Able to add trailer hitch to GL?
Junior Member
Joined: Mar 2021
Posts: 54
Likes: 4
2015 GL350 Bluetec
Thank you Jeff. Always helpful. I appreciate all your support.
My Acura MDX has 12V on pin-4 once the ignition is ON. The GL has 12V on pin-4 even with the ignition OFF. I know that some cars do have live 12V on pin-4 even with the ignition OFF, yet I didn't expect that the GL (German car) will do so. I can't think of any way I messed up with the wiring. Everything goes on the trailer recognition box so maybe when the car gets programmed it will not always have 12V on pin-4 anymore.
I am only worried that it might drain the battery if something went wrong with the harness cabling.
L-L
My Acura MDX has 12V on pin-4 once the ignition is ON. The GL has 12V on pin-4 even with the ignition OFF. I know that some cars do have live 12V on pin-4 even with the ignition OFF, yet I didn't expect that the GL (German car) will do so. I can't think of any way I messed up with the wiring. Everything goes on the trailer recognition box so maybe when the car gets programmed it will not always have 12V on pin-4 anymore.
I am only worried that it might drain the battery if something went wrong with the harness cabling.
L-L
MBWorld Fanatic!
Joined: Oct 2009
Posts: 6,987
Likes: 2,187
From: Colorado
'21 AMG53 wDPP & ARC, 19 GLC300 - Former-03 C240,2 ML BlueTecs,20 GLE450 E-ABC,15 Cayenne D,17 Macan
Thank you Jeff. Always helpful. I appreciate all your support.
My Acura MDX has 12V on pin-4 once the ignition is ON. The GL has 12V on pin-4 even with the ignition OFF. I know that some cars do have live 12V on pin-4 even with the ignition OFF, yet I didn't expect that the GL (German car) will do so. I can't think of any way I messed up with the wiring. Everything goes on the trailer recognition box so maybe when the car gets programmed it will not always have 12V on pin-4 anymore.
I am only worried that it might drain the battery if something went wrong with the harness cabling.
L-L
My Acura MDX has 12V on pin-4 once the ignition is ON. The GL has 12V on pin-4 even with the ignition OFF. I know that some cars do have live 12V on pin-4 even with the ignition OFF, yet I didn't expect that the GL (German car) will do so. I can't think of any way I messed up with the wiring. Everything goes on the trailer recognition box so maybe when the car gets programmed it will not always have 12V on pin-4 anymore.
I am only worried that it might drain the battery if something went wrong with the harness cabling.
L-L
I also think there are diodes in the source, to prevent charging from the trailer circuit to the car. I.e. one-way flow of electricity.
If that's not the case, you may have some aftermarket wiring going on, and it's worth checking.
Junior Member
Joined: Mar 2021
Posts: 54
Likes: 4
2015 GL350 Bluetec
Both harnesses are new, OEM Mercedes, bought from local MB dealership. No aftermarket wiring for sure.
If pin-4 shouldn't have 12V with the ignition switch OFF, then it is probably something wrong with the Trailer Recognition Module. Either because the car is not coded yet, or the module is faulty.
The communication goes like this:
CAR harness (brake controller/fuses-F4/SSAM/CAM) <---------->Trailer Recognition Module <---------> Hitch harness (7-pin)
Except a green plug that has the yellow/red cable (power to trailer brakes from brake controller) and black/red thin cable (SAM)
So basically, all signals/wires (except the yellow/red and black/red thin cable), to the 7-way plug come from the Trailer Recognition Module. If 12V just passes through the "trailer recognition module" to the 7-way plug, then I shouldn't have battery drain issues, because basically there is nothing to use power (just like the 12v aux sockets in the car). If it is via the electronic circuitry of the module, (meaning the module is always ON), then I will have battery drain issue. That is why I am worried.
Lights and signals don't work either, yet I noticed that the lights on my 7-way tester are LEDs, so maybe it is just the known MB issue with trailers having LED lights.
Also, I plugged in my Prodigy P3 brake controller, and when I apply the brakes manually, (using the lever on the brake controller), the "trailer brakes" LED on the 7-way tester lights up, meaning the module responds to the signals from the brake controller....yet, the car is not coded with option [550]. (towing pack option). I guess this is normal though, since the yellow/red cable runs from the brake controller, directly to the 7-Way hitch plug, bypassing the Trailer Recognition Module.
The car is going to MB today for the AEM recall as well as for the tow pack programming...I think by Friday I should have it back, so we will know about the pin-4/12V concern.
L-L
If pin-4 shouldn't have 12V with the ignition switch OFF, then it is probably something wrong with the Trailer Recognition Module. Either because the car is not coded yet, or the module is faulty.
The communication goes like this:
CAR harness (brake controller/fuses-F4/SSAM/CAM) <---------->Trailer Recognition Module <---------> Hitch harness (7-pin)
Except a green plug that has the yellow/red cable (power to trailer brakes from brake controller) and black/red thin cable (SAM)
So basically, all signals/wires (except the yellow/red and black/red thin cable), to the 7-way plug come from the Trailer Recognition Module. If 12V just passes through the "trailer recognition module" to the 7-way plug, then I shouldn't have battery drain issues, because basically there is nothing to use power (just like the 12v aux sockets in the car). If it is via the electronic circuitry of the module, (meaning the module is always ON), then I will have battery drain issue. That is why I am worried.
Lights and signals don't work either, yet I noticed that the lights on my 7-way tester are LEDs, so maybe it is just the known MB issue with trailers having LED lights.
Also, I plugged in my Prodigy P3 brake controller, and when I apply the brakes manually, (using the lever on the brake controller), the "trailer brakes" LED on the 7-way tester lights up, meaning the module responds to the signals from the brake controller....yet, the car is not coded with option [550]. (towing pack option). I guess this is normal though, since the yellow/red cable runs from the brake controller, directly to the 7-Way hitch plug, bypassing the Trailer Recognition Module.
The car is going to MB today for the AEM recall as well as for the tow pack programming...I think by Friday I should have it back, so we will know about the pin-4/12V concern.
L-L
Last edited by L-L; Mar 30, 2021 at 10:37 AM.
Junior Member
Joined: Mar 2021
Posts: 54
Likes: 4
2015 GL350 Bluetec
Hello All,
The car is at the MB service center for the emission recall as well as for the Trailer pack coding.
They just emailed me that the technician reached to Mercedes and they verified that no coding is necessary!! "From what the tech has gathered it would only need to be coded if I had a brake assist module added as well."
Do you guys know what code(s) need to be coded/enabled for the car to understand that the towing kit has been installed?
Thank you
The car is at the MB service center for the emission recall as well as for the Trailer pack coding.
They just emailed me that the technician reached to Mercedes and they verified that no coding is necessary!! "From what the tech has gathered it would only need to be coded if I had a brake assist module added as well."
Do you guys know what code(s) need to be coded/enabled for the car to understand that the towing kit has been installed?
Thank you
Junior Member
Joined: Mar 2021
Posts: 54
Likes: 4
2015 GL350 Bluetec
Correct...it is just 550. Local MB figured it out eventually.
The car recognizes the trailer. Also when I have the trailer hooked up, it disables the Distronic Plus or some fuctions I believe.
When it comes to comand and reverse, the trailer option wasn't showing. I had to go to the hidden functions menu and enable the "RVC Trailer" option. I had to wait 5 minutes for the coding to become active. It then showed the option to choose the trailer camera view, but for some reason it did not change the view (guide lines) on the screen...I am wondering if GLs without the factory towing pack have different reverse camera..
The car recognizes the trailer. Also when I have the trailer hooked up, it disables the Distronic Plus or some fuctions I believe.
When it comes to comand and reverse, the trailer option wasn't showing. I had to go to the hidden functions menu and enable the "RVC Trailer" option. I had to wait 5 minutes for the coding to become active. It then showed the option to choose the trailer camera view, but for some reason it did not change the view (guide lines) on the screen...I am wondering if GLs without the factory towing pack have different reverse camera..
Newbie
Joined: Oct 2021
Posts: 1
Likes: 0
2013 GL450
We purchased a 2013 GL450 about a year ago for the comfort, ride, power and size. Now, we are looking at adding a 2-axel travel trailer to the mix. I am new to towing and just joined the forum and discovered a lot of the information I have been digging around the internet to find. I was under the impression that installing the OEM hitch and brake controller was all that was needed but could not find any wiring diagram to find/mount said components. Then I find several hitch options, most without the sway supports, wiring, bolts or directions. I was so confused and flustered and found the information provided here has been most helpful in answering a lot of my questions. Thank you all for your contributions.
If you have any suggestions before I start this endeavor, please share.
If you have any suggestions before I start this endeavor, please share.
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Member
Joined: Mar 2021
Posts: 125
Likes: 11
From: Boston, MA
07 GL450
Did you guys see this? It also comes standard on some GL's. Which ones I wonder?
https://www.youtube.com/watch?v=QUvSKIyTtuA
https://www.youtube.com/watch?v=QUvSKIyTtuA
Newbie
Joined: Nov 2021
Posts: 2
Likes: 0
2014 GL 450
bike rack only
I have done a lot of reading on this thread and others, and wanted to install a tow hitch receiver to support a bike rack only (no towing). Two eMTB weighing about 55lb each.
So the question is, can I install the OEM Tow Hitch WITHOUT the long member braces (https://mbworld.org/forums/gl-class-...ml#post7076722) since I won't be towing? Can the 8 bolts on the frame handle the bike rack weight and bikes? I imagine it should be fine, especially since the Stealth Hitch option didn't have those braces (I have the Stealth Install PDF instructions if anyone wants them).
A Uhual/Curtis hitch (https://www.curtmfg.com/part/13190) would have been ideal, but according to the Curtis website (and visual inspection of the bottom of the car) I can't install that hitch because our model has "Active Curve System (SPC)", which gets in the way of the "Mounting Strap". Though, I wonder if that Curtis hitch would support a bike rack without the "Mounting Strap", since there won't be the horizontal forward/back load from the usual towing scenario. What do you guys think? I don't know how strong that mounting area is since it differs from the OEM hitch install. Curtis install instructions: https://assets.curtmfg.com/masterlib.../13190_INS.pdf
Thanks!
Thanks for the feedback!
So the question is, can I install the OEM Tow Hitch WITHOUT the long member braces (https://mbworld.org/forums/gl-class-...ml#post7076722) since I won't be towing? Can the 8 bolts on the frame handle the bike rack weight and bikes? I imagine it should be fine, especially since the Stealth Hitch option didn't have those braces (I have the Stealth Install PDF instructions if anyone wants them).
A Uhual/Curtis hitch (https://www.curtmfg.com/part/13190) would have been ideal, but according to the Curtis website (and visual inspection of the bottom of the car) I can't install that hitch because our model has "Active Curve System (SPC)", which gets in the way of the "Mounting Strap". Though, I wonder if that Curtis hitch would support a bike rack without the "Mounting Strap", since there won't be the horizontal forward/back load from the usual towing scenario. What do you guys think? I don't know how strong that mounting area is since it differs from the OEM hitch install. Curtis install instructions: https://assets.curtmfg.com/masterlib.../13190_INS.pdf
Thanks!
Thanks for the feedback!
Last edited by TaylorN; Aug 22, 2023 at 12:32 PM. Reason: more detail
Newbie
Joined: Nov 2021
Posts: 2
Likes: 0
2014 GL 450
I have done a lot of reading on this thread and others, and wanted to install a tow hitch receiver to support a bike rack only (no towing). Two eMTB weighing about 55lb each.
So the question is, can I install the OEM Tow Hitch WITHOUT the long member braces (https://mbworld.org/forums/gl-class-...ml#post7076722) since I won't be towing? Can the 8 bolts on the frame handle the bike rack weight and bikes? I imagine it should be fine, especially since the Stealth Hitch option didn't have those braces (I have the Stealth Install PDF instructions if anyone wants them).
A Uhual/Curtis hitch (https://www.curtmfg.com/part/13190) would have been ideal, but according to the Curtis website (and visual inspection of the bottom of the car) I can't install that hitch because our model has "Active Curve System (SPC)", which gets in the way of the "Mounting Strap". Though, I wonder if that Curtis hitch would support a bike rack without the "Mounting Strap", since there won't be the horizontal forward/back load from the usual towing scenario. What do you guys think? I don't know how strong that mounting area is since it differs from the OEM hitch install. Curtis install instructions: https://assets.curtmfg.com/masterlib.../13190_INS.pdf
Thanks!
Thanks for the feedback!
So the question is, can I install the OEM Tow Hitch WITHOUT the long member braces (https://mbworld.org/forums/gl-class-...ml#post7076722) since I won't be towing? Can the 8 bolts on the frame handle the bike rack weight and bikes? I imagine it should be fine, especially since the Stealth Hitch option didn't have those braces (I have the Stealth Install PDF instructions if anyone wants them).
A Uhual/Curtis hitch (https://www.curtmfg.com/part/13190) would have been ideal, but according to the Curtis website (and visual inspection of the bottom of the car) I can't install that hitch because our model has "Active Curve System (SPC)", which gets in the way of the "Mounting Strap". Though, I wonder if that Curtis hitch would support a bike rack without the "Mounting Strap", since there won't be the horizontal forward/back load from the usual towing scenario. What do you guys think? I don't know how strong that mounting area is since it differs from the OEM hitch install. Curtis install instructions: https://assets.curtmfg.com/masterlib.../13190_INS.pdf
Thanks!
Thanks for the feedback!
