Help! MSD tach adapter install

[NOT A TA]

Can't get tach to function using MSD 8920 Tach Adapter.

Car is 70 Firebird non gauge equipped originally. I am running an MSD pro billet dist, MSD 6A box, and MSD coil. I installed a factory tach from a later year Trans Am (76 I think) and have the portion of the factory tach harness that plugs into the tach. It appears to use stand alone tach wiring as it has its own harness.

Original harness appears to be a 2 wire setup where one wire was 12v and the other wire went to the neg side of the coil. Research says the light colored wire was 12v and dark to the -coil. Does anyone know if the tach itself has to be well grounded?

I hooked up the tach adapter using the instructions for the voltage triggered tach and got no tach movement. Double checked connections as well as power and adapter ground with no results.

Tried changing the connections of the adapter for current triggered tach. same results and doublechecked.

Am I using the wrong tach adapter? (summit guys thought this would be the best one to use). Is there a housing ground for the tach I didn't know about? Is there a way to test the tach using the orig harness to verify that it should work without disassembling the dash and removing it or taking the car out of the garage? If I could I'd park a car with stock ignition next to it and make some long jumper wires to check it but thats not practical where the car is.

Thanks John

[NOT A TA]

Just tried hooking up the trigger wire for the tach directly to msd box tach output and the tach power wire to 12 v , no tach movement.

[HILROD]

My Camaro has the same thing done to it, except a repro factory tach installed in the gas gauge spot. The MSD6A tach output runs the tach direct, with no filters or adapters. I used wiring diagrams for a tach equiped car to trace the wires. The tach does need to be grounded. One of the terminals on the backside goes to ground, one to power, one to tach signal.

[NOT A TA]

There's only two terminals on the back of the tach where they normally plug into the harness on the TA tachs. I've only got wiring diagrams for 71 and it doesn't show tach wiring at all. I'll try one to the MSD box tach output and one to ground when I get back to the shop in the morning.

[HILROD]

If there are only 2 terminals, the body of the unit must be grounded directly where it attaches. I found a diagram of the printed circuit from the back of the dash, that's how I did my wiring. I think I found it in the factory manuals, I don't remember.

[HILROD]

I just looked on E-Bay. There are pictures that show 2 terminals where wires connect, and one small stud that looks like it connects to ground. The other 2 should be power and signal.

[NOT A TA]

Thanks Hilrod. I couldn't see the ground stud from under the dash. Had to take the whole dash back apart to get at it.

[Buswrench1]

I am having the same exact issue John . Same year car same pieces . How did you solve this issue ?