Voltmeter signal

Where does the voltmeter get it's signal from? Mine has been reading low for awhile but recently has dropped down into the red. I don't think there is a problem with the alternator as the vehicle starts just fine every time, so I think it is just a bad signal to the gauge. It could be the gauge itself as I have had connection problems with the gauges before but typically when it happens it is either the 4 small gauges, the 2 big gauges, or all the gauges. I have not had only 1 gauge have a connection problem, but I can't rule it out.

It gets it juice from the dash side of the firewall, at the fuse box somewhere.

I know the the tach, speedometer and voltmeter all ground on the passenger side of the engine block near where the ignition coil bolts in, maybe the one for the voltmeter has broken off?

It may not be a ground issue. Mine shows low when cranking because of where it is taking power from. Of course it is an 86. But believe me, two batteries that are hot totaling over 200 cranking amps doesn't give a low voltage. The positive side though comes from the fuse panel area. I am not sure if the gauge comes through a fuse or not, but probably. My point is there are a number of connections there it goes through and can show a low voltage. Have you checked the battery voltage and alternator voltage?

Not since it really dropped, my multimeter isn't working. However when I first noticed the voltmeter was reading lower than normal I did check it and voltage at the battery both with and without the engine running was fine.

My TJ was doing crazy things with the voltmeter and it ended up being the ECU

Was there any other symptoms?