OEM hitch wiring - should center pin be backup lights?

I have the factory hitch on my 2015 2500. I want to add an LED light to augment the reversing lights on the van. I want to use the hitch wiring socket for this.

My understanding after googling 7-pin hitch wiring layouts is that the center pin on the socket should be either aux or backup. Mine shows no voltage at any time - either with the engine on or off, or with the trans in reverse. The other pins seem to function as expected.

Before I go looking for loose wires/etc., can anyone confirm that I should be seeing 12v on the center pin when the transmission is in reverse? Thanks.

Yes the center pin is the backup lights.

MikePML said:

Yes the center pin is the backup lights.

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Damn. Now I have to go hunting for loose connections. Thanks for letting me know!

It may not be hooked up. Doesn't seem to be on my T1N and I don't see it on the trailer wiring diagram for my unit. If not you can run a wire down from one of the tail lights.

I think this may be it. Up inside behind the camera on my 2016. If I am wrong, this must be the ejection seat wiring.I do not have the trailer hitch.
https://goo.gl/photos/SPrGAUUYqAsLSo4z5

anoldfireman said:

I think this may be it. Up inside behind the camera on my 2016. If I am wrong, this must be the ejection seat wiring.I do not have the trailer hitch.
https://goo.gl/photos/SPrGAUUYqAsLSo4z5

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Sadly not the ejection seat wiring. That's a second location for your courtesy light with the 3-position switch. I moved mine from the front to this location so I can operate the courtesy lights from our platform bed.

Because I have the hitch wiring factory installed, I was really hoping to avoid tapping into any wires. I know I *can* but I don't want to. I'm going to trace the wiring back through the system and make sure everything's attached properly.

DieselFumes said:

I have the factory hitch on my 2015 2500. I want to add an LED light to augment the reversing lights on the van. I want to use the hitch wiring socket for this.

My understanding after googling 7-pin hitch wiring layouts is that the center pin on the socket should be either aux or backup. Mine shows no voltage at any time - either with the engine on or off, or with the trans in reverse. The other pins seem to function as expected.

Before I go looking for loose wires/etc., can anyone confirm that I should be seeing 12v on the center pin when the transmission is in reverse? Thanks.

Click to expand...

Hi DieselFumes, I am curious if you ever solved this problem. I have been looking into doing the exact same thing (adding auxiliary reverse lights, using the trailer hitch wiring as the power source) and I ran into the same thing. My trailer lights work fine when plugged into the 7-pin socket, yet when I unplug the connector and check voltage at the center pin I get nothing. I have found ~1.6 VDC between some other pins when the ignition key is turned on, but still no voltage at the center pin. My Pollack connector shows that the center pin should be the reverse light. So I, too, am confused. I am wondering if this is a CAN bus thing... I know when I plug in my trailer that the distance warning is deactivated (the visual mirror indicators and beeper when another car is in a blind spot). So between these two things, that there is low voltage at some pins and the warning deactivation, I'm wondering (guessing, actually) if the CAN bus only activates that pin when a connector is plugged in or a load is sensed on other pins.

Scratching my head... Any input or successes you have will be greatly appreciated!! Thanks a bunch.

This is a 3rd party wiring diagram for a 2007 NCV3.
The reverse light wire (white with a dark blue stripe) comes from the Trailer Tow Module.
It would appear to be a CANbus-talks-to-module system...



--dick

autostaretx said:

This is a 3rd party wiring diagram for a 2007 NCV3.
The reverse light wire (white with a dark blue stripe) comes from the Trailer Tow Module.
It would appear to be a CANbus-talks-to-module system...

View attachment 79659

--dick

Click to expand...

Thanks, Dick. That helps. Now I know that it is tied to the CAN bus. Not sure if that leaves me dead in the water or not, though, since I cannot tell from the schematic if the trailer harness socket (or the backup light wire going to the socket) will power reverse lights only, or if other lights need to be in the circuit in order for the auxiliary reverse lights to work. Mmmm, I guess there is one way to tell!

Thanks again!

I have a 2011 NCV3 . I have retro fitted the factory hitch wiring. (Although, I haven't made to the dealer, so they can flash the computer (Trailer plug isn't on,,, yet). I have the wiring diagram from the WIS(?) (official MB manual), for my VIN. The ONLY connections between the trailer plug & the truck are a CAN bus connection & a direct, from the brake pedal signal. (No waiting on the CAN bus, for the trailer brakes?) Both of these run into the trailer module, under the drivers seat. The trailer plug connects directly to the trailer module.

I would say to try "spoofing" the module. Try a plug with a trailer light, to give the module the idea a trailer is attached. LED lights don't draw a whole lot of power. This could also be an issue. I believe the module is set for regular bulbs. Although, I think it may only check for Open / Short conditions. This would be for Message Center, to tell you if a bulb is burned out. A digital meter has a VERY high resistance, much more than an LED. Try an old fashioned test light. I.E. it may need a taillight or brake light, to turn on the back up light.

I have an LED light wired up to a 7 way plug and it works just fine on my 4runner but it will not work on the Sprinter. I pulled out the Fluke DMM and discovered that under no circumstance does the center pin get voltage. Every other pin will receive voltage without a load on it so it does not appear to be a load required type of situation. Did anyone figure out how to use the 7 way plug for additional reverse lights?

I was going to try using an inline resistor but never got around to it. Talking to the service writer at a dealership they suggested that the LED light just doesn't draw enough load to trigger the system. Maybe try the resistor and report back?