MORE>>>>> 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! MORE>>>>>>>> So, here’s what I’ve learned about OEM tachometers and HEI distributors. Spoke to redline gauges in California about it and he said the tach might work a bit, but will fail from too high amplitude on the tach terminal. It’s designed for a points distributor only and the tach, OEM or OER reproduction will fail if used like this, but they do offer the service to modify it to work properly for all ignition systems. Instrument specialties also offers this service. Hope this helps with our inaccurate and dead OEM tachometers with electronic ignition systems. All this came from the Team Chevelle site. This article is all about the GM HEI but doesn't touch on the tachs. HEI facts