Tach problems??
 

This is on 72 GMC. Switched to HEI. Tachs wasn't hooked up before so don't know previous if it worked or not.

Tach is reading about 1100 rpm at idle when truck is actually running 550-600 rpm at idle. Then the tach only goes to 3500 rpms. Then it bottoms out at 3500 rpm and wont rev any higher. Then it's at about 1600 rpm. A friend at a shop put it on an oscilloscope. He's says the tachs is just faulty and may be but wish to check here before I get an aftermarket. Truck is all original survivor, we rebuilt motor and tranny and put the HEI in it. We tried a tach filter and no change in tach, still same.

Any ideas?

Re: Tach problems??
 

I could be wrong but i dont think a points tach can work with HEI

Re: Tach problems??
 

Ditto.

Was the truck a 6Cyl. converted to an 8Cyl. or the opposite 8 to 6?

Bill

Re: Tach problems??
 

No it's an original big block 402 truck. Turbo 400

Guess will pull dash and see if it's been changed but don't think so. Even the houndstooth seat is original and in decent shape. Tilt, etc.

Re: Tach problems??
 

You would be wrong. The tach doesn't care if you have a points distributor, HEI distributor or have updated you points to Pertronix or something similar as long as you connect the tach to the correct location. The HEI has a built in tach output plug.

Sounds to me like you either don't have the tach hooked up correctly (it should have a 12v power wire hooked to an ignition switched outlet at the fuse panel and the trigger wire to the TACH output on the HEI cap and the tach body must have a solid source to ground, the pink wire is 12v, the brown wire is the trigger signal wire) or the tach is simply bad. They don't last forever. You can have it repaired with modern electronic guts or replace it with a reproduction.

FYI, my truck has an HEI and the tach works fine. My two Chevelles both have factory tachs and both have Pertronix ignition in place of the points. The old GTO has a factory tach and has had the original points replaced with the Crane XRi. All these tachs work just fine.

Re: Tach problems??
 

Can you recommend someone that repairs them with modern electronics? Thanks

Re: Tach problems??
 

Redline Gaugeworks. Not cheap, but excellent work and fantastic customer service. I mean FANTASTIC customer service.

Their website is not very useful, best to call them.

Re: Tach problems??
 

Thank you much!

Re: Tach problems??
 

Thanks for input will check power wire, i have already checked it is plugged into the tach position on the distributor. So that end is fine. Will check power source wiring next and ground. Will post back, it won't be til I can go pickup truck on Monday. Just don't want to have to buy another tach til I make sure it's tach itself.

We have an aftermarket tach in the 68 gmc with an HEI. It is fine with no filter it works just like it's supposed to. Just can't seem to figure this one out. Dont want to go buy another tach to find out it's screwed up somewhere else.

Re: Tach problems??
 

If you have a known properly working tach you could hook it up to the truck and see what you get. That would tell you if the tach output on the HEI is perhaps at fault.

Re: Tach problems??
 

I have another complete tach dash for another truck I am eventually building. Gonna like you said try that dash. But am only gonna ground dash with a wire somewhere close. Then just unhook tach wires and plug them into new dash as long as it has a good ground I think it should work. That should tell me if it's something in truck or distributor or wiring or tach itself. If it works good have to get tach fixed. If it does same thing its in truck. Should be easy enough. Thanks for idea. We will see if it works.

Re: Tach problems??
 

Let us know what you find out.