Hello. I have 1988 K5 Blazer. My fuel gauge works only when the dimmer switch is turned down, with or with out the headlights on. I cant seem to understand why the headlight/dimmer switch would effect the fuel gauge? Can someone help me figure this out?

EDIT: fuel gauge does not work. Went on a long ride today and it never moved from 3/4 of a tank. So is the sending unit bad?

 

Sounds like another chevy electrical gremlin. IME erratic fuel gauge behavior is often a fuel pump starting to go, but to quit entirely? Sending unit sounds plausible. FWIW I have never owned a pre '90 chevy with a working fuel gauge except my current 89, (which it so happens has a host of other problems ) so I generally just ignore it and use the trip meter.

Where it was affected by the dimmer switch, I would wonder about a short somewhere under the dash. I'm not an electrical guru, but I would guess that since the dimmer switch increases draw it might only be shorting enough when turned up to casue a problem with the fuel gauge. Where it later broke completely it may have finally shorted and broken something. Dunno, just throwing out alternate ideas to look at before going through the trouble of replacing the sending unit or tracing wires all through the truck.