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Old 06-15-2019, 08:42 AM   #1
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Trailer plug wiring problem

We are getting ready to tow a toad behind our IS. As a first step I wanted to make sure the plug on the IS was wired correctly. Using a volt meter I found that every terminal except for the ground terminal on the 7 pin receptacle is hot all the time even with the key off. The only one that is supposed to be hot all the time is the charge terminal.

I can see where maybe the charge wire may have worn thru to another wire but not all of them.

We bought the IS used and I am pretty sure the previous owner towed a golf cart so I am surprised to have this problem.

The MB dealer is 3 hours away so I am hoping not to have to go that route.

Ant thoughts?
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Old 06-15-2019, 08:53 AM   #2
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That's an unusual situation, but my approach would start by taking the plug apart and working backwards.

If I thought the problem went all of the way back to the crazy Chrysler/Mercedes "buss" well...
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Old 06-15-2019, 09:15 AM   #3
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I did separate the receptacle by pulling the plug out of the back of it. I then tested the plug with the same results. The next step would be to pull the wires out of the plug and test the individual wires but I do not see how to do that without cutting the wires.
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Old 06-15-2019, 09:50 AM   #4
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Since MB has its own way of doing things is it possible that it is supposed to have voltage to each terminal and then it is managed by a controller somehow? Seems unlikely but has anyone else checked the voltage on their MB receptacle?

I know my Ram and GMC do not have constant power except to the charge terminal.
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Old 06-15-2019, 10:50 AM   #5
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It may be a ‘phantom’ voltage caused by the electronics in the vehicle. A sensitive multimeter May see a voltage that the trailer may not.

Try using a test light instead to trace wiring in this case. Also, even though it’s MB, the 7 way plug should be dead standard. I’d be more worried about getting the Toad wired right.
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Old 06-15-2019, 12:16 PM   #6
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The connectors are monitored by the Sprinter through the CAN bus. The high voltages you see probably very high impedance.

When I plugged in my transmitter for my wireless tow lights, the turn signals did not work unless I stepped on the brake pedal, as well. The transmitter did not pull enough current to let the Sprinter know it was there. I bought a plug with built-in resistors. Voila! My tow lights work.

For the Sprinter to monitor the connectors, there is probably a high impedance voltage there.

Try it with a trailer hooked up and it will probably work.
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Old 06-15-2019, 02:34 PM   #7
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I had been reluctant to plug it in in case there was a cross wire issue. However I went ahead and plugged it in to my boat trailer (the toad is not wired yet - tomorrow’s project). Everything worked.

So it must be something with the MB system that allows some voltage to always be there.

Thank for everyone’s input. I am still going to run it by MB to see if there is an issue. If so I will post it back here.
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Old 06-16-2019, 12:33 PM   #8
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Quote:
Originally Posted by Pahaska View Post
The connectors are monitored by the Sprinter through the CAN bus. The high voltages you see probably very high impedance.

When I plugged in my transmitter for my wireless tow lights, the turn signals did not work unless I stepped on the brake pedal, as well. The transmitter did not pull enough current to let the Sprinter know it was there. I bought a plug with built-in resistors. Voila! My tow lights work.

For the Sprinter to monitor the connectors, there is probably a high impedance voltage there.

Try it with a trailer hooked up and it will probably work.
PASSINTHRU - I had similar experience a long while back when I was trying to use the 7 Pin connector/plug on my 2018 to test out the very teeny 2nd rear cam with also a very low current draw wireless transmitter as JOHN). The only diff is I remembered my voltmeter measurements were exact opposite of your experience,( i.e. I could not force a good few of them to go hot regardless of where I plugged the low-current wireless cam on); except I think on the terminal that is dedicated to be the (+ Hot). JOHN is right in that a lot of those terminals are monitored by the CANBUS. As soon as I plugged in a load with enough resistance that generated enough of a current draw, those terminals cam alive with their respected triggers (i.e. LF turn, RT turn, Stop, Reverse). Good luck
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Old 06-18-2019, 11:12 AM   #9
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Trailer Plug Wiring Problem

During March driving over a high-centered railroad crossing, the OEM Pollak Trailer Wiring Connector, and its bracket were torn off their location on the draw bar, and crushed. Note, that the location of the connector was originally poorly located, and that later AI models relocated the trailer plug well behind the bumper/hitch. This crushing resulted in the hot 12v shorting out, which blew the fuse in the #14 20a position in the fuse array under the driver's side seat, see photo, and also blew out the trailer wiring control unit, see photo, which is mounted in a rack inside the box which supports the driver's seat. Replacing the Pollak connector and the control unit did not restore the trailer wiring. As it is a MB system for US requirements, assistance from MB Sprinter Dealerships was sought in the Phoenix area, but was not immediately available as they had no idea how to go about repairing the issue, but would happy to take the AI in for lengthy diagnosis. As these dealerships are at a distance from us, and as much effort and $$ have been spent already, a MB/Chyrsler trailer wiring kit was purchased from the Sprinter Store for less than $200, and all is well again!
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