Fire Department's 92 F-350 trailer tail lights shorting out

[dieseldane]

Like the title says, my department's 92 F-350 trailer wiring is having issues. We use this truck to pull our dive team and swift water rescue equipment trailer, and with the flooding in our area we have needed it a lot more than usual lately.

The tail light pin on the truck side of the trailer receptacle is shorting to ground somewhere, and will immediately blow the fuse when the lights are turned on. Everything else works as far as trailer lighting. I have checked the receptacle itself, and it is fine, the short is somewhere ahead of that. Unfortunately with work, calls, trying to buy a house, plan a wedding, and now some minor health issues, that is about all I have had time to check. I have followed the loom up to under the hood, but then without a good electrical book for that year that is as far as I got, but did not see any issues that jumped out at me.

Are there any common issues or problem areas that I should look at first? Thank for the help ahead of time guys.

 

[Schnot]

It only blows the fuse with the trailer plugged in right? If that's the case then the short is on the trailer.

Start with the plug and work your way back.


Sent from my iPhone.

 

[dieseldane]

Nope, blows it with trailer unhooked, and is shorted to ground unhooked. Trailer works fine on other trucks.

 

[RangerSVT]

At the plug end, check with a voltmeter for ground. Use a pair of jumper cables ran to the back to give your voltmeter power. This will verify you have one or more grounded wires. Check the wire connections at the plug, making sure no strands made its way loose contacting other wires/pins. If everything checks out and you still have the short, disconnect the wire(s) that's shorted, and try again (this is to determine if the plug itself went bad internally). If the short still exists, disconnect that wire up front where you traced it to, test again. If it doesn't blow the fuse, then your problem lies between the two test spots. Check near moving parts, like E brake cables, and check for any rubbing or exposed/unprotected spots. Electrical diagnosis can be time consuming, but remember the basics and you can get through it...

SVT

 

[dieseldane]

Already did just what you decribed svt, with the exception of once the loom that the wire is is reaches the underhood area, then I don't know which smaller loom the wire I need is in, so I don't know what to disconnect to try and isolate it further. I used a continuity tester (which is pretty much a self powered test light, and will light up when a circuit is complete if you're not familiar, several people that have seen me use it weren't) and clipped the ground to the bumper. When I touch the ground pin, it lights up as it should. Then I touch the tail-light pin, lights up as well, meaning it is grounded. There is no wire under the bed of the truck to disconnect the trailer wiring, so I clipped the wire to the tail-light pin just in front of where it goes into the plugin so that I could isolate it. The plugin no longer lights up the tester, but the wire heading to the front of the truck does. I followed it loom up the frame (although I did not open it up to inspect the wire itself) and could not find any damaged areas. I follow it up until it splits into several smaller looms under the hood. I don't know where the wire goes from there, since I do not have an EVTM or even a good diagram for this truck. I am pretty good with wiring and electrical problems, but without some good info, I am kind of stuck at this point. I would consider removing that section of the harness, but that would sort of leave us in a pickle if this truck needed to roll out. Everything else works (blinkers, brakes, aux power) so if we had to roll without tail lights, we could, but it would be nice to have them working just in case we had an encounter with an unforgiving state trooper. They are out there, as hard as that is to believe.

 

[dieseldane]

If you have anything else for me svt, I'm all ears, I know you play around with vehicle electrical systems a fair amount. I have a doctors appointment this morning, so maybe I can go mess with it after lunch and see if I can find anything else out.

 

[Jim Oaks]

I would think if it was shorting it would be at the plug jack. I can't imagine a problem in the harness unless it was rubbing on something. Have you removed it just to see?

I actually rolled up on a crash one time as a State Trooper and the fire truck on scene didn't have tail lights. But I'm forgiving

 

[dieseldane]

Some of the guys in my neck of the woods aren't quite as nice as you Jim, lol.

I didn't remove the plug completely, but did cut the tail light feed going to it right before the plug. The plug checks out to be good. I was sure it was going to be the plug as well, and almost went and picked up a new one before I went to look at it yesterday. But sure enough it that's not the problem.

Another thing, the truck only has about 5300 actual miles on it. That's correct, only two zeroes, and up until a couple years ago only ran calls in city limits, so it's not like it's bounced around a lot to chafe a wire.

Another another thing, it is a 1992 F-350 crew cab, 2wd, 5.8L truck if someone can find me some diagrams for it.

 

[RangerSVT]

.... The plugin no longer lights up the tester, but the wire heading to the front of the truck does...

If I'm reading this correctly, your problem area lies here. If you disconnected the wire from the tail light, and all is good upstream, then it would have to be downstream. Consider the tail light socket a possibility. If there is no plug to disconnect the trailer plug from the rest of the vehicle harness, then it had to have been hard-wired, so check this connection...

SVT

 

[RonD]

Ford uses Brown wires for running lights, so pretty much any brown wire you see will be for running lights.

Wiring should be:
Battery----Fuse------light switch-------running lights/trailer connector

Light switch is a common failure point since it is a mechanical switch.
With fuse removed, turn on light switch, brown wire on trailer connector should now show a ground(short), turn off light switch, if ground goes away then replace light switch.
We are assuming the short wire from the fuse to the light switch is good, not shorted to a ground.

Front running lights tend to get moisture inside more than rear, also cab top running lights if so equipped, so best to start there, remove connectors/bulbs and check for corrosion.
Don't forget about the license plate bulbs.

To test for wiring shorts you can use a 12v light bulb in place of the fuse, it will light up brightly if there is a short in the system, some use a head light bulb since it can handle higher amps, but I have used a good 12v test light without blowing it.
Turn on light switch, the light bulb is now passing the power to the circuit.
Running lights on good wires might glow a bit but not always, lights "after" the short will not at all.
Start moving wires around and unplugging connectors(with brown wires) if short is lifted test bulb will dim and running lights will get brighter.