I have an auto meter voltmeter in my car. two things are happening and not sure why. the illumination light is not working, I can move the plug to another gage and light works. second is when i turn on the turn signal the needle jumps back and forth, but when car is running it's at a constant. the gages are very old as well, my guess is over 20 years, that should not matter in my mind.

any suggestions?

I would start looking at where you grounded the gauge for it jumping.
the turn signal would ground through the dash, if you did the same for the volt gage, then that would probably be the issue.
something changed in the grounding of the dash.
not sure about the gauge illumination. not sure what you mean by change the plug and it lights up.

for the gage illumination I can take the oil pressure socket when lit up and move it to the volt gage and the bulb turns off then i put it back to oil pressure gage and it lights up.

I will move the ground for the voltmeter from the dash to somewhere else and see how that does.

this light socket thing just has me scratching my head. could be an issue with the ground of the gage, I have no idea but stranger things have happened.

thanks for the tips

Are you saying that when the engine is running the voltmeter is steady when the turn signal is flashing and when the ignition is off the needle jumps when the turn signal is flashing?

If so this is normal since the alternator isn't compensating for the voltage drop of the lights flashing when the engine is off.

The voltage drop will be more or less noticeable at the needle on the gauge depending on.

Is the gauge movement dampened? (Fuel gauges do this to prevent sloshing fuel from causing a wild gauge reading)
How good is the battery? (Strong fresh batteries will compensate for voltage fluctuations better than old weak ones)
How good is the alternator? (A bad alternator / voltage regulator will not compensate for voltage drops very well).
How good are the connections, including grounds? (bad connections lead to bad voltage problems)

As for the light in the gauge I had a Autometer tach made back in the day and it had a permanent internal light bulb. When it went bad I contacted them and they said send it back and they would replace it. Not sure if they meant gauge or bulb but it was many years ago they made the offer so it may not be an option any more.. I got busy and the car got parked so I never got it fixed.

IMGTX is correct on the fluctuation.
On the light a couple of ?
1. Is the Volt gauge housing metal or plastic?
2. Is the light socket 1 or 2 wires
3. have you tried the V gauge light in the OP gauge?

you could have
1. a flakey 12V connection to the socket
2. A flakey ground connection
3. Flakey connection between the socket contacts and bulb.
try moving the wire( with the bulb in place.
You are on the right track. either the 12v source or ground is causing the problem and it can be a loss of contact anywhere from the source to the ground (socket included).