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Hi folks!

Service Trailer Brake System warning comes on every time I start the truck.
I've scrubbed the internet and found a lot of references to a relay in the rear of the truck.

2015 LML
Original batteries
123,000 miles

The weather was below zero with snow while my truck sat for 5 days outside and the batteries were dead as a doornail when I went to start it. I jump-started it and drove it for 40 minutes. It had codes for ECM and TCM power- I thought from the attempt to start it with dead batteries pulling all power to the starter. They cleared fine. Runs great when running. It started fine the next day. This was two days ago.

After a short trip (5 miles) the truck almost didn't start last night. It would not start this morning.

I found 10 volts at the trailer brake pin inside the plug at the bumper while the truck was turned off.

I cannot figure out why the factory Trailer Brake Controller would be sending power to the plug.

Is it a bad controller or a bad relay?

Can keeping that relay operated be enough of an amp draw to kill batteries overnight?

Or are the batteries old, been dead once, cannot take the strain of the small draw overnight anymore and the TBC is making the relay operate?

Does anyone know if the relay gets power from the battery or from the TBC? Is the TBC hot all the time?
 

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Just to make sure-- you measured at the brake pin and not the aux power pin because it will have power all the time
 
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Discussion Starter #3 (Edited)
Just to make sure-- you measured at the brake pin and not the aux power pin because it will have power all the time
Yeah- the pin at 7 o'clock on the truck plug in. It reads 10v while the truck battery is too drained to start it.
I pulled the trailer brake fuse, no power at that pin yesterday. It started fine this morning.

I dont have a schematic, but if the relay everyone talks about is connected directly to power all the time and normally open, maybe the snow/ freezing rain and ultra low temps (for Missouri) tricked the relay, caused it to stick full on?
But with nothing to pull the 12v off that pin, is the relay being told to operate enough draw to kill battery?
 

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If the relay stays powered up due to the short to voltage, it’s going to keep a bunch of modules awake in the truck which will kill the battery.
 
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If the relay stays powered up due to the short to voltage, it’s going to keep a bunch of modules awake in the truck which will kill the battery.
Thank you for the input!

Do you think the dealer removing and replacing the bed a few months ago could cause this or should I start throwing modules and relays at it? It's finally warming up out there. Is it easy to see if the trailer wiring harness is pinched somewhere?
 

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Thank you for the input!

Do you think the dealer removing and replacing the bed a few months ago could cause this or should I start throwing modules and relays at it? It's finally warming up out there. Is it easy to see if the trailer wiring harness is pinched somewhere?
There could most definitely be some wires pinched together. I would look for that, first.
 

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Discussion Starter #7
All the wiring under there looks OK. Nothing stressed or crushed.
 

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Connect your meter to the brake terminal and the ground terminal in the truck side trailer connector. With your short to voltage showing on the meter, start disconnecting connectors one at a time from the plug working your way forward. That is one way to locate a short like that.
 

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Just to make sure-- you measured at the brake pin and not the aux power pin because it will have power all the time
Yeah- the pin at 7 o'clock on the truck plug in. It reads 10v while the truck battery is too drained to start it.
I pulled the trailer brake fuse, no power at that pin yesterday. It started fine this morning.

I dont have a schematic, but if the relay everyone talks about is connected directly to power all the time and normally open, maybe the snow/ freezing rain and ultra low temps (for Missouri) tricked the relay, caused it to stick full on?
But with nothing to pull the 12v off that pin, is the relay being told to operate enough draw to kill battery?

Triple check that you're looking at the right pin.

In the diagram below note that the "notch" is facing upward, while on our trucks' receptacles it is downward.

1084794


I.e.,

1084796


The trailer brake pin is at 11 o'clock and the auxiliary power pin is at 7 o'clock. You said that you were looking at the 7 o'clock pin so just make sure you're not getting mixed up. The auxiliary power pin, as sambor mentioned, is always "hot" and it showing 10V is consistent with you being unable to start your truck but doesn't necessarily suggest that there's any kind of power-suck in the brake controller.
 

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Triple check that you're looking at the right pin.

In the diagram below note that the "notch" is facing upward, while on our trucks' receptacles it is downward.

View attachment 1084794

I.e.,

View attachment 1084796

The trailer brake pin is at 11 o'clock and the auxiliary power pin is at 7 o'clock. You said that you were looking at the 7 o'clock pin so just make sure you're not getting mixed up. The auxiliary power pin, as sambor mentioned, is always "hot" and it showing 10V is consistent with you being unable to start your truck but doesn't necessarily suggest that there's any kind of power-suck in the brake controller.
Good catch. Use to my current truck and not K2s as of late.
 
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