Originally Posted by vicrod
That model car/year had an ammeter not a voltmeter.
I believe you are seeing an amp draw that is causing the ammeter to go below 0 amps.
Check the dome light for shorting to ground. I would expect that a fuse would blow on the dome light circuit or you could remove the fuse and the problem will go away.
Makes sense, but my meter doesn't read -40/+40, rather it goes 0/18, and pretty accurately reads terminal voltage according to my DVM. Sure that's an ammeter?
On the other hand the deflection is about the same as an ammeter, I'd guess. Straight up with a charged battery and the ignition off, right-deflect with the alternator working, left deflection with the car shut off and the headlights on. So maybe just a faceplate repaint? Still, if it is just a repaint and it's an ammeter showing a short, that short would be drawing over 40 amps! Plus if I disconnect the battery under those conditions it's still pegged left. I'd think ammeter springs would pull it to the center. How confusing. I'll pull the lead off the meter and see if anything still works.
In any event it's sure worth a try checking the dome wiring!