Towing with the Mercedes-Benz Sprinter Van: It’s No Mule

I spent the last couple of updates telling you what our yearlong 2024 Mercedes-Benz Sprinter AWD crew van has been doing (beagles and fires), and I’ve neglected to tell you how the Sprinter has been doing, including that time out of service without a loaner, as noted in the spec sheet below. Most of this had to do with our trailer brake controller, but it also involved a couple of new axle shafts and a check engine light. I thought installing the controller would be simple. Man, was I ever wrong.

The Quest to Control My Trailer Brakes

For those unfamiliar, a trailer brake controller (TBC) is an electronic gizmo that triggers the electric brakes on a trailer. Most pickups and many jumbo-size SUVs now offer factory-installed TBCs, but for other vehicles, including the Sprinter, it’s an aftermarket add-on. While electric trailer brakes are common here in the States, European trailers favor surge braking, which uses a mechanical linkage on the trailer that responds to forward pressure on the tongue. Surge brakes require no equipment on the tow vehicles, which might explain why the Sprinter doesn’t offer a built-in TBC.

Most TBCs are all-in-one units that bolt to the dash, a deck-of-cards-sized box with a squeeze switch for manual brake application. Mercedes (which owns our Sprinter) asked us not to drill any holes in the dash but told us the easily replaceable upfitter switch blanks were fair game. We found a great solution: the stealthy RedArc Tow-Pro. The box with the Tow-Pro’s guts hides away behind the dash, with only a small control dial with a ring of status lights breaking the surface. Although the Sprinter isn’t on the list of supported vehicles, RedArc reckoned it would work just fine with the proper wiring adapter, and it sent us a Tow-Pro Elite model. (Retail price is $250; RedArc sent us the controller gratis for evaluation.)

I recruited news editor Justin Banner, an auto mechanic in his past life, to help with the installation (and by “help” I mean “do most of the work”). Banner fabricated a beautiful mount for the control dial while I set about Frankensteining the wiring harness. The Sprinter comes pre-wired for a brake controller, but I was unable to find a wiring harness for the 2020–2024 model, which should have been my first warning. I bought a wiring harness for a 2015–2019 Sprinter, which uses the same plug, then dug up a Mercedes wiring diagram showing the pinouts and set about figuring out which wires on the Sprinter corresponded to the ones on the Tow-Pro.

Once we got the wires all hooked up, we plugged in the trailer connection and got nothing. No lights, no click of applying brakes. We called RedArc tech support, and it patiently walked us through the process of testing the controller and checking the wiring on the Sprinter’s trailer socket with a test light. The RedArc folks determined the controller wasn’t getting the signal that indicates a trailer was attached and recommended a trip to the dealer to have the wiring checked. Simple, right? Not so fast, Charlie.

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Just Call Me Ingrid Bergman

The Sprinter went to Mercedes-Benz of Los Angeles, and a couple of days later I got a call from the service manager. Officially, he told me, the dealership doesn’t work on third-party brake controllers. Does Mercedes have a recommended brake controller they could install, I asked? Nope. While there are dealer-installed TBCs for several Mercedes SUVs, there is, officially, no supported TBC for the Sprinter.

This struck me as rather silly. Here’s a vehicle rated to tow 5,000 pounds, enough to warrant a brake controller. Mercedes advertises its towing abilities in a country where brakes are required for trailers over a certain weight (3,000 pounds in most states, 1,500 pounds in several). The Sprinter comes from the factory wired for a brake controller. But, according to Mercedes, there is no supported brake controller for the Sprinter. Anyone seen Gaslight recently?

Our friends at Mercedes PR seemed equally surprised by this development and helped us find another dealership willing to work on the controller. When I got the van back, the controller was indeed installed, but when I plugged the trailer in, I still got no trailer lights and no green light on the Tow-Pro dial. Worse, the techs had done a sloppy job reassembling the interior panels, the parking sensors no longer worked, and the check engine light was coming on intermittently.

I asked our PR contacts if I could return to the first dealership. They hadn’t fixed our original problem, but the service manager had been straight with me, and they hadn’t returned the Sprinter any more broken than it was when I dropped it off. I asked them to fix the broken sensors and check engine light and to look at the steering and the passenger door, both of which had been out of alignment since the van was new. I also mentioned that the parking brake was no longer holding, even if I yanked the daylights out of it.

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A Bright Idea About LEDs

As for the trailer, the service manager had come up with something: Did my trailer have LEDs or incandescent bulbs? LEDs, I told him, and he thought that might be the problem. Some LED setups don’t draw enough power for the Sprinter to detect that a trailer is plugged in, an issue for which Mercedes had a software fix.

The dealer had the Sprinter for the better part of two weeks (which included me taking it on a three-day furlough for a hay run), but when I got it back for good, the check engine light was off, the parking sensors all worked, the steering wheel was now centered when the van was going straight, and the parking brake held.

According to the dealer, the check engine light was due to an underhood wiring harness that was chafing against a bracket and causing a short circuit. The non-op handbrake was the result of faulty axle shaft seals, which had allowed differential oil to leak into the parking brake mechanism and contaminate the shoes. (The parking brake uses separate drums integrated within the brake rotors.) The mechanic replaced both rear axle shafts and the parking brake hardware. They verified that the passenger door was slightly crooked, but aligning it would require a trip to a body shop. While this wreaks havoc with my sense of order—underdeveloped as that may be—I didn’t want to be without the van long enough to have it repaired.

What was most important to me, though, was a working TBC. The paperwork said a software update had been applied and the lights checked out on their test rig. I hot-footed up to the ranch, plugged the trailer into the Sprinter’s seven-pin harness, and flipped on the parking lights. Nothing. But then I remembered that it took a hard shove to get the plug in far enough for the cover to latch down and hold it in place. I pushed with all the might I could gather, and the lights popped on. Up in the cab, a green ring around the RedArc controller dial showed the trailer was detected. At long last, we had trailer brakes!

Finally, I could tow with the Sprinter—but first I had the beagle rescue trip and a crap-ton of writing to do. November slipped away from me, and then December, and then the Los Angeles fires were upon us, and the Sprinter was forced into the role of tow vehicle, ready or not. Luckily, we did not have to evacuate our mules. I did have to take the empty trailer to the tire shop, and that seemed fine, but we really needed a mule on board to put the Sprinter to the test.

The Little Engine That Couldn't

Last time we tried to put our half-ton mule, Ruby, into the trailer, she said “Nothin’ doin’,” and this time she was only slightly more amenable. She walked into the trailer but was clearly not happy about it, shaking both the trailer and the van as she danced out her discontent.

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The Sprinter’s 2.0-liter turbodiesel I-4 (good for 208 hp and 332 lb-ft of torque) has always amazed me with its ability to shrug off heavy loads, but the turbocharged overachiever finally met its match in Ruby. The problem with live loads is that you can’t just floor the accelerator; all inputs must be gentle so that your four-legged passengers can balance themselves. Rather than our usual highway loop, we turned onto a long, steep, uphill street with a 45-mph speed limit. I got into the throttle very slowly and gently, getting the pedal to the floor just as the hill was at its steepest, and it was clear we weren’t going to get much past 40 mph. Even with a running start at the next hill, the van could just barely hold the speed limit.

The downgrades were worse. While the transmission kept the revs high even without help from the paddles, there wasn’t much engine braking to be had, just as one would expect from a diesel without an auxiliary exhaust brake. I was taught that the safest way to use friction brakes on a downgrade is to decelerate sharply to 10 mph below your target speed, let the rig accelerate, then brake again, but remember what I said about driving smoothly with live cargo? Obviously, riding the brakes is to be avoided at all costs, but that’s practically what I found myself doing.

“Let’s not try the freeway,” I said, and my wife, Robin, agreed that was a fine idea. Stability is usually our first concern; the tow vehicle needs to feel firmly planted and not let the trailer push it around. The Sprinter checked that box, though there was more bounce in the rear suspension, particularly at higher speeds, than we like. Power is usually our last concern, as we’re not looking to drive 85 mph with animals in tow, but in the case of the Sprinter, it was the lack of acceleration and engine braking that sealed the deal.

Finally, a Job the Sprinter Can’t Do

“It’d be fine with a boat, I think,” Robin said, “because you could accelerate and brake more aggressively. But there’s a big difference between dead loads and live loads.” I agreed: Over in Europe, where they have the option of back roads with lower speed limits (and where they sell horse trailers lighter than our 3,200-pound steel-and-aluminum Circle J), the Sprinter might be fine. Towing in the western United States means 60 mph over steep mountain grades, and the Sprinter’s not up to that job—at least not with two horses in the trailer. Or one mule.

Robin and I agreed: Like our previous long-termer, the Nissan Frontier, the Sprinter would an acceptable horse hauler for local trips to the vet or the fire evacuation center. But for long-distance travel, like an equine camping trip to the Central Coast, it would not be the best choice. A two-wheel-drive Sprinter with less suspension travel might quell the ride issues, but acceleration and braking would still be a problem.

So far, we’ve found lots of jobs the Sprinter van does better than a full-size pickup truck. Unfortunately, it looks like towing the horses across the American West ain’t one of ’em.

More on Our Long-Term 2024 Mercedes-Benz Sprinter:

• Our New Pickup Truck Is... A Van?
• Off-Roading in the Sprinter—There Is No Turning Back
• This Van Is a Pretty Great Pickup Truck
• Our AWD Sprinter is Kind of a Pain in the Ass
• A Mission From Dog
• Trial By Fire

Relatively stable with a (relatively) heavy trailer hooked on.