On my 78 pickup. Just last night I noticed that the high beams are on even when they aren't "on". This truck has the four headlights. The high beam light is lit up on the panel, and all four lights are on, even when I don't have the switch on.

Going over bumps will make them flick on & off. REALLY annoying driving through town with my brights on, flickering on & off.

PLUS I'm actually trying to sell it right now. Kinda bad timing...

I'm also blowing blinker fuses. That started when I got the whole camper covered in mud trying to get out of a worksite during a snow storm earlier this spring. The wiring for the tail lights is a known issue on these and I assume I've got some mud back there and there's a short. But not sure if it could be part of the same issue causing my high beam problem.

Any ideas of where to look?

 

For your headlights sounds like you have a bad switch. Disconnect the harness to the column and then determine which pin is supposed to have power when on the supply side. Once you find your power source use a small length of wire to figure out which pin supplies power to the headlamps. Then look at the harness going to the switch and test for continuity between the supply wire and the headlight wire with a switch in both positions. If you have continuity between both on and off positions you have a bad switch.

As for the blown fuse yeah I would suspect the short at the trailer harness to be the culprit

 

Ok thanks. I'm starting to think I'll check all the grounds, too, since that was an issue before with the tail lights.

 

Being a novice with a multi-meter, humor me for a bit...

I found the switch (I think), right behind the fuse panel in the cab. I can hear and feel it click when I pull back on the lever which turns on the brights. When you push forward on the lever, it turns on the brights completely. Pulling back on the lever just flashes the brights.

When I pull the lever back to flash them, the switch clicks. When I pull the low beam knob, the switch clicks. When I push the high beam lever forward to fully turn on the brights, the switch does not click.

Would that mean anything to you? I guess it doesn't necessarily track down the problem...

At this point I'm just trying to narrow it down to an issue in the cab, or in the engine bay, I guess...

It's cold outside!

 

I'm not certain if it is the same electrical setup or not, but I had a high beam issue that resided in my actual headlight stalk on the column. If I pulled the high beam stalk back I would be able to flash the high beams like normal, but if pushed the high beam stalk forward I would have no lights at all. Now I know that you have separate highs and lows on your truck so I'm not sure if my solution will be the same for you, but here's what I did...

I removed the two screws that hold the stalk in place in the combination switch on the column so that I could remove stalk and gain access to the switching mechanism - be careful because there is a small detent ball and spring in there that you don't want to lose! There was a bunch of what I assume to be electrical grease gummed up with some 30+ years of dust. I used some carb cleaner to clean out the old grease and gunk, re-lubricated the switch with some lithium dry lube and reassembled the stalk. From what I can gather, I think I basically was not getting a good contact on my switch with I flipped the high beams on. So given what you are saying, and assuming that the 78 switch is similar to the 83 switch, you may actually be having somewhat of the opposite problem that I had - the switch is contacting the high AND low beams at the same time. Clean it up and see what happens.

 

Thank you. Someone else mentioned checking the switch in the steering column, so you're probably on to something.

If the deal goes through today, the camper is actually sold, so it'll be his problem now. But I'll pass this advice on to him.

 

I was about to post this question too, but looks like Dave nailed it. I don't have high beams when pulling or pushing, but already checked the terminals at the lights and got nothing- boy were they dirty, though. So I cleaned that up as best I could and moved on to what I think are the headlight relays, down below the steering column, two little round modules on a bracket, which I broke. Whoops, zip tied that. I swapped them out in each other's places and still nothing. So I'll check out the guts of the stalk this weekend if I have time.