis NC
A quick check to see if you have 12 volts and ground is to turn off the ignition. The tach should move to some number (example 800rpms), then when you turn the ignition on, the tach should jump to zero. If it's doing that, but not moving off zero when runninig, it's in the tach board, the tach, or the distributor feed (maybe the filter). If it's not doing that, follow back to find which is missing. It's connected by printed circuit, so it would be easy to lose your voltage or ground if you have a problem with the larger flexible printed circuit behind the tach/speedo. The other two guys gave you good information. Post what you find.
Le Mans Master
Joined: Oct 2005
Posts: 6,247
Likes: 471
From: Glen Allen, VA
I took mine out on my 79 and verified with a column mounted tach that mine was reading wrong. I wired the column tach into the inline wiring that connected to the back of the factory tach and got it to work. I then sent mine to ACECO as suggested above. He does great work and is a stand up guy. He fixed mine for less than the cost of a new tach board.
Thread Starter
5th Gear
Joined: Nov 2008
Posts: 5
Likes: 0
How to test 1978 tachometer and circuit board
OK thanks to all of you I have jumped out the condenser no difference. I now have taken out the tach. Circut board looks good (but what do I know). There are three contact in dash. One at 1;00, 4:00 AND 9:00. I believe the 9:00 to be ground. I get 12v from th other two with key turned on. The tach has 6 post on it. Tach is still on zero out of dash. How do I test the tach and circuit board. Also is ACECO a company, member? How do I contact if need be?
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
The white wire comes from the dizzy, pink/black is hot lead from fuse and the black is ground.
Most all the Vette parts suppliers sell the circuit board. Here is the ZIP one.
http://www.zip-products.com/Zip/prod...55241A6EE519AD
Most all the Vette parts suppliers sell the circuit board. Here is the ZIP one.
http://www.zip-products.com/Zip/prod...55241A6EE519AD
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8th Gear
Joined: Sep 2013
Posts: 8
Likes: 0
76 tachometer
I pulled the circuit board of my tach and there was a broken wire plate so I replaced it. I started the engine and another wire smoked and broke on the board. I have a new circuit board on order but I want to make sure that I am not getting too much amperage from the HEI. Should I be concerned about installing the new tach board? thanks for your help.
Le Mans Master
Joined: Apr 2008
Posts: 9,433
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From: Powassan ON
I pulled the circuit board of my tach and there was a broken wire plate so I replaced it. I started the engine and another wire smoked and broke on the board. I have a new circuit board on order but I want to make sure that I am not getting too much amperage from the HEI. Should I be concerned about installing the new tach board? thanks for your help.
To many unknowns to diagnose the issue.
I'd be testing those wires before swapping in a new board.
Former Vendor
Joined: Aug 2006
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Le Mans Master
Joined: Sep 2002
Posts: 5,236
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From: Myrtle Beach SC
Get the Wilco board! I would not trust the repair of a dated board unless you like the prospect of tearing into your dash...frequently!
Original boards used thinner copper paths that crack with time or burn out while the components, especially the black and white chip that calibrates the tach. This calibration chip is a fixed resistor Duel In-line Package (DIP) ruthenium oxide chip last used in 1996. New boards use a potentiometer that can be adjusted for calibration requirements. Old boards that used the DIP were etched. Once calibration goes...no way to re-calibrate the board!
If you need background on recycling old boards versus investment in a new board, this technical article describes how the old ones were made versus what to look for in a Twenty-First Century style tach board. See the section "Buying a Tach Board: Buyer Beware!"
Tach Boards
Hope this helps!
Original boards used thinner copper paths that crack with time or burn out while the components, especially the black and white chip that calibrates the tach. This calibration chip is a fixed resistor Duel In-line Package (DIP) ruthenium oxide chip last used in 1996. New boards use a potentiometer that can be adjusted for calibration requirements. Old boards that used the DIP were etched. Once calibration goes...no way to re-calibrate the board!
If you need background on recycling old boards versus investment in a new board, this technical article describes how the old ones were made versus what to look for in a Twenty-First Century style tach board. See the section "Buying a Tach Board: Buyer Beware!"
Tach Boards
Hope this helps!
