I just towed my 6800lb (loaded), 32 foot Ultra-lite travel trailer over Mt. Hood with my 2010 GL450. No problem and smooth. I have a weight distribution hitch with sway bars, a brake controller, tires at 45psi, and extension mirrors. It's not as powerful going up the steep grades as my 2013 GL450 (with the twin-turbo) was when towing the same trailer, but it still was able to stay at 55mph with the AC on. Without the turbos the engine stays cooler so the fans stay on lower speed so it actually runs quieter. My 2013 was annoying loud sometimes with the fans trying to keep the hot turbos cool. Downhill braking on high speed corners is where bad towing setups get into trouble, and the 2010 GL450 did great there too. At the dump station of the campground, a guy walked up to me and asked if the Mercedes had airbags because he was surprised it wasn't squatting in the back. I explained the adaptive air suspension to him, and how I'd adjusted my trailer to reduce tongue weight. I felt like Han Solo when Princess Leia said "you came in THAT thing? You're braver than I thought". Han knew what the Falcon could do, and I know what the GL450 can do, even if people driving around in giant bouncy loud unrefined Dodge diesel trucks don't understand.

 

Hey forum,

I'd appreciate some help figuring out what went wrong towing with our new (to us) 2015 GL 450. I overheated the brakes descending a minor mountain pass. This was unexpected as we were well below the tow rating.

The setup was as follows:
2015 GL 450
Uhaul Utility Trailer with ramp: 2290 pounds
Kawasaki Mule: 1500 pounds
Internal load: 2 people + dogs + stuff: Estimate 800 pounds fully loaded including fuel

We towed from Novato to Calistoga via Petrified Forest Road. The descent into Calistoga is approximately 1 mile and 700 feet of elevation drop at a sustained 10%.

We were not going fast. There is no fast on that road. When we arrived in Calistoga we had to pull over to allow the noticeably overheating brakes to cool down.

Does anyone have any insight into what exactly went wrong here?

 

Usually anything over about 3000 lbs should have trailer brakes. Did the Uhaul trailer have surge brakes or not? And if it did, were they even working properly? I've towed lots of heavy trailers with both my X164 and X166, and they always have trailer brakes that I adjust to help take some of the braking load from the GL. Once I towed my 4000 pound boat without trailer brakes due to a wiring issue, I could feel the difference on downhill braking so I would strongly recommend trailer brakes for heavy trailers. And I overheated my brakes on my X166 once while towing 7000 pounds over a mountain, to the point where they were visibly smoking, on a long downhill grade in heavy traffic where I was constantly riding the brakes. After pulling over to let them cool, they were fine. I'm wondering if in your case, if the road was curvy, your GL was activating the rear brakes to control stability which can add extra brake heat to what you were already creating by using your foot. And remember when braking downhill, it's better to brake with short, firm periods of braking between sections of coasting instead of just constantly riding the brakes. It's not as smooth of course, but it gives the brakes time to cool off while coasting.

 

Trailer brakes are a must. I tow a 27 ft trailer with mine. I also put a set of cross drilled / slotted rotors on mine to help with heat issues.