Way Outta Control
Join Date: Sep 1999
Location: The Palouse
Thanked 3 Times in 3 Posts
Re: HEI and tach
Well, if no one else will tackle this problem I will give it a try. I just installed a GM HEI two weeks ago with my Howell EFI System.... and it seems to be working fine. So, let's look at the TAC…
A TAC is driven by current. The current is supplied by the pulse from the ignition module. The ignition module differs in VOLTAGE levels from manufacture to manufacture, however the job the module does is the same. In fact, in a pinch, if you know all the connections of each of the OME modules, GM, FOMOCO, and DODGE, you could make up a set of swap connectors and connect a GM ignition module into a FOMOCO system and vice-versa. Things will work just fine.
Now the rub….if the pulse produced by the module is insufficient in width, duration or amplitude, the TAC may not work…
If you want to make sure the OEM TAC works with the GM HEI distributor, run the two modules in parallel. Here's what I mean….
Leave the OEM FOMOCO ignition module in place, connected to the harness, but with the FOMOCO Coil out of the circuit..
Now run a spade lug/wire connector from the GM HEI TAC connector down to the COIL TAC pigtail and connect the two together. Start the Jeep, the OEM TAC should be working.
Now turn off the Jeep and then unplug the OEM FOMOCO ignition module…
I'd bet the tack will still work… however unless you did the Nutter Bypass, the OEM Computer will be brain dead!
Let me know how it works.