Want to install a battery isolation relay in my 04 duramax with trailer towing package. Does anyone know where the fuse for the 12 hot lead that goes back to the trailer connector is located? Now please don't say under the hood, left side, under the fuse panel. I need something a little more specific. I would like to disconnect it and jumper it to the isolation relay with switched power so it only has power when the key is in the on position. Pictures are a plus. thanks

It is under the hood on the drivers side under the fuse panel cover http://www.dieselplace.com/forum/images/smilies/smile.gif It is the fuse that is in the very front passenger side corner of the fuse panel.

On my 2004, that one is not used. It is empty and has a red plastic plug which says B+ on it. It is not a fuse, it is just a place holder protecting the circuit I guess.

On my 2004, that one is not used. It is empty and has a red plastic plug which says B+ on it. It is not a fuse, it is just a place holder protecting the circuit I guess.
My truck also has the camper wiring harness, so am wondering whether this makes my configuration different? Any other suggestions?

The fuse goes where that blank is. The fuse is in the brake controller bundle that was put in the glove box on under the seat.

To get to the wire that goes to the trailer charging circuit, you will have to unbolt the UHFP and flip it upside down to get to the wire. I don't know the color, but a DVM and some patience should find it.

If there is a shunt there you simply don't have any power to your camper.

The wire color is red.

First I do want to thank all those who have responded. The picture was great. However comma, my OEM trailer harness connector (looking into the truck connector, the first spot to the right of the keyway) is hot all the time, so it has to be getting power from somewhere. I guess I could short it out to ground and see which fuse blows, but there must be a better way.

OBTW, what does UHFP stand for? I came up with Under Hood Fuse Panel, but am not sure. I took the cover off, and then the metal brace and unlatched the cover around the fuse panel. But I don't see the wire that goes from a fuse to the wire harness that goes back to the trailer connector on the bumper. I don't know where it is getting the feed, but it is not from the upper left one on my fuse box, unless it is not fused?????? The brake controller cable supplied with the truck has been installed into the second from the left receptacle under my left foot up under the dash. I did not use the 40A fuse because I have two batteries, and when you have two batteries, the book says not to use the 40a fuse. So I did not, but this is for the trailer brake controller, and it is getting its power from somewhere, because my trailer brakes work.

Surely there has to be another GMC 2004 2500HD out there with the same issue?

The book says that for gassers. Diesels are different. If I pull that fuse out on my 04 I don't have power anymore. That is the way it is wired from the factory. If yours is different, someone altered the wiring.

I have worked on my countless 03, 04 and 05's and they all have been the same way mine is.

Just did mine today. Hooked up everything for trailer and all was well except no inside courtisy lights inside the trailer. Pulled the red dummy plug out and installed the 40 amp. fuse and" bingo" lights. Must have 40 amp. fuse in per the pic above.:grd:

First, you are correct that UHFP stands for Under Hood Fuse Panel.

Next, as Max Power said, the statement in the manual is for gassers, not diesels.

Finally, when the 40 amp fuse is not installed, you will have full functionality with exception of the trailer charge circuit. I believe that is the center connector. So, you will have brakes, lights, etc., but you will not have power to the trailer's interior circuits and power to charge the on-board battery. That is why you have to install the fuse in where that red "blank" is.

I hope that helps you.

I believe that is the center connector.


Center pin is for Reverse lights. Upper right pin on the truck is for power. My factory trailer plug has a diagram on it.


OCDUNE

I think alleghenyrose (http://dieselplace.com/forum/member.php?u=14978)<SCRIPT type=text/javascript> vbmenu_register("postmenu_529099", true); </SCRIPT> was asking how to isolate the truck battery when the ignition was turned off. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

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Today I wired my 5r plug and installed the 40-amp fuse into stud #1 (Thanks to all you guys for the info. I love this forum) and now I have full time power to my trailer batteries. Problem is I also want to isolate the trailer battery from the truck when the ignition is turned off. You don't want to be draining down your truck batteries if something goes wrong at the trailer.<o:p></o:p>

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I won't have any problems removing the fuse box and reconnection wires behind it. I gather I will find a buss bar on one side of the 40amp fuse and a wire going off the other side. However, I would like to know if any of you know a good place to take a positive voltage source (on when the ignition is also on) for when the ignition is turned on. That way when the ignition is turned on I can use that source to close a relay to provide power to the trailer batteries from the 40-amp source.<o:p></o:p>

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It would be best if the positive power source were in the engine compartment. It’s a ***** to get through the firewall. I have already run wires through for driving and backup lights but an engine compartment source would be nice. <o:p></o:p>

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Thanks in advance for an answer. Like I said "I love this forum"<o:p></o:p>

There is a switched 12 volt source available in the UHFP, I think it is a pink wire, (i know, there are a lot of pink wires under there).

I have a fuel filter heater that I have not hooked up and I remember that one of the wires will be hooked up to a switched 12v source to activate the relay that will provide power to the thermostatically controlled heater.

Greg Lundyt at http://lubespecialist.com/ made the Racor kit that I use and he is also a member here. If you give him a PM, I'm sure he can tell you exactly what you need to know.

I was checking a few things. I had a Lance camper installed by the dealer, so thought this may be why I had the red plug instead of the 40A fuse. So I pulled the lance camper receptacle and followed the leads back to the factory camper harness tucked up under the truck just aft of the cab. The Lance dealer only used four wires in the harness and capped the rest. He used the yellow (L Turn); Dark Green (R Turn); Brown (Lights) and Light Green (Backup lts). He ran an 8ga wire from stud #2 direct to the receptacle, and another 8Ga white wire to ground. When I checked for power at the connector mounted inside the bed of my truck just aft of the driver, it was hot all the time, and not fused. I removed the connector yesterday, and cut the 8 ga wire.

So then I went back to the trailer towing OEM 7 pin connector and checked for power again with the truck off and key removed. I have power on the 1st pin to the right of the keyway when looking into the connector. This came from GM that way. I did not have to do anything to get power to the trailer tow connector, it was just there. So when I am connected to my trailer, that hot lead feeds the two batteries sitting on the trailer. If I keep connected to the trailer, and draw down the two trailer batteries, I am also drawing the two batteries in the truck down. Makes sense to me.

What I think people are saying, is that if I install the 40 amp fuse that came with the brake controller cable, into stud # 1, which right now has a red plastic plug, that somehow the power to the connector will not be there when I shut off the engine, but will be there when I have it on so it charges the trailer batteries? Is that the consensus?

Here is my test: If you guys with the factory tow connector, pull the 40amp fuse, do you still have power on the 1st pin to the right of the keyway?

First of all I am not a Tech. I did as you asked. Before pulling the 40 amp fuse checked the 1st pin tot he right of the keyway (aux. power to trailer) and it is hot.....then pulled the fuse and no power (just as it's supposed to be). In your case either the factory wired it incorrectly or your source of power is coming from somewhere else.

I haven’t checked the trailer hitch 7-pin plug yet but the one behind the cab is controlled from the 40-amp fuse. It is on all the time. Even if the truck is running or not. In my view that is a bad situation especially if you leave your truck connected to the trailer. I want to be able to start the truck more then I care about lights inside the trailer.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

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I am in the process of connecting up with a later post to find a good ignition connection in the engine compartment. Then what I will do is find a good quality 40-amp relay to use to disconnect the power from the 40-amp fuse when the ignition is turned off.<o:p></o:p>

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My guess is that disconnecting the power from the 40-amp fuse will shut off both the camper supply and the trailer 7 pin supply.<o:p></o:p>

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I'll let you know.<o:p></o:p>