Note: The 5 volt instrument regulator used to be (and sometimes still is) called a voltage limiter, which served to differentiate it from the 12 volt alternator regulator. To avoid confusion, I will use limiter from now on.
On OEM gauges, the fact one limiter serves the three gauges represents a single point of failure. If the fuel, oil, and temp gauges are all operating erratically or fail to operate, the instrument limiter is probably the cause.
In this case, the limiter may have gone bad or its +12VDC supply may have been broken. Identifying the cause of the malfunction typically requires removing the instrument cluster from the dash, which can be a bit of a PITA.
Instrument voltage limiters tend to be fairly durable so a broke +12VDC supply is a more likely cause. A loose or disconnected +12VDC supply at the instrument cluster should be relatively obvious. Less obvious causes could be a blown fuse or circuit breaker, broken supply wire, or disconnect at the other end of the supply wire.
Use a 1156 bulb and socket, two alligator clips, and a length of 16 gauge wire to make a continuity tester. (You could buy a continuity test but the wire leads on these are often too short to test all circuits.) You can connect one end to a known ground but I usually just attach one of the alligator clips to the battery's negative terminal. Connect the other end to the instrument voltage limiter. If the bulb glows brightly, you have a good +12VDC supply. If the bulb glows dimly, you've found the +5VDC side of the limiter and it's associated wiring.
Two notes: (1) The ignition key must be in the run position for the above test to work. (2) If the bulb glows dimly, your limiter is working and your problem is elsewhere.
I'm going to take a break here to go get some other things done. When I come back, I'll continue gauge troubleshooting and repair.
