[Turbo Raccoon]

but here there are no old carbed 2.3 turbo mustangs. i would bet money im the only person in this town who has one.and no it is in a 79 mustang.

[GT40Capri]

If I am reading this right, you snagged another module off of another vehicle and now the car runs. The concern now is that the tach is inop, right?

Sometimes tachs blow fuses. There is one in the under dash panel (behind that cover under the steering column).

If that checks out you could look for a poor connection out under the hood. The tach signal wire connects to the Negative wiring on the coil, but not necessarily at the coil itself. IIRC, the connection is back up the harness a bit unless the negative side of the coil has two wires right there. If only single wires to the coil then trace the harness back up a bit and I wanna say there's a square grey two-, or four-wire, connector that has the coil negative wire going in one side and two out the other. One of those two goes to the module and the other to the tach. Sometimes a poor connection develops here. Wiggle the harnes a bit and see if the tach responds (engine running of course.

If it all looks good then it's likely the tach died, which was not uncommon when these cars were newer. You could install an aftermarket tach, scrounge for a used replacement, or perhaps get yours repaired. I found the company below on a Google search. Have never dealt with them myself.

http://www.autoinstruments.com/

[Asimov]

Is the tach one of the things that will function oddly (or not at all) because of the ignition switch going out?

[Turbo Raccoon]

well see the thing is when im crankin the motor the tach will jump with the RPM the motor is turnin and when she fires it wont move but every once in a while it wil jump. so im thinkin it problanly the connection under the hood. and ive read that the stock ford tachs arent real accurate anyway. is this true? even if i get my old tach workin again im still gonna put an aftermarket. just to be safe.

[GT40Capri]

The 79-86 tachs are better than the 87-93's.

In both cases the real problem is with response time more than +/- accuracy at steady RPM. The 87-93's are pretty sluggish and lag behind engine acceleration up to several hundred RPM.