Just got my 75 off the stands and on the pavement (drivable). Now with running, I am finding all new problems.

Tachometer works but idles at 5500 rpm. When you rev the engine the needle runs the opposite direction (counterclockwise). I have heard mentioned a filter and or a Circut as possible cures. Has anyone had this issue and what course should I take.

 

I have a 76 and am also have Tach problems. I have been reading the other post and trying to figure mine out. I guess this is a stupid question but where is the tach filter located, is it on the engine next to the dist? I purchased my car used of course and do not know what the previous owner may have taken off, tach was not working when I got the car. Thanks

 

Ive been following these posts for a couple of months.
(My tach didn't work either and I had to fix it.)

There has been some discussion as to the purpose of the filter.
My belief is that the tach filter was put in to reduce the 'noise' entering the cockpit. This noise was probably being picked up by the radio resulting in a whine on the speakers.

It does not offer much additional protection from high voltage spikes. The circuit board on the back of the dash already has a filter on its input which should provide adequate protection against that.

The Tach filter can be replaced with an RC network to see if it has any effect. This can be done easily without paying $30 for a filter. A schematic of the tach filter can be found in some previous posts. I have it somewhere and could send it if needed. Exact values are not that critical, and I think you could achieve the same thing with a 10K ohm resistor followed by a .1uF cap.

However, the symptoms you describe are kind of wierd. I can't imagine how a defective tach filter would make the tach run backwards. A defective filter usually means a wire has vibrated loose and no pulse gets to the instrument panel. That would make it read 0 RPM. My bet is that the board on the back of the instrument panel is blown.
You could have a bad ground, but I think the odds of that are kind of slim.

The tach posts that I have read concerning 76's in the last couple months have all said - "Where's my tach filter?" I'm starting to think that there was no tach filter in 76. It is supposed to be beside the distributor on the passenger side. Its a cylinder that's about 1/2" in diameter and an inch long. The wire from the distributor goes in one end and out the other.

Not working how?
If the tach board is functioning, but the tach signal isn't getting up to the dashboard, then it should go to 0 RPM when the ignition key is turned to run. No signal - 0 RPM

If the tach is pegged at some other value (Say 3000 RPM)
then its probably your board. Watch it carefully, if it moves even little but pegs at some odd value, then its probably the board.

If the tach does nothing (no movement at all!), then it could be power.

Repair? Anything that one man made, another can repair. However, the success rate by those who are not versed in electronics is extremely low. Parts are obsolete and hard to find. Your better off buying a new board.

 

It's right here (arrow) on my 76.

 

Well there you go, a 76 does have a tach filter. (or at least some do)
Great pic that answers a common question, do you mind if I use it?
(I'll put "Pic by Lucky76 on it")

 

The tachometer on these models are electronic, and the way they function is pulses from the distributor are sent to a printed circuit board located on the back of the tach gauge. This circuit board has a single I/C (integrated circuit) chip that converts these pulses to voltage output. The tach gauge is functioning as a volt meter, the higher the voltage, the higher the reading. The most common mode of failure for electronic circuits is to short out. That is what has happened in this case, the shorted i/c is outputing the full ttl voltage which results in full meter deflection ie: "pegged out" the backward deflection you are seeing is caused by increased load on the circuits power (from the shorted chip) causing the voltage to drop which results in a lower tach reading. So to make a much too long story shorter, your printed circuit board needs replaced, usually about $60 bucks and a half hour of your time.
Good Luck

Bill.

 

As always, you're getting some good advice on this forum. I went through this with my 75 last year, so here's a few gotcha's to watch out for. The circuit board is behind the tachometer gauge, and I removed the tach to get to it. When you remove the tach, you'll notice there is an inner bezel that is plastic and blue in color. Treat these bezels gently. Their mounting tabs will break VERY easily and if they do, you're screwed because I had a lot of trouble finding those inner bezels. Also, I bought a circuit board from a reputable vendor that turned out to be crap. There was a post on it just recently so you can probably find it in a search. At the time, after I installed the new circuit board and saw that it did not behave properly, I started suspecting my tach gauge was bad, so I bought a complete tach and circuit board assembly from another vendor. Worked like a charm. For grins and giggles, I changed out the second new circuit board with the first, and that's when I discovered the first replacement board was bad and that I had wasted more than a hundred bucks on a tach gauge! Moral - it does matter where you get your circuit board. If you want to know where I got mine, send me a PM.

 

why cant just post where you goT it????

 

Peer pressure is too much. I got chastized for mentioning a non-supporting vendor in an earlier post. What the heck - Corvette Clocks by Roger is where I got the tach assembly including circuit board that worked perfectly. The faulty circuit board was purchased from Mid-America.

 

Thanks, for all the advise. Looks like a new board will be required. Thank you to Renowa and Razorhorn, you were most enlightening.
Any suggestions of who is the most reliable to purchase from?

 

The time that has been mentioned for this job is a bit short.
Takes about 2-4 hours to take the whole dash apart and change the tach PCB.

DON'T touch the face of the tach as the inks can smear.

 

and worth repeating !

 

Thanks to everyone on my question also. I have a place to start looking this weekend, maybe I will not need anymore help but its good to know it's here if I need it.