Howdy all,

I'm having trouble with my trailer wiring when I tow with my GL320. The car has the OEM 7-pin outlet, and my utility trailer has a 4-pin set-up wired to the 4-pin standard. I have a 7-4 pin adapter, but the trailer lights won't work, and the car keeps giving me trailer bulb fail warnings.

Funny thing is, my Volvo XC70 has a 6-pin harness, and I use 6-4 pin adapter that works fine. The trailer electrics also work with my neighbour's Colorado and 4-Runner.

What's my problem? Bad adapter? Scratching my head here.

Cheers,

BV

 

Your problem started me thinking. I just checked my new 7 pin to 4 pin adapter on my GL320 and it worked fine with my trailer. I would suspect the adapter in your case since the trailer lights work on you friends vehicles.

 

The wiring configuration is pretty universal for the 4 pin flat connector.

Green = right brake / turn
yellow = left brake / turn
brown = both tail lights
white = ground

There was a diagram of the MB 7 pin here somewhere in a related topic. You can check to see if they correspond or if you have a light tester, you can see if you are getting current before and after the adapter.

 

I had the same problem with my GL the first time I towed a trailer. Lights worked fine with my F250, but not with the GL, kept getting error messages. What I found was that the GL is much more sensitive to ground than the Ford was. I had to make sure I had a good ground wire on the trailer wiring harness back to the trailer, with a good connection, and then it worked fine.

 

That makes sense. It happens all the time when the ground wire on the trailer gets corroded or broken. Sometimes they will still ground through the ball, but you will see the lights flicker. The GL probably doesn't like that.

 

Thanks for the responses, guys. I'll go over all the grounds this weekend.

I tried to check the pins on the MB connector with a friend doing the brake / turn signal thing in the car, but some of the voltages seemed wonky. I think I'll try a new adapter and play with the grounds. Stay tuned, and thanks again.

Brad

 

Just a thought - the trailer has LED lights. Could it be that they don't draw enough juice and the car thinks they've failed?

 

This is a known problem with Cayannes, so I would not be shocked to learn that you are right.

 

Anyone know the best place to get the 7-to-4 pin adapter?

 

I can't speak about where in Breukelin, NY, but when I need something like this, I usually go to an auto parts store, or to a U-Haul store, or to a truck bumper sales shop... Here in Arizona we have a place called Quality Bumper that sells this kind of stuff.

Alternatively, if you Google "7-to-4 pin adapter" there are numerous online stores where you can order one to be shipped to you.

 

I've always thought of the 4-pin configuration as the "uhaul" configuration and that's where I got mine.

 

I discovered on my GL320 that you had to hold the flap way up while inserting the adapter in all the way. Then let the flap down. There is a lip on the flap that holds the adapter in. If you don't hold the flap up then the adapter hits the lip and won't go all the way in.

 

I guess I should have been more detailed with my question.

Is there a tried and true 7-to-4 pin adapter that GL owners have used (brand, model, etc)?

Additionally, is there on that works seemlessly with and LED light kit because I read on another post that there might be issues using LED instead of incandescent?

Thanks in advance.

 

Josey,

I believe an adapter is an adapter is an adapter... Most likely folks are just using what they could find locally.

Its not the adapter that has the issue with LED lights. Its the GL... Mercedes built in some functionality to auto detect when a trailer light is out and display a warning to the driver. This functionality is based on the fact that incandescent lights draw a certain amperage when they are in use... If not enough amperage is drawn, the "smart" GL things that a light is burned out.

LED lights, by nature draw much less amperage than their incandescent cousins... Thus the GL will think the light is burned out since it is drawing less amperage.

The only way around this is:

A.) Use only incandescent lighting on the trailer.

B.) Ignore the trailer light out warning.

C.) Wire in either a resistor or an incandescent light in series with each of the LED lights.

 

Thanks for the insight Danno. Very helpful. I guess for as little time that I will need this (motorcycle carrier as opposed to a trailer), I can live with the message. I thought that it would actually not power the LEDs.

Thanks again.

 

I think this would work for the LED problem.
http://store.uhaul.com/Wiring__light...Trailer_Module

 

I rented a Jetski over the Memorial Day weekend and had to borrow the owner's Wiring Adapter to convert the 7 pin wiring receiver on my 2008 GL320 to the 4-Flat pin plug that was on the Jetski trailer. He pulled it off of his BMW X5 and it worked fine on my GL.

I didn't get a chance to look at it but decided I should go ahead and purchase my own for future use. I found a site that sells these (http://www.etrailer.com/wiring-adapt...spx?vw=7-Blade) but not sure if the Mercedes 7-pin is a "7 Blade" or "7 round pin" type. Does anyone know?

 

I hate to say it, but Wal-Mart is the easiest place to pick up trailer stuff like adapters, *****, etc.

 

"Wal-Mart" and " trailer " ...seems I've heard the two used together before .

 

http://www.peopleofwalmart.com

 

 

Product Description

Part Number:39008For use with European model vehicles.

• Allows VW's, Mercedes, Porsche's and Audi's to pull a trailer with LED Light System.
• Use only with trailers equipped with LED lights.

 

Grease, this kind of thing always interests me - I'm curious... how/why did you end up posting on a 12-year old thread? I mean, you must have searched for this thread to add that product link, right? Again, just curious!

 

Ya, old thread.. found it in a Google search in the midst of 3 hours trying to rent a U-Haul trailer this morning, sweating in the hot sun under the rear bumper..burning $5.25/gal gas driving around and buying different 7 to 4 pin adapters, and finally happened to find someone at the local MB dealer who knew what I needed from U-Haul... Hoping the picture will help someone in the future. Although there was a link earlier in the thread I overlooked it and there was never any resolution posted until now. This was for my 2008 ML350, and that U-Haul adapter allowed their trailer lights to work with my factory setup.