Connecting 12v Lead For Trailer Harness

c4cruiser · August 22, 2008

Just bought an 08 Silverado with the ITBC and I need the 12v lead powered up at the trailer harness connector.

I found what I think is the wire taped to a harness just to the outside of the fuse center under the hood. There are two posts at the forward end of the center; one has a red wire attached to it and the other does not. There is a fuse installed for Stud 2 (trailer brake) which I assume is the post that currently has a wire attached. There is no fuse for Stud 1.

So does the wire for the 12v lead attach to the unused stud? There is a metal strap from that post leading to the inside of the fuse center and it appears to point right at the location for the fuse for Stud 1. That is marked as fuse 68 in the diagram and labeled as (Stud 1 (Trailer Connector Battery Power) (Optional-40A fuse required).

I assume that a 40A J-Case fuse has to be installed in order to get power after the wire is connected??

minibike132 · August 22, 2008

Auxiliary Power

Circuit #742 for Auxiliary Power at the 7-way trailer connector is no longer connected by the GM Assembly Plant. If the customer desires auxiliary power at the trailer connector location (i.e. refrigeration, battery charger or interior light in the trailer), complete the following steps to connect circuit #742:

<LI type=1>Locate the red/black wire, looped and taped to the chassis harness, below the brake master cylinder. <LI type=1>Break the tape and route the wire to the front of the underhood electrical center.

<LI type=1>Place the terminal on the smaller of the two studs on the electrical center and secure with an M6 fastener. This is circuit #742 to stud #1 for auxiliary power to the 7-way trailer connector.
ONLY for vehicles without RPO TP2 - Auxiliary Battery, install a 40 amp fuse to power the circuit. Important: For vehicles equipped with RPO TP2 -- Devices powered by this fuse will drain the vehicle battery if left connected with the vehicle not running.

5Alive · August 22, 2008

Wow, that is very similar to how the previous generation (GMT800) trucks work.

There is a 10-12 guage red wire that goes to Stud#1, with no fuse installed, but a 40 amp fuse supplied.

Stud #2 also has a red wire, about 14 guage, that feeds the trailer brake, and has a 30 amp fuse installed.

So even while the stud locations are different in the GMT800, and while the red wire is no longer hooked up in the GMT900, the wiring design is similar between the two generations of trucks.

I'll be removing my 40 amp fuse once I get my second battery installed, following the RPO TP2 wiring method. A positive lead from the auxiliary battery will go to Stud #1, the same stud as the Red Wire that feeds the trailer. With the 40 amp fuse pulled, the trailer wire will work soley off of the auxiliary battery, so should a light be inadvertantly left on in the trailer while still hooked up in a parking lot having dinner at a restaurant, it will not drain the main battery. If that fuse wasn't pulled, it would drain the main battery.

Of course, without an auxiliary battery, one needs to have that 40 amp fuse in place, otherwise there would be no battery power to the trailer. But once the second battery is added, following the TP2 RPO (there are other dual battery RPO's and SEO's that are wired differently), that battery will have it's own 40 amp fuse located in a separate megafuse holder nearer to the battery itself. The lead to feed Stud #1 will therefore already have 40 amps of protection.

I say all this not to pretend to teach... but merely to reaffirm my own understanding under the public scrutiny of any gm techs who read this. And I also wanted to explain more about what was meant by "TP2" should the OP consider adding a second battery down the road (like I'm in the midst of doing right now) and be at all confused about whether the 40 amp fuse should be in or out and why.

c4cruiser · August 24, 2008

sparkstech-- Thanks for the quick reply. Got the wire hooked up to Stid #1 and now I just have to go back to the dealer to get a J-case 40A fuse. Seems like none of the local chain parts places carry that fuse type.

It's too bad that the factory doesn't include the fuse like they used to do when they included the wiring harness for a trailer brake controller. You would think that if a truck has the ITBC option (like mine) it would be reasonable to add the fuse for the 12v power lead.

I don't plan on a second battery as I only tow an open car trailer, but it's nice to see that a battery tray is in place already. But it seems strange that GM would add the battery tray and not include a single fuse for the traierl wiring.

Markma230 · January 19, 2009

I know this is an old post however I have a 2004 k1500 and cannot locate the 2 red wires my trailer brake controller(prodigy is plugged into my harness direct to the factory connector and lights up. I just got my trailer and my 1500

My problem is my Cargo light is not coming on I used a meter and found no power what else should I check?

builderpaul · January 19, 2009

I know this is an old post however I have a 2004 k1500 and cannot locate the 2 red wires my trailer brake controller(prodigy is plugged into my harness direct to the factory connector and lights up. I just got my trailer and my 1500

My problem is my Cargo light is not coming on I used a meter and found no power what else should I check?

Above post is for an 08. Your 04 is already wired. Just need to add a the fuse. A dummy fuse may be in the socket now. This will have nothing to do with the cargo light.

Cargo light power comes from interior dome light circut. Is your dome light on?

Markma230 · January 19, 2009

My inside truck light was on I checked stud #1 and found a dummy fuse just went to autozone picked up a 40amp for $4. inserted and miricle the light now works.

pagerboy · March 15, 2009

Very strange but the location mentioned above for the 12v lead is actually for the traler brake on my truck. I know this because I have the factory brake controller and it was wired this way and says so on the fuse list.

My 12v aux power is to the right of the fuse box and it is marked that way, just not sure why I still have no power with my test light at the plug(only powers when trailer hooked up?).

sgtjetdrive · August 30, 2011

Very strange but the location mentioned above for the 12v lead is actually for the traler brake on my truck. I know this because I have the factory brake controller and it was wired this way and says so on the fuse list.
My 12v aux power is to the right of the fuse box and it is marked that way, just not sure why I still have no power with my test light at the plug(only powers when trailer hooked up?).

Pagerboy,

I too have a factory brake control. I hooked the 12V on the stud and tested for power at the stud, 40 amp fuse was already there. Power checks good. However I do not have power at at the trailer plug. I need this circut to charge the camper battery enroute and would like to know how to test this. Any sugestions anyone?

MikeNH · August 30, 2011

I am nowhere near my truck at the moment but to get power to the plug, you need to locate the red wire under the brake booster and connect it to the left stud when you're standing in front of the truck. Sorry I can't be more help but I'm in DC and the truck is at home.

markw8 · August 30, 2011

Very strange but the location mentioned above for the 12v lead is actually for the traler brake on my truck. I know this because I have the factory brake controller and it was wired this way and says so on the fuse list.
My 12v aux power is to the right of the fuse box and it is marked that way, just not sure why I still have no power with my test light at the plug(only powers when trailer hooked up?).

Pagerboy,

I too have a factory brake control. I hooked the 12V on the stud and tested for power at the stud, 40 amp fuse was already there. Power checks good. However I do not have power at at the trailer plug. I need this circut to charge the camper battery enroute and would like to know how to test this. Any sugestions anyone?

It sounds like you hooked up the power wire to be used with an aftermarket brake control. It is the easiest to see, but not the right one.

Mark

wavery · August 30, 2011

If I were you, I wouldn't bother using the factory wire. I ran a new 8G wire all the way back to the trailer plug.

The factory wire is either 10 or 12G wire (I don't remember). With a 50' run, that wire is too small to carry the voltage required to charge a battery. Remember, when dealing with DC current, you must measure the run, round trip. If you run your fridge on 12V en route, you may find the battery dead in a few hours.

Very strange but the location mentioned above for the 12v lead is actually for the traler brake on my truck. I know this because I have the factory brake controller and it was wired this way and says so on the fuse list.
My 12v aux power is to the right of the fuse box and it is marked that way, just not sure why I still have no power with my test light at the plug(only powers when trailer hooked up?).

Pagerboy,

I too have a factory brake control. I hooked the 12V on the stud and tested for power at the stud, 40 amp fuse was already there. Power checks good. However I do not have power at at the trailer plug. I need this circut to charge the camper battery enroute and would like to know how to test this. Any sugestions anyone?

sgtjetdrive · August 30, 2011

Very strange but the location mentioned above for the 12v lead is actually for the traler brake on my truck. I know this because I have the factory brake controller and it was wired this way and says so on the fuse list.
My 12v aux power is to the right of the fuse box and it is marked that way, just not sure why I still have no power with my test light at the plug(only powers when trailer hooked up?).

Pagerboy,

I too have a factory brake control. I hooked the 12V on the stud and tested for power at the stud, 40 amp fuse was already there. Power checks good. However I do not have power at at the trailer plug. I need this circut to charge the camper battery enroute and would like to know how to test this. Any sugestions anyone?

It sounds like you hooked up the power wire to be used with an aftermarket brake control. It is the easiest to see, but not the right one.

Mark

Mark,

You were exactly right. I looked again and found the correct wire under the brake booster. I removed the afore mentioned wire and installed the correct wire on the stud. I now have 12 volts to the correct pin in the trailer plug with key off or on.

Pagerboy, you may want to look at yours again. I have a wire for aftermarket brake control even though I have the built in brake control option. Not sure where that wire goes to with the IBC option.

Thanks to all problem solved.