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Topic: tach circuit board question

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tach circuit board question (1/8)
9/15/11 10:28pm

daveo76
Standard Member
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Vancouver, WA - USA

Vette(s):
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.

Joined: 8/25/2005
Posts: 869

I've read many times that replacing the tach's circuit board will solve many tach-related issues. Well I have the dash partially apart (enough to get to the speedo cable which I've already replaced) and I'm trying to decide if I should replace the board. Sometimes my tach works perfectly normal, but other times it seems to get "stuck" at a lower RPM than what the engine is running at, or it will jump around. Are these issues indicative of a faulty board or is it just time to rebuild the whole thing?

So if I do need to replace the board, where is the darn thing? Do you get to it from below or above? Do you have to do any soldering to replace it? Thanks guys...

Dave

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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.

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Re: tach circuit board question (2/8)
9/16/11 4:24am

lukesvette

Lifetime Member
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HOWELL, NJ - USA

Vette(s):
1979, Targa Blue (72 Color), Pace Car rear spoiler, L88 hood, Dark blue factory interior, 525HP 406, HD 700R4, 370 gears,Steeroids, composite rear spring, TT IIs wrapped in T/A Radials.

Joined: 5/18/2004
Posts: 6812

On my 79, I was able to remove the lens and bezel from the front (I think I lowered the steering column) and was able to remove a few screws and the tach pulls right out from the front. The tach board is held in place with 3 speed nuts - very easy to replace. The tach then just pushes back into the harness through the dash (also very easy) and reverse the rest of the process. My tach was doing things similar to yours and the new board resolved the problem... Paul

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Re: tach circuit board question (3/8)
9/16/11 5:44pm

rraider1
Limited Member
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Woodland, WA - USA

Vette(s):
1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1171

Take a look at this info You may need to do a zero adjust

http://willcoxcorvette.com/repairandinstallhelp.php?hID=252

Re: tach circuit board question (4/8)
9/16/11 11:13pm

Thanks for the info, guys. I think the 78-82 folks are pretty lucky in this case. The tach does not pull out from the front on the older cars. I took the entire main gauge cluster out tonight - whew! stressful cause you don't want to break anything and the whole time you're wondering if this will ever go back together. Once it's out, though, it doesn't seem like it will be too bad. Bob, that link is interesting and helpful, but it's for '78 - '82. I wonder how much is applicable for the older cars. I think I am going to order the board from that company, though.

Now that I have the left cluster out it seems like it will be pretty easy to get the center one out? I might try to get the clock working and possibly do some stuff to the radio. I know there are bolts on the center bezel that are hidden behind the console, but do you need to get the console out to remove them?

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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.

Re: tach circuit board question (5/8)
9/17/11 4:16am

nosal1
Former Member
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Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.

Joined: 3/21/2006
Posts: 262

Good Morning Dave,Before you go to far may I suggest you check the ground (black wire) on the TACH filter located near the distributor.If the ground is not good the TACH will work when you first start the car then it will fall to the 900-100rpm and stay there or bounce occasionally..Check and be sure the ground wire is connected and giving a good ground to the metal holder of the TACH filter or filters(my Vette has two filters which,one is a back up filter)....Take care and good luck.. Sal C

Re: tach circuit board question (6/8)
9/17/11 9:19am

rraider1
Limited Member
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Woodland, WA - USA

Vette(s):
1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1171

It looks like the willcox tach board has an zero adjustment, the instructions for the Midamerica has you turn the key on and if not at zero remove tach needle and reinstall it. something to look into to make sure

Re: tach circuit board question (7/8)
9/17/11 11:15pm

Bob - yep, I agree that the Wilcox one has a zeroing adjustment. Just hoping it can be calibrated without a running engine and by just applying a voltage to it.

Sal - The gauge cluster is out so I think I'm going to replace the circuit board no matter what. I don't want to have to get that thing out again any time soon. I'll replace the gauge lights while I'm at it. But good advice on the filter. I'll probably do that, too. I think that's the unit that looks like a capacitor, correct?

Thanks to you both.

______________

1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.

Re: tach circuit board question (8/8)
9/18/11 1:29am

Yes Dave, it is the canister that looks like a capacitor with one wire in and one wire out and is attached to a metal bracket which should be grounded..One side is connected to the TACH side of a HEI distributor and the other to the wiring harness..Take care and have a great day....Sal C

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