My '86 D150 just developed an odd light problem. My right brake light and right rear 4 way flasher quit working. The right tail light and right rear turn signal work just fine. I have traced the wiring back to the plug in the engine compartment where it breaks out from the main harness and it all looks good but I get no power for those circuits at the plug. I suspect the problem is under the dash. Anyone ever run into this one?

Both rear tail/brake light sockets and bulbs are new and tested OK for continuity. Problem is definitely at the other end.

I did a search and did find where someone said the brake lights and 4 ways share the same circuit but nothing else. My FSM isn't here yet and the diagrams in the Haynes manual are next to worthless, when I was a kid Haynes manuals were THE best, now I don't know why I bother to buy one.

 

Hi,

If i had a wiring diagram, i might be able to help but like you said, the Haynes manuals are pretty worthless when it comes to wiring diagrams. All i can find for that circuit is a 20amp fuse and a flasher.

Do you know where the feed comes from for that circuit? And did you test the flasher?

I could access Alldata for you on Tuesday if your still at a loss as thats the soonest i could head back to college.

 

i just thought of this. if your left brake light and 4way work and your right one does not the problem must be where they connect at the back of your truck.

 

They don't split at the back of the truck which surprised me, I assumed they would use the same lead until it got to the back. Never assume..... I traced the power lead for the right side all the way up to the connector on the LH inner fenderwell. I disconnected the plug and checked for power at the main harness plug. I get power on every terminal except the one for that wire. I got that far yesterday but ran out of time.

I was doing a lot of work under the dash recently so maybe I accidentally snagged a wire. I hope it's that simple. I'm expecting my FSM in today's mail. Wherever the problem is it almost has to be between the connector on the fenderwell and the point where the flasher and brakelight circuit come together. I hope........

 

Found it and the cause is even weirder than the problem.

I traced the wiring clear back to the harness coming out of the steering column and still no juice on that circuit and then it hit me. When I got the truck the left turn signal wouldn't cancel, the mechanism had the tip broken off. While searching eBay I came across a repair kit that replaced just the mechanism without having to replace the whole switch, it was about $14 with shipping. I installed it but the right turn signal didn't seem to go all the way over but it worked for now. It bugged me though and was on my list of things to correct down the road. I had a suspicion that might have something to do with it so I pulled the wheel and started playing around. The replacement arm was off just a hair to the right which is why it didn't feel right and it was also just enough to break contact for the 4 way flasher and brake light to the right rear. If I moved the lever just a hair to the left they would work and I literally mean a "hair"! I had an '83 switch that was the same except for wiring colors and the connector so I spliced my connector onto it and problem solved.

 

I Having close to the same problem with my 81 d150. My rear right lamp wont work when just the lights are on. It will work with brake, turn signal, and flashers.

Im a little confused on this, and its 10 degrees F and snowing outside. So I would like to do most of the thinking indoors. I do have a multimeter and access to some other electronics.

 

Don't blame you a bit on the "indoor thinking" LOL. It was a balmy 29 and windy with light flurries here yesterday and I wasn't having a lot of fun sitting underneath my truck tracing wires in that little heat wave.

Sounds like a broken power wire or bad contact in the socket since all the lamps share a common ground. I'd start at the socket and work my way forward. The harnesses from the sockets have connectors about a foot forward on the inside of the frame rails. There will be two connectors with a pair of wires each. One connector will be yellow and the other black or gray. The yellow connector has the power leads for the tail, brake, turn and emergency lamps, one wire is for the tail lamps and the other for the other three. The other connector has the power wire for the backup lamps and the ground wire for all the lamps. Your highest probability of a problem is between that connector and the bulb.

 

Twitchr, have you checked the bulb? The parking light filament is different than the stop, flasher, and turn filament.

 

Good call, sometimes we overlook the obvious.

 

i think i have a loose or screwed up wire on my left rear marker light. once and a while it will not go on when you turn the lights on. to fix it i hit the fender and it comes back on. i have had this problem for as long as i can remember. some one has already replaced this socket and i think that is the problem. it looks like someone used the socket from a ford.