Tach Troubles

I have a 75 with the electronic tach, after doing a pile of work on the car including carpets, seats, molding etc., the tach naturally decided to crap out. With the key off it reads 500 RPM, with the key on it goes to zero so it obviously an electronic/electrical problem - well maybe not so obvious - I just can't think of anything elese it would be. Anyone had the same symptoms and fixed it - where to look ?

Thanks !

75 Coupe

Does the tach function with the engine running?
Most GM cars with electronic ignition w/tach will not go to zero with the key off. It will drift up a few hundred RPM's. But when you turn the key to on, it should read zero. My '81 does the same thing. It's normal. As long as it gives you RPM readings as the engine speeds up or down smoothly, nothing to sweat about.
Sarge

I am having the same problem. Also self-induced, I suspect, when I pulled the instrument cluster back.

I have checked all the terminals in the back of the cluster and all are intact. I also get the jump on start, but no functional use out of the tach. It is frozen at around 1500 with the motor running.

I was wondering if I fried the circuit board and have contemplated replacing that, but it is a $60 experiment.

Let us know what the solution is. I will do the same if I can ever nail it down.

R,
David

mine does the same thing no problem works great,that what a electronic tach is suppost to do<,I put a new after market tack in my truck and it does the same thing instructions even stated it should do this .
are you sure it dident allways do that and u just noticed since u worked on it

75 stringray

I gotta 75 also,,tach does same thing..no problem

david my 76 goes to 1200 with the key off you may have a bad filter jim

Jim:

Thanks for the tip. That sounds a whole lot easier than replacing the circuit board. Any idea where I can find one? I just checked Zip and Bair's. Neither one have them listed in the catalog.

R,
David

Problem Solved ! - Well Sort of

I did a little more looking and found that while there was indeed a wire attached to the 'Tach' connector on the distributor, it only went to a filter capacitor - then I found the other end ! I suspect it attaches to the other end of the filter cap - but I could not detect any residual wire ends on the cap - so I just attached it directly to the tach connector and the tach is now working. Before this occured I noticed the tach was junping at low RPM. In retrospect this is what I would have expected as the wire was obviously fraying and the engine vibration was at its worst. Now it is smooth as silk and Yes, it does read between 500 - 1K when the engine is off, and returns immediately to zero when the power is switched on. Can someone with a 75 confirm wether or not it is supposed to go through the filter cap or not ?

Thanks all !

Do you know if a 1979 Vette does have to have a filter hooked up ?? Mine hasn't worked, or I should say my Wife's, since I bought it for her about 7 or 8 years ago. Haven't got a clue as to what to do to it.

She just absolutly loves the car as it is, but I'd like to fix it if at all possible.

It does however have a filter attached.

Thank You for your time in this matter.

My 75 rides up to 400-500 RPM when you turn the key off. I was hot-doggin' it once coz there was a chicky in the car and I revved it up to 3000ish just before shutting it off and the tach dropped to 0 and stayed there while the car was off. However, I'm almost certain its not good to rev the car and then shut it off

Scot