2003 Ford F250 Trailer Wiring Harness Diagram
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Discussion Starter · #1 ·
I have a 2003 F250 with the 6.0L diesel. I cannot get the running lights to work on a trailer. I do not have power at the plug on the truck. I think what started this was I had a wire come off in the trailer plug, and it may have shorted out on other wires. I found 2 fuses that were blown. I have switched them with good fuses and still nothing. I have a wiring diagram that came with the ford wiring harness for installing a brake controller. It shows that there should be a relay in the running light circuit. The diagram shows 5 relays for the trailer wiring. I cannot find where these are located. The relays in the fuse panel are not listed for the trailer wiring. Is there another location of relays? Any other ideas?
Thanks for your help
Nathan
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Check your Owner's Manual first and verify that you replaced the right fuse. I believe the Tail/Marker lights will be in the box located under the steering wheel. Take a test light and be sure that you have power to the fuse. If no power at fuse, start checking relays. My guess is that there is still a fuse that is blown.
Did the plug on the truck get damaged? They are generally trouble free is OEM.
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Discussion Starter · #3 ·
The fuse for the trailer running lights was not blown. The fuses that were blown were the trailer back up lamps and the trailer battery charge. I checked every fuse and the rest were good. I don't use the back up lamps on the trailer. When I reconnected the wire that came loose in the plug I did put it together wrong. I aligned it 180 degrees from what is should be. I figured that out when the running lights and the brakes were on without activating them. I don't know if that could have caused a problem or not. I could see that it would have blown the fuses. I have checked relays that I think they might be a problem. I don't have a relay listed for the trailer park lamps in the manual, but a wiring diagram shows a relay for them. The plug on the truck is fine. When I test the plug, it shows the trailer battery chage terminal is hot when the key is off. Should that be hot when the key is off?
Thanks
Nathan
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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: FarmerK</div><div class="ubbcode-body">Check your Owner's Manual first and verify that you replaced the right fuse. I believe the Tail/Marker lights will be in the box located under the steering wheel. Take a test light and be sure that you have power to the fuse. If no power at fuse, start checking relays. My guess is that there is still a fuse that is blown.
Did the plug on the truck get damaged? They are generally trouble free is OEM.
</div></div>
I don't know about a 2003 model, but on my 96 the fuse and relays for the trailer are under the hood.
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Ditto for my 2000. There was a whole package of relays for the large trailer plug that came with my truck when it was delivered, all of which plug in under the hood.
short_stuff
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Yeah, check your power distribution box under the hood. Owners Manual should have everything in it.
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Discussion Starter · #7 ·
The owners manual does not say anything about any fuses or relays under the hood. It does say there is a 15 amp mini fuse located in the underhood relay block only on diesel models. I never found a relay block with a 15 amp fuse. I found 2 other boxes with at total of 3 relays and 2 mini fuses. The fuses were a 10 and 20 amp. Nothing was bad in there. I have looked all around for another box but found nothing. Is there another box I am missing? The 2 I found are on the drivers side back towards the firewall.
Thanks
Nathan
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Ford861,
You may have melted a wire during the various stages of your repairs. I had this happen on my old '92, and I traced the problem section to a stretch right above the rear fuel tank. Not wanting to drain/drop the tank (I had just done this on the inboard tank to replace the float ball), I simply spliced in a new wire where I still had +12V and reconnected after the tank.
What you need is either a multimeter, or a test light, and start walking down the distribution line for the running lamps beginning at the battery, and working your way back. At some point, you will lose voltage, and then at least you know where to focus on to find the problem.
Good luck!
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Discussion Starter · #9 ·
I believe I finally figured out what was wrong with the running lights. I traced it back to inside the fuse box on the circuit board. There are 5 relays loacted inside. The one for the trailer running lights went bad. The fuse never blew it just burnt the relay. A new relay is only about $3, but there will be a lot of time replacing it.
Nathan
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640 Posts
. The plug on the truck is fine. When I test the plug, it shows the trailer battery chage terminal is hot when the key is off. Should that be hot when the key is off?
Thanks
Nathan
Nathan,
You still have something wrong in addition to the lights. The battery charge terminal should only be hot when the truck is running.
Tom
Source: https://www.thedieselstop.com/threads/trailer-running-lights.183497/