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Just bought an 04 Super duty with the factory plug 7pin and 4 pin plug. Checked the fuses and finally tracked down the 2 relays under the hood but can't seem to find anything wrong. Everything on the trailer works except for the running lights. Is there another fuse or something I have missed?
Trailer works fine on my buddies Duramax and worked fine on my old 97.
If the fuse is ok ,check back at the plug harness about a foot under the truck at the factory plug you may have crostion in that plug or a wire in that area that can get brok by stones,or stick or if you live in canada like my self you'll have ice that plays havick with the wiring.
Fuses are alright but put new in anyway. Checked the connection from the 7 pin to the harness and it seems to be fine. The ground going to the frame looked dirty so I took it off and cleaned while I was under there. Tried it on the trailer and still no running lights. Tried it on a different trailer today and it had the same problem, everything works correctly except for the running lights?
I am looking at a diagram for a 2003 superduty. It does have a fuse 38 in the underdash fuse box. I believe this setup is the one that has the running lights relay built into the fuse box and is not serviceable. You should have a brown/white wire coming out of the fuse box and going to the trailer plug. There is a solid brown wire feeding the relay inside the fuse box. When you turn the running lights on, the brown wire should have voltage. This triggers the relay and then the relay hooks fuse #38 to the brown/white wire, and sends the #38 voltage back to the trailer plug.
I would get a meter or testlight, find the brown/white wire at the fuse box, and see if you have power with the running lights on.
The 03 and prior (including 04 Heritage "Leftovers") use a completely different physical layout for the CJB from the 04 and 05. It changed again for the 06 MY.
Edit for Correction: The 03 SD looks the same as the 04 and 05 layout. Guess they made that transition earlier than with the LD series. I think you may be on to something with your statement about the relay not being serviceable. I infer the same from the Workshop procedures.
Dang! There isn't a lick of data in either the 04 or 05 SD service or owner's manuals as to where the "Trailer Tow Relay, Parking lamp" is physically located. Every other relay's location is documented, this one isn't. The relay is in the schematic and is shown as being in the CJB. I guess it's located in either position 210 or 212.
THE TRAILER LAMPS ARE INOPERATIVE TRAILER PARKING LAMPS
Key in OFF position.
Disconnect: Trailer Tow C4098 (4-pin).
Disconnect: Trailer Tow C4099 (7-pin) .
Disconnect the trailer from the vehicle.
Check the operation of the vehicle exterior lighting systems for any concerns. Does the exterior lighting system of the vehicle operate correctly?
Place the headlamp switch in the OFF position.
Disconnect: Central Junction Box (CJB) C270j.
Place the headlamp switch in the ON position.
Measure the voltage between the CJB C270j pin 15, circuit 962 (BN/WH), (component side) and ground.
I checked the voltage at CJB C270j pin 15.. with the headlights off there was no voltage, with the headlights on there was no voltage, when I turned the key on and the head lights I have voltage.
At the plug there is 12.5 volts between the ground and the running light pin,but the trailer lights still won't light? The test light lights across the ground and running light pin as well. For the heck of it I tried lighting my spot light across them and it would not light? So I put the volt meter across the two while the spot light was attached and the volts drop below 1 volt when there is a the load of the spot light. Tried it the same way across the all time hot and even the turn signal pins and the spot light lit and the voltage stayed around 13 volts. Everytime I try the running light pins it drops below 1 volt?
Not sure if I am explaining this very well.
If the built in relay was bad would it do this? I have redone the grounds on both the truck and the trailer thinking it may be the problem but it didn't make any difference.
Thanks for all the help so far, any ideas what I need to try next?
The state of the ignition shouldn't affect anything as this is supposed to be an ALWAYS HOT circuit.
Your testing indicates that there is an excessive resistance in the circuit. Since you got it at the output of the CJB, I can only think of one additional check to make before concluding that the relay has failed. With your spot connected to pin 15/ground, check the voltage at both legs of fuse 2.38. If both legs read battery voltage, then I'd conclude the relay has failed.
I have a terrible time finding anything on these DVD's. How I found that was by going into the diagrams, and working my way to the diagram that has the trailer tow relay. Then I went up to the upper right corner where the little boxes with the pictures are located, and clicked on "component view" or something like that. It will bring up a little window that gives you a list of most of the components of the diagram you where looking at, and it will give you description of where it's located, and some of them have a spot to the right you can click on, and it will bring up a another window that will have a location picture. It was in the first little window that came up, where it said it was located in the fuse box and not serviceable.
If it where me, and my relay was bad, I think I would add an aftermarket relay instead of changing the whole fuse box. I think I would use a fuse tap and come off fuse 38, and then run over to the new relay contacts, and then intercept the wire going back to the trailer, and hook it to the new added relay. Find the brown wire going into the fuse box, and tap into that to trigger the relay.
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