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Diagnosing a mysterious trailer turn signal issue


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I have a used Legend cargo trailer that is new to the fleet. I plugged 7 way into my camper truck - 2012 F350 with flatbed. Flatbed has LED lights, trailer has LED lights. Right side turn signal no worky on trailer, all other lights fine. Right turn on flatbed headache rack working, but not the skirt right turn. Then all lights on flatbed quit. Lights on cab working, did not quit.  Immediately disconnected trailer.... 

No low amp fuses in passenger kick panel box blown. 30amp "trailer" fuse in high power fuse/relay box under the hood blown. Replaced.  All lights on the truck now work properly.  The truck powers 2 other trailers and a slide in truck camper properly. 

There is no emergency battery connected on the trailer at the moment. 

I did a continuity check on the trailer turn signal circuits at the 7 way plug and found:

Left turn signal/brake lamp (that was working) 3 ohms.

Right turn signal/brake lamp (that was not working) open circuit. 

I am chicken to plug the truck into this trailer due to might not get so lucky with a $4 dollar fuse next time....

What do you electrical experts think?

 

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Without laying hands on the victim, I'd start with a battery and some jumper wires. Battery ground to trailer ground, Pos to signal light cct. in question. This will let you work your way back, towards the lights.

The truck lights sounds like a bad splice somewhere in the bundle on the truck itself. Did the truck upfitter use a splitter box, or a taped mass?

Agree with Ray, keep an eye on the grounds the whole time you're testing.

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3 hours ago, Darryl&Rita said:

Without laying hands on the victim, I'd start with a battery and some jumper wires. Battery ground to trailer ground, Pos to signal light cct. in question. This will let you work your way back, towards the lights.

The truck lights sounds like a bad splice somewhere in the bundle on the truck itself. Did the truck upfitter use a splitter box, or a taped mass?

Agree with Ray, keep an eye on the grounds the whole time you're testing.

Yes agree with Ray about grounds. 

The truck deck itself connects via a 4-way flat trailer connector for quick disconnect for service - 6 bolts and the flat 4 and 4 people or a hoist can lift the deck off the truck to access fuel tank and chassis components. The flat 4 is wired into a Phillips junction box because the first body was uninstalled by cutting the Ford harness. Because 1st body up fitter was too stupid to use Ford up fitter harness / weatherpak kit supplied with the truck... but I digress. This wiring work on my deck was done carefully while using Ford wire diagram for the truck etc. The only tape is around the flat 4. The installation has been flawless until the test plug in to this trailer. It is working flawlessly since once 1st post fuse was replaced.  The momentary weird light display was like there was stray voltage escaping for a couple seconds. 

I rounded up supplies to make a fused jumper wire set to test with battery power rather than risking a truck...

The Legend trailer has a molded to the cord really nice 7-way plug. It is run up into the interior of the V-nose into a Phillips junction box. Wire runs take off from there along the top of the wall frames to the rear. I see I can remove the aluminum interior wall to ceiling 45 degree angled panels along the walls to access all the wiring.  So getting eyes on wires is possible except where the drop about 3ft down the rear corners to the light fixtures. 

I will report findings of live wire testing.... 

Thanks for ideas and help so far. I know a guy should just "get bizzy and fix it" but hopefully these trouble shooting posts can assist others sometime. 

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Ok - fused jumper wire built. Battery drug over to the patient. 

Installed 15 amp fuse and connected battery - to 7 way - , battery + to fused jumper.

Connect left turn brake. Bright light - OK

Connect running lights. All illuminate - OK

Connect 12v Aux - all interior lights and rear loading lights illuminate. OK

Connect offending right turn / brake circuit - BZZZZZZZZZZZZZT sparks poof goes the 15 amp fuse. Dead short. 

All diodes in the right lamp illuminate when running lights are powered. I am thinking lamp is ok but I will get a look at the back of it tomorrow. 

So it is 28C / 82F here right now and the trailer is sitting in the western sun with not one bit of shade so it's a bit warmer inside for a not quite unwinterized Albertan right now - never fear it is going down to -5C Weds night - so looking for the short begins in the cool of the morning. 

I can isolate the cord at the junction box inside to test it. 

Then I can remove 3 or 4 panels to expose the circuit on it's way to the tail light. 

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1 hour ago, noteven said:

Connect offending right turn / brake circuit - BZZZZZZZZZZZZZT sparks poof goes the 15 amp fuse. Dead short. 

It would certainly look that way. I suggest that you remove the bulb from the offending light socket and then measure from connection point to ground. If you see a very low to 0 ohm reading, you are moving in the right direction. At that point I would start tracing the wire to look for possible rubbing points where the insulation may have been damaged. Blowing fuses is always excessive current going somewhere. 

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Not Even, it sounds like you did a good job of troubleshooting. That right turn/stop circuit sparking and blowing a fuse is indicative of a short (to frame or the ground wire). Sure it takes patience but starting at the plug working your way back to the light looking at each and every connection, splice or junction box and where the wires are routed (open or inside conduit) should find the short. You're looking for bare wiring (stripped or burned insulation or sharp pinch etc.) allowing the circuit to come in contact with the frame ground or the ground wire conductor. No telling where the short is (plug to light) look inside the light fixture itself and the bulb and socket to make sure its not there. Ive seen shorts inside the plug itself !!! Hey you're getting there keep up the good work....

 

John T  

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  So you know there is a problem. If it is not at the front end, then check the rear end. As in connections and for wire problems, where access is easy.

  Next thing I would do is to find where the the right and left split and look there . While you are thinking about said subject try and find a easy access to determine if the the problem is to the front or rear of where the wires split.

 

  Just thinking,    Vern

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9 hours ago, noteven said:

The brown wire smashed under the new ground wire is the short circuit.

Good troubleshooting. Did you repair the brown wire or replace it with a new one? You may be able to use zip-ties or something similar to keep everything where that can't happen again. 

 

 

 

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I mentioned there is a junction box. 

I removed both ends of the brown wire circuit from the stud in the junction box. I tested the cord to vehicle side. Good.

I tested the chassis side. Short to frame.

The wiring except for brakes is all enclosed inside the trailer. I removed the led tail light unit. I disconnect the wire from the light. I tested the isolated wire. Short to frame.

I removed upper interior trim panels from back to front, inspecting. 

3rd panel off I found the oops. 

Fix:

The wiring in this trailer is all supported by attached zip ties and run through grommets. Replacing it would be a bit of an involved job.  There is also not enough slack at the spot to cut out and a splice...

The wire was not bared. There was a tiny spot that was shorting, hardly visible to the eyes. Bare aluminum is a pretty good conductor. 

Anyways I freed the wire and tested - bending, wiggling, shaking - and decided the wire is not broken inside so I installed the ground screw and insulated the brown wire and laid it back loose where it was.  Reinstalled the interior trim panels. 

Lights worked fine on a 30 mile drive on rough roads. 

The brown wire resting on the frame is not visible by anyone less that 6' tall standing on the floor... so if you were adding that white ground and reached up and run the screw in you may not see the interference...

So be sure of your target when you are driving the self tapping screws :)...

Thanks to everyone for input and ideas. 

 

Edited by noteven
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