06 Pete 357 headlights

OK, I try ti help our truck guys when I can, but this one makes mo sense to me. Maybe someone can explain the operating theory of the headlight operation. They tell me that on this truck, with 4 lamps, one on each side a dual filament, that sometimes, everything is good. 4 hours sitting with lights on, high beam, low beam switching normal. Then an hour later, the drivers side only, something goes haywire. With the headlights on, and brights, about 100mv runs to the single filament lamp, and nothing to the dual filament lamp. It simple goes off. The single filament lamp you can barely detect any actual visible light. Pass side is normal - bright and dim as expected. They have replaced all 4 lamps and new pigtails (crimps, I do not approve) and traced all wires across the bumper and back to the firewall. There seems to be two entirely separate circuits running the drivers and passengers side lamps. That makes no sense. Naturally, since the pass side work correctly, their first idea was to abandon the original drivers side harness and jump back across the bumper and run the drivers side. The breaker it's on won't handle it. They have no schematic and I don't have a lot of time to dig into it. It would seem to me to be a connector issue like corroded/sprung/burned or something. They do look clean at the firewall connector, at least on the engine compartment side. I am told that one wire goes from ground (at battery "-" potential) or chassis then switches to +12 volts when they hit the brights. Normally I would suspect that the return path for that particular wire is already at +12, and it runs on low beam, and switching to +12 merely eliminates any potential, rendering the circuit effectively neutral as supplying any current. I don't understand how it could operate like that....and in fact it doesn't. That's when the high beam dies. Apparently the pass side does not act that way. Every relay in the truck has been tested....they actually bought a relay checker.

That's about all I know about it. A schematic might help, but it seems to have a lighting control module, and being intermittent, I suspect there is a problem with it. I'm not positive about a separate module, just going by what I have been told. Any ideas?

Sorry Tony, but I HATE TRUCKS! All I can do here is make you a stiff drink.

That couldn't hurt. I'm glad that's not my thing. I just try to help if they get bogged down. About the only thing I hate worse is forklifts (add skytrak family). They have to be so compact there isn't room for tools or hands.

Don't know if your issue is resolved yet but check the spades on the switches really close, even though there is power to the spade the wire is connected to they tend to heat up and the connection on the back of the switch will get loose which in turn from the loose connection causes more heat and starts cooking the wires/connectors as well,give em a wiggle and check for any play, had a couple of that vintage Petes scratching my head before, one minute the lights are on then they're out..had the same issue with old FLR's as well..maybe worth a gander..

Thanks for the tip. It is still unresolved to my knowledge, just pushed back in the line. It will be back in the shop soon though.

I’ll post the fix when we find it.