Stupid Pyro tricks

[AJ650]

My pyro gauge acts oddly on a regular basis, and the problem appears to be electical. It's a new autometer gauge, + wire directly to driver side battery, grounded under the dash. The gauge acts normally with the engine running and no other electrical acc. running. Turn on the AC and the needle sometimes bounces up and down about 200 degrees. Turn AC off, fan still on, no problem. AC on and touch the break pedal and the gauge bottoms out (not at zere but mechanical bottom limits). AC off, no problem. Windshield wipers, every time the wipers fire the gauge bottoms out then returns to normal until the next time the wipers move.

I don't have probelms with any other gauges, tach... all read normal. My alternator has been tested at 100amps at idle, but only 13 or so volts, probably a regulator problem. Side note, internal or external regulator? The pyro's problem seem to be lessened at interstate speeds, making me think it's somehow related to the charging system. The only other thing I can think of doing is grounding directly to the battery and seeing if that changes things.

Ideas?

[ucdavis]

I have autometer also. Got 12v from fused gauge spade on block under dash; did a single conductor to spade into block, then splice to two wires-one to other stock dash gauges and one to new gauges. Ground is to common dash ground (mine is different than yours physically but same electrically).
Ground to batt would be good test; I wouldn't go hot from batt w/out fuse.

[patrick m.]

i agree with ucdavis, try another ground location.
strange electrical problems are often a poor, or faulty ground.
also, some grounds may be electrically "noisey" due to a different component having a poor ground.