Mercedes/Trailer Lights Compatibility Issue

Question

Hi all,&nbsp;I'm having a strange electrical issue and would be grateful for some advice.&nbsp;We recently purchased a used boat trailer and a 2020 Mercedes GLS 450 suv to tow it. The Mercedes has a 7-pin plug and the trailer light system is 4-pin. I bought a 4-pin to 7-pin adapter from Curt. With the adapter, the trailer lights come on, but only solid red. The turn, hazard and brake signal functions do not work.&nbsp;The trailer is pretty old and corroded, so I had the lights replaced with a new Kenway system from Harbor Freight. Same result, lights burn solid, won't signal. So to test the system I plugged the trailer into my Tesla's 7-pin plug, and everything worked correctly.&nbsp;We had a trip coming up in a few days, so in a desperate move, I bought a magnetic clip on trailer light system (also Kenway) and...everything works correctly with the Mercedes using the magnetic system. The magnetic lights got us through the trip, but taking them on and off during the launch/haulout is an extra step I'd be happy to skip if possible.&nbsp;So basically both the Mercedes and the trailer lights work correctly, they just don't work together. I'm assuming there is some kind of voltage/resistance compatibility issue between the Mercedes and the trailer, but don't really know where to start. If anyone has suggestions on what to try next, I'd appreciate it!&nbsp;Thanks,&nbsp;David&nbsp;

optimistic · Answer

Thanks way2roll. I'll try that one next!&nbsp;

way2roll · Answer

Not sure what this has to do with RVs. Assuming you're coming here in case someone has a similar issue. I had the same problem with my Mercedes based motor home and LED wireless lights on our toad. Tow lights require a certain voltage. The Mercedes equipped with LEDs sends too little voltage to the trailer wiring harness and they don't behave as a result. Sounds like you need a resistor adapter. It will trick your trailer lights into getting the proper voltage to work. Worked in my case anyway.

optimistic · Answer

I know what you mean.&nbsp; However the guy who installed the new lights on the trailer tested the LED's alone before connecting the running lights into the system, and it was the same deal, solid illumination but no blinking, while the Tesla was able to work correctly.&nbsp; Not clear to me why can my Tesla handle that setup but the Mercedes can't.&nbsp; Too bad my Tesla can't pull a 5000# boat trailer combo!Do you happen to know the name/model of adapter you had success with on your Mercedes&nbsp; RV?

way2roll · Answer

Here's what I bought.https://www.etrailer.com/Custom-Fit-Vehicle-Wiring/Tow-Ready/20142.html?feed=npn&amp;gad_source=1&amp;gad_campaignid=753605683&amp;gbraid=0AAAAAD_vMtlPL_Y6A_xDbav8OevXl2muD&amp;gclid=EAIaIQobChMIvsz44_bGjwMVy0Z_AB004DLmEAQYByABEgKkdPD_BwE

blt2ski · Answer

Not saying this is the case, but "SOME" manufactures use a separate wire for the brake and turn signals per side vs the left side pole being both brake and turn, same with the right side. I had this issue with a few Japanese - Toyota and Nissan trucks in the past. I needed an adapter that made the wires combine to work. Despite having an ML320 for 20 years, never wired it nor towed with it, I don't know if their wires are like the ones I described.

The adapter is rather easy to find at any auto/RV parts outlet.

Marty

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Mercedes/Trailer Lights Compatibility Issue

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