jpatrick636

My tach has worked fine in the past but it quit, indicating a bad filter. (Kinda went spaztic)
I have been told that 75 tachs must have a filter and they are located in the engine compartment.
When I put this motor in, there was no tach filter.
So I assume it was put under the dash some time in the past. However, when I pulled the dash last winter I did not see one. That doesn't mean it's not there, I was not looking for it at the time.
So here is the big question: can I put a new filter in series with the old (mystery) filter so I won't have to tear out the dash until next winter? When a tach filter fails, does it become just another strand of wire i.e. passing whatever signal it gets right through it?

vette75406

I too have a 75 with no tach filter , my assembly manual shows it bolted to the intake , my tach works fine though , im going to replace it when i find one should not be hard to find one several mid 70-80 had the same filter .

wombvette

It will work without the filter, but the filter is desirable to handle any voltage spikes etc. that could cause a problem. My guess is that yours was just removed sometime ago. They are harder to find than you would think, but you can fabricate one if you want.

vette75406

I've had my car since 1988 its never had one since I owned it , its always worked maybe im just lucky .

Schmucker

They normally have a plug that goes to the distributor and another that goes to the tach, and then the body of the filter is bolted to the intake and that's used as a ground. My tach bounced at about 2600rpm+. While working on the car for the past year and a half I found the filter and found that it was unbolted from the intake. I bolted it back down and when I finally got the car to where it could be started the tach moved smooth. There is a diagram somewhere on making your own. Some resisters and such.

80TexasC3

I agree with the others... my unit is still installed (given on an 80) but the basic design has not changed... bolted via a bracket to the manifold then wire runs to the TACH port on the distributor.

ACECO

Patrick,

If you don't need the factory style for correct restoration purposes, just build one from a couple caps and resistors. Here's the diagram for reference.




I put together one for my 75 and it works fine. I also built a couple extras just because I had the parts lying around here. If you send me your address, I'll put one in the mail for you if you'd like.

Good luck either way.

jpatrick636

Thank you for your responses.

ACECO, I will IM you with my address.

Corvette Central has them but they are 50 bucks.

If it will work without a filter, why do think mine went spaz? I had the tach checked and it's OK. I have checked all the grounds and the coil. If sonething else in the dash is feeding back to the tach, will the filter handle that?

ACECO

The ones sold for $50 are in a nice little "can" with a mounting tab for ground and two studs/nuts at each end for attaching the wires. The ones I built are on a piece of circuit board with heatshrink around them and three wires sticking out which have to be attached into the harness. Very simple, just an in from the distributor, an out to tach and a ground wire.

Not sure why your tach is not responding beyond something is "open". (assuming the tach is OK as indicated) Either the harness/wires are open/not connected or the filter is open. If there is a filter and it is open, then just adding the replacement will not help. Based upon the circuit and assuming you do have a filter somewhere in line, then I'd say it was highly unlikely that if it were to fail, it would fail shorted. (in other words now acting like a piece of wire). Most likely it is open so you will have to find the old one and replace it to get the tach to work again. If you do find the old one, first try just connecting the two wires together to see if the tach starts working. It should if the issue is a open filter.

Of course if you really don't have one anywhere inline, then the only thing beyond the tach would be something in the wiring harness or at the distributor itself.

I will put the package in the mail on Monday. You should see it later in the week. Let me know how you make out.

Have A Great Weekend!

jpatrick636

With the tach connected, it works but it intermittently spikes up 500-1000 rpm. Then last week, while idling, it went to 7000. I disconnected the tach wire at the coil. Now it either stays on zero or runs up to 1000-2000 with the wire disconnected??????

ACECO

Well unlike a mechanical tach, these electronic ones will stay wherever they are at when the signal is removed. This is why when all is working normally and you turn the car off the tach stays where it is at. As soon as you turn the key and apply power, even before the engine cranks, the tach jumps back to zero.

Sounds like perhaps the tach is OK but the "jumping" is misc spikes occuring which is what the tach filter is supposed to filter out. I'd leave it unhooked so as not to damage the tach circuit board until the filter I'm sending you arrives then install it inline and see if the "spiking" goes away.

As pervisouly indicated, the filter will ship on Monday morning. You should see it either Wed or Thur.

jpatrick636

THANK YOU AGAIN

ACECO

Your welcome but save that thought until you let me know it works OK.

Have A Great Week!

OkeeGuy

when my tach started spiking it turned out to be a bad modulator in the distributor...

jpatrick636

SIGNGUY, Good point. What the heck is a modulator?

PS. Your ride is looking good!


[Modified by jpatrick636, 4:32 PM 6/28/2004]