Trailer wire-running lights

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Sep 27, 2018 | 09:17 PM

BridgerDog

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Trailer wire-running lights

I got all my wiring connected for the trailer plug, but no running/taillights are working on the trailer. They work on the truck, but not the trailer.
Is it possible to connect the brown wire of the regular body wiring harness and use that to connect to the trailer plug?

Sep 27, 2018 | 11:06 PM

Colorado350

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From: Colorado Springs

Welcome to FTE.
‘I would start trouble shooting before I’d modify wiring. If you use a test light do you have power on the truck side AT the plug? If Yes, then start checking the trailer wiring. If NO, then how about IMMEDIATELY before the plug…as wires insert into plug?

Sep 27, 2018 | 11:39 PM

slpcth

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From: Foresthill, CA

Did you check the fuse? There are separate fuses for the trailer wiring.

Sep 28, 2018 | 02:35 AM

Shovelheadrob

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From: Cambs, England

Quote:

Originally Posted by slpcth

Did you check the fuse? There are separate fuses for the trailer wiring.

This /\ /\ if if you're truck didn't come with the tow package you will need to add the fuses, possibly a relay too.

Sep 28, 2018 | 05:00 AM

Just Strokin

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From: Tallassee, ALabama

What year is your truck?

Are you sure it is the truck wiring? Have you checked the trailer wiring for the problem?

Sep 28, 2018 | 07:55 AM

BridgerDog

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Update .

It's 2000 F250 with tow package.
Trailer lights work with another truck.
Fuses were ok.
Relays? I've read about relays, but am hearing two locations. Under the dash buried up by the radio, or in two additional boxes by the power distribution box. I've located the ones under the hood. If these are present, am I still looking for something under the dash?

Sep 29, 2018 | 08:37 PM

SuperDutyScaler

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From: South Jersey

Get a test light and check the terminal at the seven pin. If you have 12volts then its your trailer, if not check the connection at the rear of the truck where the seven pin connects and also where the harness plugs into the back of the seven pin. On my 2004 I kept losing all lights on my trailers, turns out it was the seven pin plug connection was bad for the ground wire. I had to disassemble the seven pin plug clean and resend the tabs and it's been fine ever since

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Oct 1, 2018 | 06:02 AM

projectSHO89

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Joined: Jan 2004

Posts: 19,768

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From: St Louis

Quote:

Originally Posted by ligito

Do you have two batteries?
That can cause this issue, as well, if your second battery is bad.

Not on an F250 which is what the OP says he has, the two batteries on a diesels are simply in parallel. E-series with the auxiliary battery are completely different. Even on those, if the auxiliary battery is bad or missing, the trailer tow running and backup lamps are still powered by the main electrical system provided the auxiliary battery relay is working (critical!). If that relay does not energize, then those two circuits won't have operating power.

On the 2000 SD, the trailer running lamp relay is in the I/P Relay block which is located above the gas pedal. Remove the radio and look down and to the left. Turn on the 4-way flashers and follow the clicking as the flasher relay is in that same block.

BJB fuse F4 supplies the power for the trailer tow harness running lamps. When the headlight switch is turned to either the PARK or HEADLIGHT position, the trailer running lamp relay is to be energized routing power from fuse F4 to the trailer light connector(s) via the brown/white wire.

The wiring harnesses going back to the rear of the truck have a history of getting corrosion in the pins and causing a loss of the circuit. You probably need to run that harness first, open up each connector and do a visual. After that, get your meter or test lamp out and test the circuit to see if power is getting to the harness on the noted wire. Follow it as needed to see where it's getting lost.