Erratic Gauges

So I just bought a very old-n-rusty 1989 Suburban to use to tow a boat back and forth to the ramp. It is, overall, a junker, but it’ll do what I need it to do. However, the gauges all read weird. The gas gauge oscillates around slowly. The voltage gauge does the same. And the oil pressure reads high for a while, and then drops to nothing, and then comes back. Any ideas on where to start with fixing this?

Start with a thorough check of the starting system. Don’t forget the battery cable connections too.

Your symptoms are symptomatic of an alternator with a fried diode in the bridge rectifier portion. But don’t let me troubleshoot it from here, check the system out from there.

Rusty? Start looking for bad grounds.