Trailer wiring - 7 pin to 4 pin problem
Thread Starter
Member
Joined: Jan 2008
Posts: 118
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From: Ottawa, Canada
2008 GL320CDI
Trailer wiring - 7 pin to 4 pin problem
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
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
Member
Joined: Apr 2008
Posts: 116
Likes: 1
From: Franklin,TN
2008 GL320cdi
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.
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Joined: Aug 2007
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From: Spicewood, TX
2008 GL 320, 2007 Silverado LTZ C3500 Duramax Turbo Diesel
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.
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.
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Joined: May 2008
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From: Terrell, Texas
2008 GL320CDI, C6 Corvette, 1969 Corvair Convertible
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.
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Joined: Aug 2007
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From: Spicewood, TX
2008 GL 320, 2007 Silverado LTZ C3500 Duramax Turbo Diesel
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.
Last edited by scottybdiving; Jun 13, 2008 at 12:45 PM.
Thread Starter
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Joined: Jan 2008
Posts: 118
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From: Ottawa, Canada
2008 GL320CDI
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
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
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Joined: Dec 2006
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From: Scottsdale, AZ
2007 GL320 CDI, 2001 TJ Rockcrawler, 2005 Scoobie STI
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.
Alternatively, if you Google "7-to-4 pin adapter" there are numerous online stores where you can order one to be shipped to you.
Member
Joined: Apr 2008
Posts: 116
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From: Franklin,TN
2008 GL320cdi
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.
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Joined: Dec 2008
Posts: 132
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From: Breukelin, NY
2009 GL450, 06 Triumph Speed Triple, 07 Triumph Speedmaster
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.
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.
Senior Member
Joined: Dec 2006
Posts: 405
Likes: 1
From: Scottsdale, AZ
2007 GL320 CDI, 2001 TJ Rockcrawler, 2005 Scoobie STI
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.
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.
Member
Joined: Dec 2008
Posts: 132
Likes: 0
From: Breukelin, NY
2009 GL450, 06 Triumph Speed Triple, 07 Triumph Speedmaster
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.
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 again.
Junior Member
Joined: Mar 2007
Posts: 15
Likes: 0
2009 GL450 2005 E320
I think this would work for the LED problem.
http://store.uhaul.com/Wiring__light...Trailer_Module
http://store.uhaul.com/Wiring__light...Trailer_Module
Junior Member
Joined: Dec 2008
Posts: 33
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From: Houston metro area
2010 E350 and 2008 GL320CDI
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 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?
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Joined: Jul 2002
Posts: 377
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From: .
08 GL450
Junior Member
Joined: Dec 2008
Posts: 33
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From: Houston metro area
2010 E350 and 2008 GL320CDI
Senior Member
Joined: Jul 2002
Posts: 377
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From: .
08 GL450
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From: Orbiting the planet
This place is a joke.
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.
Last edited by E55Greasemonkey; May 27, 2022 at 11:46 PM.
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Joined: May 2015
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From: Allentown, PA
'08 GL 320 CDI
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!
I mean, you must have searched for this thread to add that product link, right? Again, just curious!
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Joined: Jul 2012
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From: Orbiting the planet
This place is a joke.
Last edited by E55Greasemonkey; May 27, 2022 at 05:08 PM.
