Trailer Battery Charge Wire

robertbick · April 22, 2008

I have seen the document on where to find the trailer battery charging wire under the master cylinder and that it should be connected to the right side stud in the front of the UBEC and a 40A fuse must be installed.

My situation is a little different since I have the OEM Aux. battery and it has a connection to that stud and there is no 40A fuse in the UBEC for Stud 1. Should I still connect the trailer charge wire to that stud AND should I still install a 40A fuse?

I have the `08 2500HD. Thanks for any help.

sonjaab · April 23, 2008

No idea with the aux. battery option but check out this link for what needs

to be hooked up and where the fuse goes on the GMT-900s:

http://www.gm-trucks.com/forums/index.php?...rake+controller

robertbick · April 23, 2008

No idea with the aux. battery option but check out this link for what needs
to be hooked up and where the fuse goes on the GMT-900s:

http://www.gm-trucks.com/forums/index.php?...rake+controller

I have seen all the info on the hook-ups and fuses for the GMT-900's but none of them are for the 2500HD with the Aux. battery. I also have the IBC so don't need to wire for an aftermarket brake controller. I just need to know if I should connect the wire that is taped up under the brake booster to Stud#1. My Aux battery has a lead to that stud and am wondering if that will then power the trailer connection with 12v for a trailer battery. I don't think I need to install the 40A fuse since the Aux battery is connected there. I would think that the Aux battery is being charged via the alternator.

robertbick · April 24, 2008

...nevermind. I found my answer. You do not put the fuse in cicuit 742 in trucks with the Aux battery.

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 steps 8.1 thru 8.4 to connect circuit #742:

1. Locate the red/black wire, looped and taped to the chassis harness, below the brake master cylinder.

2. Break the tape and route the wire to the front of the underhood electrical center.

3. 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.

4. 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.

brycer867 · April 6, 2010

I know this is a super old post but i was hooking up auxiliary power to my 7 pin trailer connector and found a second wire taped up under the hood right between the fuse box and the fender, It is the identicle color and stripe as the one taped up under the master cylinder but there is a tag on it that i cant make out very well but it appears to say "aftermarket trailer 12 volt feed" Does anyone know what it powers up?

Gregorio · April 7, 2010

I know this is a super old post but i was hooking up auxiliary power to my 7 pin trailer connector and found a second wire taped up under the hood right between the fuse box and the fender, It is the identicle color and stripe as the one taped up under the master cylinder but there is a tag on it that i cant make out very well but it appears to say "aftermarket trailer 12 volt feed" Does anyone know what it powers up?

Sounds like the trailer brake controler wire.

brycer867 · April 7, 2010

I know this is a super old post but i was hooking up auxiliary power to my 7 pin trailer connector and found a second wire taped up under the hood right between the fuse box and the fender, It is the identicle color and stripe as the one taped up under the master cylinder but there is a tag on it that i cant make out very well but it appears to say "aftermarket trailer 12 volt feed" Does anyone know what it powers up?

Sounds like the trailer brake controler wire.

The truck has the factory ITBC so its not for that, thanks though

Gregorio · April 7, 2010

I know this is a super old post but i was hooking up auxiliary power to my 7 pin trailer connector and found a second wire taped up under the hood right between the fuse box and the fender, It is the identicle color and stripe as the one taped up under the master cylinder but there is a tag on it that i cant make out very well but it appears to say "aftermarket trailer 12 volt feed" Does anyone know what it powers up?

Sounds like the trailer brake controler wire.

The truck has the factory ITBC so its not for that, thanks though

Doesn't matter. All trucks have the wire for an aftermarket TBC regardless if there is an ITBC.