Hello!
I own a 1971 Mustang and in all Fords pre-1974 they use a Current Sensing tach (the tach gets the signal thru the positive side).

Anyways, I installed a HEI and I KNOW that HEI dizzys DO NOT WORK with Current Sensing tachs. So I installed a after market tach and left the factory tach wire unattached (wrapped end in electrical tape).

When I fired up the car to test the HEI I was shocked to see that BOTH tachs (aftermarket and stock) were working fine and reading exactly the same! How could this possibly be? I’m very surprised to see this and may have accidentally figured out how to wire a factory tach to an HEI. However, I have no idea what is causing it to work!

-Here’s how I have it wired-

HEI WIRING
1. Positive wire - keyed ignition on starter solinoid
2. Negative wire (tach) - signal wire on aftermarket tach

AFTER MARKET TACH WIRING
1. Positive wire - fuse for accessories (power windows, defrost, etc)
2.Negative wire - Grounded to bolt underdash
3. Signal wire - Negative side of HEI

FACTORY TACH WIRING
1. Signal wire - hanging loose in engine bay with electric tape on the end to keep it from grounding out

I have no idea how the stock tach is working but I want to find out so I can help other people on the forum! Any help would be awesome! I’m not familiar with how the current sensing tachs work so someone with more knowledge on the subject will probably help me out!

Thanks!
Reed