toddlaj

Does anyone know the correct part number for the tach circuit board for a 76 Automatic. My original burned up and since then I have ordered two (I burned up the first one because I forgot one of the plastic washers) from Ecklers which didn't look like the original, say my idle is around 3000 RPM, and don't even fit into the tach due to a very large capacitor soldered onto the board. Unfortunatly I tossed the original after it burned up. Also the original said the idle was below 1000 RPM.

original76

I have a '76 auto that I bought new so I know it is original. Any idea where I'd find a part number and is it something that I can get to without tearing out the dash?

toddlaj

Thanks for replying. You would have to tear out the front dash, disassemble the tach, and it can be a real pain in the butt. Thanks anyway though.
Todd

original76

That is what I thought. I completely redid my interior two years ago with new dash pad. Thought maybe you had an easy way to get numbers. Sorry I can't help.

toddlaj

I would like to thank everyone for their advice. My tach starts out at 0 then goes to around 3000RPM once the engine is started and climbs from there with acceleration. I adjusted the variable resistor to try to bring it down but discovered it is already at it's lowest setting. It looks like it works overall just not calibrated to zero.

John, I'll be contacting you to find out more about your CCA, but I'm pulling two jobs right now so I'm about to run out the door. One job for the wife, one job for my 76 mistress. Thanks again.

jlpb1954

I came across one of your postings and need to know if you have a prosess or steps to trouble shooting tachs while still in dash?
Thanks
John

NORMAND

hey guys new to this form thought id seek out some info on this as i was readin the previous posts.... im having the same issues... im just wondering so i dont burn out another tach wich wires are the correct ones to hook up to the 3 on the plug. i have running into the car from the distributor.. a pink wire.. 3 wires i have on the plug are pink brown and black... first i assumes black was ground an that pink was ur signal wire, and brown was switched is this correct or no?

NORMAND

anyone?

Red 69

Hey-Norm, I wish I could help you, but my 79 must use different color wires. My tach only has one white wire going to the tach filter, white-to-white at the connector. The other side of the tach filter is brown and goes to the neg side of the coil (HEI) Dist. The pos side of the coil is red and goes to battery.

My tach died suddenly, and in solving the problem, I have replaced the tach filter and circuit board twice. Non of these changes have solved my problem. I'm starting to think my original circuit board was OK, but I replaced it first, then the filter. I was assuming aftermarket circuit boards were all good, but with what John said, maybe reinstalling my original CB will fix it. Ecklers said they would take my circuit board back, but if that isn't the problem, the tack rebuild is $200. I'll buy an Auto Meter Tach before I do that. I guess that's why so many guys have gone to aftermarket gauges in their C-3.

Thanks for taking the time to write the trouble shooting instructions, John. I printed it out, and when I get a chance I will go back to the tach and see if I can finally get it working properly.

NORMAND

tach filter? duno what or where that is... and colors dont matter, just need to know if its the same 3 prong plug might be able to tell me what wires go where..

Red 69

My car does not have a three prong plug to the tach in the engine bay, only the one white to white wire connector. That one white wire goes to the tach. The tach filter is a silver cylinderical thing with a wire out either end. It is mounted with one bolt to the intake manifold on the driverside and to the left of the distributor. If you look at your distributor and see the coil on the cap. The front wire clipped to the negative side goes to the tach filter, the wire is brown on my 79. When you find the filter, the wire on the other end goes to the tach and is white on my 79. It has a connector between it and the white tach wire on my car. The red wire I mention has nothing to do with the tach and goes to the pos side of the coil and battery. I hope this helps, if there is anything else I can provide just ask. I've had my tach out a few times replacing the circuit board, changing calibration and moving the needle orientation, and I'm still no better off. I printed John Gardner's instructions and will try again when I get a chance. Good Luck!!

NORMAND

not sure that my car has one of those... then again ur setup is differnt them mine too...am yet to figure this out will let u know if and when i do lol thanks.. cheers!

hunt4cleanair

John...we certainly appreciate your contribution and expertise. I just replaced the circuit board in my 78 and once I worked through an electrical glitch, the tach fired up smartly and finally settled in at 900 (4 speed set at 900). If I had known about your boards, I would have jumped on it but suspect I settled for a China made version. But some followup questions:

Can you rebuild original boards? My original shows the burn spot.

Were original tach boards, well did they come with washers on the backside to provide insulation from the unit?

This is a bench test:

Good to know

What should be used than?

Finally a question for others...the redo dash in 78 permitted access to the tach through the lens cluster which makes it quite easy to replace the tach board. So for owners with model years 75-77...how much of a bear is it to replace the board? I'm assuming you've got to pull dash, drop steering column?

Kilroy1024

Talk about a thread coming back from the dead...

Pink +12V
Black Gnd
Brown Tach Signal

Yes, actually you can. I have recently discovered that the chips for the 76 Tach board are still available. It's an LM3900.

This is an in car test using a modified A.C. adapter:

NORMAND

very good thank you much.. will see if i can get around to that today need to get that done lol

NORMAND

just a question about these tachs... got mine to work thanks guys.. but just wondering, why do they fluctuate so much is it cuz there old school as u guys would say ? an i mean fluctuate to as how there not so steady as to when u rev it up they kinda flop an get all wacky when u rev it, but is ok if u do a steady rev up to a certain rpm.

Red 69

I now have a working tach, but the solution took a different direction. A fellow member sold me his L48 tach with original circuit board for $30. It worked perfectly, but isn't the correct replacement for my L-82 model. By using the same CB with the tach, it didn't need to be recaliberated. This was several months ago and I have since seen a few used working tachs at similar prices. It would be wise to get the original circuit board along with the tach to make life a little easier. Don't toss your original curcuit board until you are positive it is your problem. Ecklers accepted return of the unused circuit board I purchased from them. The board and filter are expensive, throw in a rebuilt $200 tach and a new AutoMeter tach looks mighty good. Used parts was my solution and stayed on budget. Good luck!

0Willcox Corvette

Readytogo!

I have a ton of info on the tachometers on our web site at this link.


I also have a video on Youtube on how to install the tach board in the tach.


We also have the tach boards on sale in the forum special section of the site too.
http://willcoxcorvette.com/advanced_...s=tach%20board


975051F 1975-1977 Tachometer Tach Printed Circuit Repair Board 75-77 $58.00

Here are some newer threads that may help you too!

http://forums.corvetteforum.com/c3-g...lp-please.html
http://forums.corvetteforum.com/c3-t...eter-help.html
http://forums.corvetteforum.com/c3-t...ease-help.html
http://forums.corvetteforum.com/c3-t...achometer.html


Willcox

readytogo

thanks. I did a double take on the size of the original board in the video but size doesn't matter. Bigger doesn't mean better either. That's good but you only implied a circuit board would solve my problem. From all I have read and my unique symptoms I might conclude that I have power; I have a ground; and I have some movement of the needle while cranking so it must be the circuit board. I have bypassed the filter and have same symptoms. Can you say conclusively I need to replace the circuit board? I don't own the franken device. Is it absolutely mandatory to check since your boards are calibrated before shipment? Really it would be nice to see the thing work but I seriously doubt I will ever need to know anything about red lining. Close to right would be nice. Will call you later regarding a purchase.

0Willcox Corvette

It sounds like a tach that is missing power, ground or both.

On your car you can reach up behind the tach and pull the wires off. Once you have the wires off test for power, and ground. Use a test light to test the power side and use a different ground other than the one on the tach connector. If you have power then use the one on the tach connector and see if the ground is good.

If you have power and ground then I would say yes, you have a board issue. If you don't have power and ground then go the the fuse.

Here is why I say to check for power and ground first. A gauge will store the last reading it had when you turn off the car. So if you have power and ground to the gauge with the key turned only in the on position the tach will go to zero.

If you don't have power and ground then you need to trouble shoot the system and I've posted the links above on how to do this.

Willcox