Tach Project
 

I'm in the process of installing a tach from a larger GMC truck in a '65 C10. I've pretty well got the installation figured out, but the wiring has me a little confused. Most every tach I've encountered has two wires for the tach to function. 12 volt power and another that goes to the - terminal on the distributor or tach terminal on HEI.
This tach is secured in the can by 3 studs with only 1 stud being insulated from the case and no spade terminals for the wires.
The case is stamped AC Div. GMC. I know the tach came out 0f a '70s-''80s big GMC gas burner.
Anyone know anything about this style tach?
Any help appreciated!
Just looked at the tach and found a date October 13,1977

Re: Tach Project
 

Oh ya, that's your basic 12-2 Alpha-Delta Charlie right there. Yep.

Re: Tach Project
 

I haven't seen one exactly like that before, but GM did have some tachometers that used a simple 1-wire hookup to the negative side of the coil (no connection to 12v power).

Here's a link to an old GM tachometer service bulletin --> http://www.oldsmobility.com/img/pdf/...air_Manual.pdf

See the non-transistorized "Group 1 inductor type" tachometers as described on pages 12 through 14. The internal schematic is shown in figure 8 on page 27.

Re: Tach Project
 

Spudder,

I have used a similar style tach in the past.

I believe that the insulated terminal goes to the negative side of the coil/distributor and one of the two case studs goes to ground. (Doesn't matter which one)

No 12 Volt power required.