Trailer wiring SNAFU

[TPCDrafting]

So, the other night I'm hauling home 5 ton of hay for the ponies. Just toodling along at 65. I'm pulling a 24ft GN flatbed. I start climbing a hill and with the ol Cummins I just add a little weight to the go pedal and it's holding. All the sudden I lose power in 5th. It seems strange so I grab 4th. Still losing power, I'm checking gauges and all seems well. I'm down to 3rd and still losing speed. There happens to be an off ramp so I take it. Once stopped I figure out that it's the trailer brakes holding me back. They are on about where I had them set, not locked. So I drag this thing into a parking lot where I can have some light. I pulled apart the wireing plug on the trailer to find 2 wires have come loose. So I put them back in and all seems fine. So I head back for the interstate and as soon as I let off the brakes making the turn onto the on ramp the trailer brakes lock up. This time they are on hard! I can't move the thing at all. It's a quiet on ramp and it's now about 11:30 pm! So I jumped out and pulled the plug again making sure I did it right. Well one of the wires just frayed when I put it back together the last time. So I cleaned it up and maticulously put it back together. Jump in the truck and it will roll! Only now I have no trailer brakes and no running lights. WTF There wasn't many people on the road so I limped home. I returned the trailer to its owner the next day with the same condition. We hooked it to his pickup and it all works fine. So it's somewhere on the truck! When I got home I hooked my truck to my horse trailer and I get the same results: turn signals are fine, brake lights are FINE, but no running lights and no brakes on the trailer. Everything on the truck works fine with or w/o the trailer. I've checked fuses once and didn't find any burnt. I'll double check, but wouldn't the lights be out on the truck as well if the fuse was burnt? I know the brake unit is getting juice. It's a Drawtite somethin or other. It has a digital number readout when the brakes are working. You can set them to a number aka 2. 5, 3. 0, 3. 5, etc. for how strong the trailer brakes need to be. When nothing is hooked to the truck all I get is a ". " (period). This is normal when nothing is hooked up and that's all I get now trailer or not. So I know it's getting juice, but its not functioning. I don't get it, if the brake lights are working and the box is getting juice, it should work right? Sorry so long, any ideas???

 

[AggieJustin]

Have you checked the fuses in the PDC?



Fuse 8 is the Trailer Tow fuse, and there's also a relay there for the Trailer Tow package. (At least on an '01) It's the first relay on the right side of the box (closer to the firewall) starting from the fender.



Another thing to check is to make sure that you have a good ground in the plug at the back of your truck. If your ground failed, then that could cause none of the electrical systems on the trailer to work properly.



Electrical problems are always fun to track down... .

 

[Scrappy]

wiring

It is definitely the fuse, I blew that fuse on my truck and those were my symptoms as well. the fuse cost me $2. 60 at the dealer.

 

[FDNYMOPARGUY]

I had a similar problem. It was the splice in junction block #2 in the power distribution box under the hood. The fuse was fine, and the brake worked, but my running lights and charge line to the trailer were dead. I metered everything out to verify where the problem was. The factory shop manual was, of course, a godsend during this process, so try to get your hands on one. A good multimeter, time and patience is all else you need.

 

[yogibear]

Make sure you check that fuse with a cont. or ohm meter. I've had them look good, but were blown. Jvan is right, I've had them blow without any wireing problems, just get soft, like a breaker will do, symptoms are no cl. lights or brakes, but br. lights and signals work. The only place I could find them were a dlr. or Wally World, no parts store had them.



Larry

 

[TPCDrafting]

Gotta love the TDR!



So when you say "fuse" you are all talking about the black block plug in deal with the 4 prongs in the fuse box under the hood? The one labeled "trailer xxx"? I'll stick the meter on it this am. Dad is coming with his good meter, he's really good at finding/fixing problems like that.

 

[RustyJC]

That sounds like the "trailer tow" relay. It provides power to the tail and running lights on the trailer and is activiated by the truck's headlight switch. The headlight switch can't take the additional load of the trailer lights, so this relay provides a higher capacity circuit for the trailer lighting.



Rusty

 

[AggieJustin]

That does sound like the relay. The fuses are in a group on the left side of the box towards the engine. In my manual, the trailer tow fuse shows to be #8. I can try to post a diagram if needed.



JM

 

[TPCDrafting]

Yeah trailer tow relay is what it's called. No need for a diagram, there is one on the inside of the box lid. Thanks for the help, it sounds like thats it!

 

[yogibear]

Tpc, I think its the fuse, believe its a 30 or 40 amp. On the lid of the PDC is a diag. My trk. is not here, or I would go look. They are square, and diff. colors.

 

[TPCDrafting]

Turns out the relay was good. The numbers on the cover match the ones for the fan blower. I swapped them and no dice. We checked the 40 amp fuse and it was bad. Can't tell by looking like one said above. Had to use a continuity meter to tell. We swapped in another 40 amp fuse and sure enough everything works fine. It must have been the trailer wires that came apart that shorted the fuse. Thanks all for your help.