My 911 tach for my 3.2 is not showing revs correctly. It does rev slowly to about 2,000 rpm when the engine is obviously reving much higher. Do I need a new tach or is there some sort of debug procedure I can check. One question I have is: does the tach unit body itself have to be grounded to the dash to work or just the ground connection to the back of the unit itself?

It worked this spring when I drove it last

The single ground connection is all that is required as far as grounds go.

The same gauge worked fine with the same engine?

Does the needle stick as well?

On our track car (1987 3.2 engine and tach), the plastic gauge lense was bowed inward so that it made contact with the needle. I swapped the plastic lense with a glass one from an early tachometer, and all has been well since.

I hope this helps and solves your problem.

Tap on the gauge to see if it goes back to normal.

I think I found the problem but....
I think it is my switch power wire. The red with white stripe. I don't think I have power from it. If I put another power source to the tach it works fine. If I hook the red/w stripe to one of the gauge lights or my fuel gauge it doesn't work. My head lights, dimmer switch works fine though.
How do I trouble shoot the switched power wire?
Thanks,
Buck