Headlights on= No Turns, Headlights off= Turns

Hello All,

I searched the forum and couldn't find the exact issue my truck is experiencing so I thought I'd start thread to see if I can get some clarity.

1987 V30 with the 6.2 and a service body. The previous owner swapped the incandescent brake/turn signals on the service body to LED replacements but otherwise nothing strange. The issue is: When I turn on my headlights I get no turn signals flashing (front or rear) and my brake lights barely light up when depressed and the turn signal indicators on the dash are both lit up (right brighter than left) but do not flash either. When I turn on just the running lights, I get something similar, except then my left turn signal will flash but it is not lit up like when headlights are on; Right turn doesn't flash and is lit up on the dash.

To me it seems like I might be getting some sort of backfeed from the brake/running light circuit into the turn signals or something like that? Has anyone experienced something similar or does anyone have some possible insight as to what I need to look at closer/ test? Electrical isn't my strong suit like mechanical is, but I can diagnose well enough with a meter/test light.

Any help or advice on where to start looking would be appreciated!


Fuses are all good, the only other electrical that doesn't work on the truck is the interior dome light (new bulb and fuse) and the horn (good fuse).

Thank you in advance!

I used to see this on euro cars when someone added a side marker turn lamp. Could be a backfeed, but like everything else electrical and 30+ yrs old.....check all your grounds.

Doppleganger said:

I used to see this on euro cars when someone added a side marker turn lamp. Could be a backfeed, but like everything else electrical and 30+ yrs old.....check all your grounds.

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Will do, I'll double check the grounds and maybe throw another ground from the service body to the frame and light to service body to rule that out or fix the issue. Thank you

if the issue is new with the LEDs, try going back to incandescent bulbs.

gmbellew said:

if the issue is new with the LEDs, try going back to incandescent bulbs.

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I wouldn't know if its a new issue or not since the previous owner was the one that installed the LEDs, probably a couple of years ago. I might switch back to the old Incandescent style to see if that solves it since the local Farm supply has the same lights in incandescent for $10/light.

I'd first start checking grounds. That half lit lights indicate that some bulbs get connected in series.

Something strange is happening before the circuit even reaches your taillamps. Flashers and brake lights should be separated until you either use a turn signal or turn on hazard lights. In normal position it lets the power from the brake light switch to go to rear lamps only. Any backfeed from taillamp should not go anywhere else, just back through the directional switch to the brake light switch.

I wonder if it isn't an issue within the directional light switch assembly.

mibars said:

I'd first start checking grounds. That half lit lights indicate that some bulbs get connected in series.

Something strange is happening before the circuit even reaches your taillamps. Flashers and brake lights should be separated until you either use a turn signal or turn on hazard lights. In normal position it lets the power from the brake light switch to go to rear lamps only. Any backfeed from taillamp should not go anywhere else, just back through the directional switch to the brake light switch.

I wonder if it isn't an issue within the directional light switch assembly.

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I am wondering that as well because my turn signal stalk is a little janky. Wipers will turn on with Hi-Beams and don't turn off unless finagled just right. I'll test out some additional grounding on the lights to the body (with some alligator clips) and see if that helps. If not I might be needing to replace my column assembly. I'm going to pickup an NV4500 this weekend so swapping out the Auto column for a manual column might be a good upgrade to go along with that.

Did you check the bulbs and sockets. Every light that shows a turn signal also has running lights. (side markers, front park/dir lights, brake lights) Any one of those places the brown wire (running light) could be touching a turn signal wire whether it is a bad bulb or socket. First place I would start is the front park/dir lights.

Jgonick said:

Did you check the bulbs and sockets. Every light that shows a turn signal also has running lights. (side markers, front park/dir lights, brake lights) Any one of those places the brown wire (running light) could be touching a turn signal wire whether it is a bad bulb or socket. First place I would start is the front park/dir lights.

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I'll check the fronts to see if there is any crossover. The rears are LED and have a 3 prong insert (not a socket per se) that I can check for crossover on. I'm starting to lean more towards my column having issues over the grounds/bulbs themselves but I'll tear into it over the holidays. Thank you!

Good luck, be sure to let us know what you find. Your issue is definitely strange. 9 times out of 10 light issues are usually a bad socket, ground, or bad/wrong bulb. So everything works correctly until you pull the headlight switch?

Add a couple grounds especially from core support to chassis

@Haydenh Same-thing is happening to me. I was told it is need for resistance. I switched all the original bulbs to LEDs, and then my turn signals would work very slowly if the lights weren’t on. When I would turn the headlights on, the turn signals would not work to either side. Oh, and the turn signal indicators inside the cab are always slightly on. I added an LED flasher that was grounded and grounded it to the bottom of the dash, but I’m told that is not a good ground. Adding that did make the turn signals flash a little faster with lights off, and now flash very slowly with lights on. I tried a different LED flasher that doesn’t have a ground wire and and it speed up the flashing when the lights were off, but now they are back to not flashing at all when the headlights are on.

Dano500 said:

@Haydenh Same-thing is happening to me. I was told it is need for resistance. I switched all the original bulbs to LEDs, and then my turn signals would work very slowly if the lights weren’t on. When I would turn the headlights on, the turn signals would not work to either side. Oh, and the turn signal indicators inside the cab are always slightly on. I added an LED flasher that was grounded and grounded it to the bottom of the dash, but I’m told that is not a good ground. Adding that did make the turn signals flash a little faster with lights off, and now flash very slowly with lights on. I tried a different LED flasher that doesn’t have a ground wire and and it speed up the flashing when the lights were off, but now they are back to not flashing at all when the headlights are on.

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Interesting, I think I need to switch the rear brake lights back to the incandescent lights. Since its a service body, the Incandescent types are super cheap and plentiful thankfully! I'll maybe give that a go and report back after!

Just wondering if there has been any resolution for this issue? I have the same thing happening on my '83 C15. Brake and turns work perfect without the parking lights on. Once I turn the parking/headlights on I have no brake or turns.

Tattoovt, I'm in the same boat. Just finished a three year long rebuild and now the electrical is giving me hell.

When I took off the bed and reinstalled it, all seemed well. Until now. No gauge lights either, so I figured it was a the common issue of a ground that everyone seems to have.

They check out from what I can tell.

Fuses. All good.

Power to the panel is good. At times its a little lower than 12v at some of the fuses, kinda a concern but eh. Traced wires along the frame, seemed OK. Fixed some connections that previous owner did and were crusty. Still no dice.

Got the meter out and started testing things. No power to the rear sockets except the markers. Pulled it off the truck checked all the sockets, replaced some, fixed/added more grounds, and put all new bulbs in. Reinstalled; still no good.

Took apart the ground on the parking brake, cleaned the connection, didn't change. Finally checked the gauge cluster, turns out all the bulbs were just being funky from when I installed the new gauge bucket. So dash lights work and aren't the combined issue as some have had.

I've been working all day on it and it has to be something dumb. My hazards didn't work so I went and got a new relay and all of a sudden things started working again. Albeit, wrong, but a step in the right direction. So I know I'm getting power to things. Here's what I discovered:

All front lights work fine. Rears are the issue. Have all grounds alligator clipped to a solid ground on the rear harness.

Headlights off/Hazard relay out:
- brake and turns work as they should at full illumination and speed.

Headlights on/hazard relay out:
- slow/dim turn signal
- no tails/brake.


Hazard button pushed in with relay out and headlights off:
- No turn signal
- dim brake light

Hazard button in/relay out/headlights on:
- Tail lights on
- No brake lights. Tails off if brake pushed
- No turn signal

Relay in/lights off/Hazard button off:
- all fine

Relay in/lights on/Hazard button on:
- slow turn signal
- no tail lights or brake

Kinda long winded and jumbled, hopefully comes across as somewhat understandable. At the end of this I have some theories that maybe someone can confirm. Seems like the Hazard button could be a culprit, or the light switch maybe? Or, the relays don't like each other. The new relay is a "LED compatible" one. However, I have no LEDs installed. Or, I need two of the same so they get along.

I do have a wiring diagram that I've referenced here and there. As well as searched out online for other's solutions. So far none have been it. It's cold and wet so I've given up for the day and came inside to type this all up.

Thanks in advanced.