Attention: TDR Forum Junkies 
Kirwin
TDR MEMBER
I posted this over on the 4th generation forum but thought in might be more important here for future problems as our trucks age.
I have been researching what could go wrong if you connect a trailer with a dead short or some other wiring problem to my 2013 Ram as I am familiar with the TIPM issues we went through a few years back. So after much research as best as I can tell the Body Control Module (BCM) located in the left side kick panel is now the brains of the system and controls all lighting functions much the same as the old Front Control Module (FCM) or TIPM. The headlights have a discreet wire dedicated from the BCM to each filament. So the BCM powers them directly via a driver circuit. If there is a relay involved I can't find it, nor is it described in the service manual. The taillights and rear brake lights and backup lights seem to have this same set up: a wire from the BCM to each filament with a common ground for the assembly. Keep in mind that the high end taillights are now LED so they draw very little power. The backup lights remain a 921 lamp and the marker light and license lights are still 194s. All of this is well within the power limitations of the BCM. I still think that the headlights themselves are a little high on the list of things that would be better served by a relay but apparently that is not the design they used. There are several fail safes built in so that the headlights remain on on low beam if the communications buss were to fail or lose connectivity.
This brings us to the trailer circuits. There are 4 relays in the Power Distribution Canter (PDC) on the PDC PC board as best as I can tell since they are not any of the relays you see when you remove the PDC cover. One for trailer tow backup lights via fuse F28 (F-27 is the fuse called out on the wiring diagram but this fuse is not installed on my truck, and the title listed on the wiring diagram matches fuse 28's title on the underside of my PDC cover), one for trailer tow tail lights via fuse F-29, one for trailer tow left turn/brake light via fuse F-53, and one for trailer tow right turn/brake light via fuse F-81. Each or these relays receives power from the battery bus (A0) in the PDC. These relays connect to individual wires that route through the body harness connectors to the 7 pin trailer connector at the license plate area of the truck. Each relays output is routed through an individual 20 amp fuse and out from the PDC through connectors in the main wiring harness and follows the left frame rail through the main body harness connector which is located just behind the left front tire to the rear of the truck. Control power for these 4 relays is provided from the battery bus (A0) in the PDC and the relay coils are individually grounded by the BCM such that the trailer wiring and its associated power is completely separate from the trucks brake, taillight, and backup light circuits. The Integrated Trailer Brake Controller receives power from two sources. Control power comes from fuse F-87 which, unlike the other fuses, is energized from the run/start relay. Power for the electric brake circuit comes from fuse F-11 (30 amp) which is powered directly from the battery bus (A0) in the PDC. There is also a dedicated fused 30 amp (F-30) charge line that feeds the 7 pin trailer connector, but it is not relay protected. Note that this circuit is also exempt from load shedding since it is connected directly to the battery bus making it always energized. The 7 pin trailer connector is also grounded via circuit G-314 to the truck body. In conclusion it seems that the BCM is well protected as the BCM only switches relay control power for the trailer lighting circuits. Any direct short would blow the 20 amp fuse in the PDC. The only issue I can see is if trailer lighting current is high enough (but less than 20 amps) for long enough you could damage the PC Board in the PDC, however I cannot confirm that this power actually flows through the PC board as it not accessible as best as I can tell. I for one, may install smaller fuses than the 20 amp ones as my trailer uses all LED lights so the total amp draw of my lights is quite small. Cheers. Ken Irwin
Below is the schematic of the circuit. I determined that the trailer tow relays are on the underside of the PC board that all the fuses reside on the top of. The fuses, along with relays K-1 through K-16 as listed on the underside of the PDC cover. On the underside of the PC board reside relays K-17 through K-33 according to the wiring schematic drawing. It also states that some of these relays are on the PC board but some don't have this designation so I assume some are replaceable like the ones on the top of the PC board.
I have been researching what could go wrong if you connect a trailer with a dead short or some other wiring problem to my 2013 Ram as I am familiar with the TIPM issues we went through a few years back. So after much research as best as I can tell the Body Control Module (BCM) located in the left side kick panel is now the brains of the system and controls all lighting functions much the same as the old Front Control Module (FCM) or TIPM. The headlights have a discreet wire dedicated from the BCM to each filament. So the BCM powers them directly via a driver circuit. If there is a relay involved I can't find it, nor is it described in the service manual. The taillights and rear brake lights and backup lights seem to have this same set up: a wire from the BCM to each filament with a common ground for the assembly. Keep in mind that the high end taillights are now LED so they draw very little power. The backup lights remain a 921 lamp and the marker light and license lights are still 194s. All of this is well within the power limitations of the BCM. I still think that the headlights themselves are a little high on the list of things that would be better served by a relay but apparently that is not the design they used. There are several fail safes built in so that the headlights remain on on low beam if the communications buss were to fail or lose connectivity.
This brings us to the trailer circuits. There are 4 relays in the Power Distribution Canter (PDC) on the PDC PC board as best as I can tell since they are not any of the relays you see when you remove the PDC cover. One for trailer tow backup lights via fuse F28 (F-27 is the fuse called out on the wiring diagram but this fuse is not installed on my truck, and the title listed on the wiring diagram matches fuse 28's title on the underside of my PDC cover), one for trailer tow tail lights via fuse F-29, one for trailer tow left turn/brake light via fuse F-53, and one for trailer tow right turn/brake light via fuse F-81. Each or these relays receives power from the battery bus (A0) in the PDC. These relays connect to individual wires that route through the body harness connectors to the 7 pin trailer connector at the license plate area of the truck. Each relays output is routed through an individual 20 amp fuse and out from the PDC through connectors in the main wiring harness and follows the left frame rail through the main body harness connector which is located just behind the left front tire to the rear of the truck. Control power for these 4 relays is provided from the battery bus (A0) in the PDC and the relay coils are individually grounded by the BCM such that the trailer wiring and its associated power is completely separate from the trucks brake, taillight, and backup light circuits. The Integrated Trailer Brake Controller receives power from two sources. Control power comes from fuse F-87 which, unlike the other fuses, is energized from the run/start relay. Power for the electric brake circuit comes from fuse F-11 (30 amp) which is powered directly from the battery bus (A0) in the PDC. There is also a dedicated fused 30 amp (F-30) charge line that feeds the 7 pin trailer connector, but it is not relay protected. Note that this circuit is also exempt from load shedding since it is connected directly to the battery bus making it always energized. The 7 pin trailer connector is also grounded via circuit G-314 to the truck body. In conclusion it seems that the BCM is well protected as the BCM only switches relay control power for the trailer lighting circuits. Any direct short would blow the 20 amp fuse in the PDC. The only issue I can see is if trailer lighting current is high enough (but less than 20 amps) for long enough you could damage the PC Board in the PDC, however I cannot confirm that this power actually flows through the PC board as it not accessible as best as I can tell. I for one, may install smaller fuses than the 20 amp ones as my trailer uses all LED lights so the total amp draw of my lights is quite small. Cheers. Ken Irwin
Below is the schematic of the circuit. I determined that the trailer tow relays are on the underside of the PC board that all the fuses reside on the top of. The fuses, along with relays K-1 through K-16 as listed on the underside of the PDC cover. On the underside of the PC board reside relays K-17 through K-33 according to the wiring schematic drawing. It also states that some of these relays are on the PC board but some don't have this designation so I assume some are replaceable like the ones on the top of the PC board.
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