This tach stuff gets frustrating and complicated, especially when you don't know which of the many parts isn't working. If you get completely fed up there is always the option of aftermarket guages, so long as you're not an originality-person. For ~200 bucks you can get a modern tachometer and a matching speedo, both of which will be easier to install/make work/troubleshoot. Search the forum for "Autometer" and "Phantom" if you want to see how nice they look in a C3. (a full set of Phantom guages are on my to-do list, as well as my christmas list )

 

I would test the tach out of the car before I went out and paid $50 - $200 to replace it.

Find an electronics guy.
The only part you can't replace is the chip.

Kilroy1024

 

Well I had my board checked and all seems to be OK.He said the only things he couldn't check were the 2 chips on the board.

So I went and bought a tach filter and I just finished installing it.Same thing.The needle goes down a little bit when I start the car and the rpm ain't working.

I was thinking of checking the wiring that was going through the firewall But I think it's a waste of time since the needle moves a little when I turn the key ON.

Should I go ahead and buy a new printed circuit?

What else could be wrong?

 

If you find you need the whole printed circuit (pictured below) I have one I won't be needing. Like I said if I get my dash that far apart I'm putting Autometer guages in anyway. I paid ~75 last year, you can have it for 30 + shipping.

 

My ptinted circuit isn't made like that.This is how it made

 

Okay, your's is just the tach one, mine is the big one that goes in back of the whole speedo/tach cluster.

 

would I have one made like yours in the back of my cluster? or do I only have one for my tach?

I didn't take out the whole cluster,didn't think I had to....

 

Yeah you have one of those too, they also go bad and have to be figured into the Tach-Not-Working equation along with about 20 other things....

...and that's why I have a 30 dollar Sun tach mounted on my steering column

 

Yeah, I too have a bad board. so out goes the old and original and a new Autometer tach and speedo go in.

 

Well I much rather try to make the original one work than having to mount one and drill holes and everything.

I'm still trying to figure out what I'm gonna try next

 

The printed circtuit board in Star79's post looks like a lamp/connector PCB.
Basically, it takes all the instrument panel connections to a single connector. Its cheaper to have one connector than 12 lamp sockets and a tach connector. (4 tach backlights, 4 speedo backlights, L Turn, R Turn, Brake, Hi Beams, tach pwr, gnd, signal) It doesn't look like there are any tach parts on this PCB.

My 76 did not have this, all the lamp wires in the harness were run to individual sockets. Not sure when they cut this in.

If youv'e got the tach out of the car then they way to test is this:

1) Place the completely assembled tach on the bench including coil and pointer.
2) Hook up the power and ground inputs to a 12 volt power source.
(A 12V/1 Amp "wall wart" should do it.)
Make sure your polarity is correct, early tach can blow if backwards.

3) Put the pointer on, pointing at zero.

4) Feed in a 0-12V square wave signal in on the Tach signal input.

200 Hz should rotate to 3000 RPM
400 Hz should rotate to 6000 RPM

 

so really the only thing left to change would be the printed circuit behind my tach needle.Ne need to take out the whole cluster 'cause there would be nothing there

 

If you've successfully tested all of the components on the board, then it begins to look like the Integrated Circuit (chip). The replacement chip is obsolete now, has been for about ten years I understand. I managed to track some down and repair mine, but it took a really long web search. Some companies specialise in obsolete semiconductor components, but will only sell you a thousand at a time!
I would have thought you could canibalise any GM tach board of that vintage, and look for a chip of similar markings - might be the easiest route.

 

the Quick and easy way would be to buy a new board...

So maybe I should pay up and shut up like some would say

 

 

Well I got the whole cluster out of my car in trying to troubleshoot my wires just to make sure they are working properly and check any fuses along the way.In hoping to eliminate other possible problems.

Will post more results after I tested some wires.

Too dark to keep working anyway

 

80_Vette Said:

"so really the only thing left to change would be the printed circuit behind my tach needle. No need to take out the whole cluster 'cause there would be nothing there"

The PCB youv'e already shown is it, the whole tach, if you got that out you don't need to take anything else out. To test it properly on the bench though, you will need a coil and pointer.

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TheoUK said
"I managed to track some down and repair mine, "

I'm curious to know what part number you tracked it down by.
As far as I can tell, the parts are made by National Semiconductor.
However, they appear to be branded with a custom part for GM.
I think it would be pretty hard to find in any catalogue.

The 76 tach used 2 chips, and 2 resistor packs.
The 80 tach used 1 chip and 1 resistor pack.
They are completely different and not interchangeable.

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Another note: If youve got the tach apart DON'T try to clean the face.
The white ink on the face of my tach has deteriorated with age and smudged a little. Blow any dust off with an air compressor.

Kilroy1024

 

Well, it took me ages to find the right part number, and you're right, the number on the original part is some form of in-house thing I would imagine. After lots of digging, I discovered it is an "air-core meter driver" chip, part number LM1819. Here's the data sheet, but good luck trying to find one, as they're obsolete now. I eventually tracked a few down but have used them in my own and other friends tachs. As I said above, I would imagine any GM tach from the time would have this chip on board, so you could easily canibalise one. This was less of an option for me, being in the uk...
I never realised the older design had two IC chips - if that's the case I wonder what they are??

 

DOH!
As soon as I saw that part number, I remembered playing with it about 10 years ago.

Thanks a bundle for the lead.
That schematic looks like the 1980 tach circuit,
I should be able cross reference those parts to the PCB.
I should be able to write up a trouble shooting guide with that info.

Kilroy1024

 

Well I double checked all the wiring and everything is in working order.So my last option is buying a new circuit board Which I did 2 minutes ago.

Now this has better work

Will post results when received and installed

Thanks alot guys for your help so far.It's very appreciated