my tach doesnt work, 1975 L-48 with Hei, I have the wire plugged in and replaced the distributer cap coil, ect. nothing.

 

Probably the tach circuit board - $40-50 at most suppliers. I have the same project on my docket (some day...)

 

do you know where it's located in the car??

 

Mounted to the back of the tach itself..

 

I had this problem once after I installed a new distributer. On my '77, there is a little selanoid on the intake manifold and a tach wire is supposed to be attaced to it. The wire had fallen off and I had to solder it back on. I don't know if the '75 has the same setup or if this is your problem, but figured I'd let you know just in case.

 

Jonny is refering to the tach filter, may be on firewall. Came undone on mine & I got a new one from Chevy while they were avail. More likely to be a connection problem on 75-77 than board.

 

the only thing I see is the tach wire that plugs directly into the distributer, anyone have a diagram on this setup??

 

On my 80 it is mounted on the intake manifold. There will be a Tan wire coming from your distributor which goes to the filter itself. The filter is a small silver object that resembles a condensor.
Available in a non correct version from www.RodneyDickman.com for $29 or the correct looking version for $49 from Ecklers, Mid America

Kona

 

Hmmm... well, hopefully my problem is at the connections as well - I will check my setup tonight, and I second the original request for a pic or diagram.

Where on the intake manifold is the filter?

Thanks guys!
JCJ

 

Between the carb and the distibutor.

 

If memory serves me the wire from the dash is red goes to a filter mounted on the intake near the distributor then a brown wire goes up to the distributor tach plug.
Gary

 

I have a brown coming from the firewall that I just plugged right into the distributer, I would imagine the other is just a jumper?? either way my tach does NOTHING!!

 

My tach gets buried when the engine comes on. Any quess as to what my problem is? As soon as I go above idle the tach goes all the way over red line.

 

I don't have a diagram, but I just went through all that crap on my '77 and they sound very similar.

First check all the connections all the way up to the tach itself. Also, make sure you don't have a blown fuse (there is a fuse for instruments, not for the tach specifically)

The tach output on the HEI goes through a small white cylinder with a lead out the other side that goes through the firewall (if its stock). This is the tach filter. You can bypass it entirely to see if it makes a difference or not. If it does, replace the filter (eckler's carries them and a cheaper non-original looking version).

If that doesn't help, then it's possible that the board has gone (and that is inside the tach assembly). I replaced mine, but the replacement board didn't work either thanks to pretty much every solder joint being junk. After touching all connections up, success!

There is a small potentiometer on the board that adjusts the "red-line" (although my experience, it acts like a signal gain). Start with it in the default location and tweak from there (you may need another tach read out to compare to).

Hopefully that'll help you with your research -- as I beat my head over this one for some time.


Brian.

 

I have a similar problem, will be interested in the replies

 

I was told the tach is shot. However there seems to be a lot of possible issues beyond it just being no good. Will be intersting if anyone has fixed this type of problem. Thanks all.

 

I'm no expert but I've recently been thru this, from a non working tach, to a faulty filter and so on. Run some searches under "tach" and "filter" and a few other and you'll have LOTS of reading ahead of you. Without writing a book here, there is no way to cover all the possibilities.

However .... if there is no needle movement, I would suggest a bad connection or a loose tach filter wire. Check them, the wires fell off in my hands when I removed the filter.

Still nothing? probably a dead board. The complete dash has to come out to get the tach out, then you can replace the board.

Test the operation of the repaired unit prior to re-install (don't ask ) Some replacement boards may have one or more adjustable resistors so you can "trim" the tach.

Have fun and work on your flexibility.....

 

The overvoltage protection on my 76 Tach was a 1W zener diode which is pretty poor. It was blown on mine, and I could see this being an issue in lots of cars. I replaced it with an automotive voltage regulator (National LM2940CS).

 

Where is this zener diode located? Is this part of the tach board itself or is it inserted between the tach signal line and the tach board?

 

Picture of PCB, with diode highlighted found here:
members.shaw.ca/gvn/

gvn