tach problem with 75

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Jan 22, 2014 | 01:45 PM

75vetteman

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From: some hole in the ground town- camp verde:) arizona

tach problem with 75

when you start the car the tach needle spins all the way around then rests facing straight downward. while driving it will sometimes move back and forth about half an inch. was hoping for a cheaper solution than replacing the tach circuit board. i've heard of a filter but have no idea where it is, if i even have one, and what it looks like.

Jan 22, 2014 | 03:18 PM

dochorsepower

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From: Laguna Niguel California

Many years ago I had a similar problem. I removed the tach and discovered that one of the little tootsie rolls on the circuit board had smoked. I soldered in a new replacement and it has worked ever since. I think it is a common problem.

Jan 22, 2014 | 04:20 PM

Case75

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The tach filter is on the intake manifold near the distributor. You can bypass it by unplugging it from the distributor and plugging the wire from the tach (that comes through the firewall) directly into the distributor briefly to see if that is the problem. Most likely the board is gone. I replaced mine last year with one from Wilcox. Problem solved.

Jan 22, 2014 | 04:43 PM

STODD

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From: Charlotte NC

I am updating to digital to match my speedo on my 75, I may sell mine, it was smooth but off some, but I have no tach filter. Removed it rests at 1500 rpm though.

Jan 22, 2014 | 04:50 PM

75vetteman

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From: some hole in the ground town- camp verde:) arizona

thanks. apparently neither of my 75s have filters. both cars have a wire coming through the firewall going straight into the distributor. aftermarket intake manifolds on both too. also niether has a functioning tach. is it a problem to not have a filter? what does it do? what years had filters?
guess i'm going with the general consensus of circuit board...

Jan 22, 2014 | 05:34 PM

Case75

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Quote:

Originally Posted by 75vetteman

In 1975 they went with the electronic tach to coincide with the switch to the HEI distributor. From '75 on they have a tach filter grounded to the intake manifold. It's purpose (correct me if I'm wrong) is to filter out electrical noise that would make the tach movement more erratic. Prior to '75 all tachs were cable driven off the distributor shaft.

Jan 22, 2014 | 05:44 PM

75vetteman

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From: some hole in the ground town- camp verde:) arizona

Quote:

Originally Posted by STODD

I am updating to digital to match my speedo on my 75, I may sell mine, it was smooth but off some, but I have no tach filter. Removed it rests at 1500 rpm though.

i'm going to order a new one in the next couple days.