Sorry to dig up an old topic. After researching pages and pages of this topic I am still not sure what to do. My factory tach reads normal at idle but the higher the revs, the farther from accurate it reads (too high). It it simply plugged into the "Tach" connector on the GM HEI cap. A lot of people report they have done this with no problems. A lot of people say there is some type of resistor conversion wire that needs to be installed (cannot find who sells that). Some others have said it may be a problem with the Control Module in the distributor (factory tach likes constant dwell which it doesn't get with HEI). Just wondering which direction I should go and if anybody has any new info?

I hope you get an answer. My shiftworks repro tach works fine also but if I get on the gas hard the tach needle goes up to about 4500 pretty fast and then drops like a hammer back to the bottom but regains itself if I let off.
I have the HEI also.

Thanks Herbie. I've found a lot of answers but they're all different!

Mike, If I find out anything, I will let you know.
Do the same for me.
Thanks

Mike, If I find out anything, I will let you know.
Do the same for me.
Thanks

Deal!

Anybody??

A few years ago we had a '72 big block Chevelle with the factory tach and an HEI. The tach seemed to work just fine when plugged into the tach terminal on the HEI.

Anybody??

You need to install a tach filter between the tach wire and the distributor. The waves that an HEI distributor puts out cannot be properly read by MOST analog factory tachs. I have heard (but never personally seen) of very few instances where a tach worked without any modification. Anytime someone buys a harness from us that is modified for HEI, we typically inform them that they will need to put a tach filter in line on that wire. Accell and Mallory both make a piece, but most any Camaro or Monte Carlo or other HEI vehicles from the 70's/80's will have a tach filter on them if they are equippped with a factory tach. A junk yard trip is possibly in order. They look like a condenser with wires coming out of each end....1 to the coil/distributor cap, the other to the tach wire. They are a plug and play deal. They show up on ebay pretty regularly and can usually be bought for 35-50 bucks. Good luck!

Donny

Donny,
Are those filters engine specific - 6 vs 8 ? Seems like it wouldn't matter, being it is just a filter, as long as the proper tach and engine match.
Thanks, ZK

I agree. I think MSD makes a tach adapter to use old style tachs too.

Thank you very much guys, I just found/bought an original one off ebay for $19.95! It has the proper Brown Tach connector on one end too and the mounting bracket. Hope it works!:hurray:

My 70 factory tach will not go past 3000 RPM with GM HEI dist.. What is the problem? Will a filter correct this?

Good deal, hope it works, Mike

Update: Installed the GM tach filter today and it took care of the problem - works perfectly now! Thanks again Donny!

Update: Installed the GM tach filter today and it took care of the problem - works perfectly now! Thanks again Donny!

You're welcome! Good deal.

Donny

Sorry to dig this up but would the Classic Instruments filter part# 2752005 work with AAW 67 harness update using HEI? I will be using a repop tach from Ausleys.