Ford advertises the 2021 Ford Bronco Sport as an off-road beast, boasting of “unrelenting Bronco capability,” and saying that the new Ford Escape-based SUV “delivers the 4x4 off-road capability that made the original Bronco a legend.” Plus: “For maximum off-road capability and long-term durability, Bronco Sport was tested in extreme conditions.” But when it came time for journalists to test the new baby Bronco late last year, multiple vehicles experienced four-wheel drive system performance degradation due to overheating. Here’s what we know about this scenario.
In my 2021 Ford Bronco Sport review, I mentioned how, during my test drive, a journalist on one of Ford’s specially chosen off-road trails experienced a four wheel-drive overheating episode.
“While on the trail ride in a group, the driver of an Outer Banks model attempted to ascend a hill,” I wrote in my generally positive piece. “After just a couple of failed attempts to climb the rather modest incline, she said over the radio that her vehicle indicated Four-Wheel Drive Temporarily Disabled.” This message, per the Bronco Sport’s owner’s manual, indicates an overheating condition:
After Ford’s rep at the press event noted that the Outer Banks model the journalist was driving didn’t have a liquid-cooled Power Transfer Unit (basically a gearbox that apportions power between front and rear axles) like the more hardcore Badlands trim does, the journalist let the vehicle sit for a few minutes. The four-wheel-drive system seemed to recover and function well, successfully managing the climb. Here’s video of that ascent:
In my story, I also mentioned Emme Hall’s Bronco Sport review on CNET—a review in which the seasoned off-road driver alluded to what seemed like a similar incident. This happened while driving the Badlands model (which, it’s worth highlighting, does have a liquid-cooled PTU that pulls coolant from the engine cooling loop post-radiator, per a Ford engineer) in moderate temperatures. Hall’s remarks from that review below:
Unfortunately, after 15 minutes of hooning around in the sand, my Bronco Sport overheats and goes into limp-home mode. I might expect this if ambient temperatures were very high, but it’s a perfect 82 degrees Fahrenheit. Thankfully, the Bronco cools back down quickly so I can keep on playing — albeit a bit less aggressively.
I chatted with Hall about this shortly after her review published. She told me that she got a warning about operating temperatures having been exceeded, and that the car should be shut down.
“I don’t think that mine actually said four-wheel drive,” she told me. “Something got too hot and it went into limp mode.” That all went down at Olancha Dunes roughly 200 miles north of Los Angeles. “It’s not a super dune-y steep area. It’s pretty small,” Hall told me, though she did admit that she was “driving [the Bronco Sport] like a freaking idiot.”
The vehicle was in sand mode, the throttle was on the floor and Hall was “just not being gentle with [the car] at all.” It took roughly 15 minutes for the car to give the overtemp warning. Hall shut the machine off, and after five minutes it was ready to go again.
Hall told me that she didn’t expect to have this issue. “I would expect maybe something like that to happen if it were super hot outside, but it was not. It was perfect.”
With two unrelated incidences of apparent four-wheel drive system overheating (though admittedly, we don’t know for sure that it was the 4x4 system overheating in Hall’s case, though I bet it was a drivetrain issue based on the situation) in what seem like relatively mild off-road conditions, I reached out to Ford to learn more.
That was a few months ago, so this story is hardly timely, but let’s talk about it anyway.
After I published my review back in December, the Ford Bronco Sport forum began buzzing, with fans growing concerned over the off-roader’s potential vulnerabilities. People wanted to know more about what was going on, and so did I.
My discussion with Ford didn’t really get to the root of the issue, largely because the vehicle engineering manager, Eddie Khan, denied that there was an issue at all. And to be frank, he wasn’t technically wrong, because the concept of a vehicle “failure” can often be highly subjective. I’ll dig into that thought further in a sec, but for now, I’ll just download the contents of my discussion with Khan.
Khan mentioned three main points: The first was that the 2021 Ford Bronco Sport has met Ford’s internal targets, which are based on projected customer usage. The second is that Ford has made sure that the Bronco Sport is competitive with other vehicles in its class. And third (this one is related to the others) is that Ford has done extreme testing on the vehicle, putting it through conditions far more strenuous than the vast majority of customers will see. All of these are fairly standard in any vehicle development process.
Whether we should consider the aforementioned overheating episodes as failures relies in large part on expectations. As an experienced off-roader and former Jeep engineer, I found the hill where the preproduction Bronco Sport Outer Banks struggled to be a rather mild climb.
”First of all, that’s not a small hill climb,” Khan told me in our interview. When I challenged that, he responded with: “Would you do that in a normal vehicle?” I responded that this isn’t a normal vehicle, it has “Bronco” in the name. Khan agreed, though the discussion seemed to get a little tense from there, as we weren’t in agreement on the severity of the driving condition.
In any case, all of this is subjective, but it does give us an idea of how my expectations may have differed from Ford’s when it developed this SUV.
What were my expectations based on? I’m not sure. Possibly Ford’s marketing, possibly my own hopes and dreams, and likely my experiences off-roading older vehicles that use transfer case-based four wheel-drive systems — a design that tends to be more thermally robust than clutch-based systems like the one in many modern front-wheel-drive-based off-roaders. Damn near any old-school 4x4 could have climbed that grade 1,000 times without overheating its four wheel-drive system (maybe the transmission would have gotten hot after a while, but I doubt it’d have been a big deal), and that includes the old Bronco. So the whole “delivers the 4x4 off-road capability that made the original Bronco a legend” line is a hard one to buy.
Though the Bronco Sport shares its “C2" architecture with the decidedly not-off-road-oriented Ford Escape and Lincoln Corsair, there are important differences. “We geared and sized it up to the needs of a Bronco Sport duty cycle,” Khan told me, saying the clutch size and number of clutches in the four-wheel drive system are higher than in the Escape, and that the shorter final drive ratio, along with changes to the halfshafts and driveshafts, are all important factors in off-road capability and durability.
“We sized our system very generously to have a lot of capacity to ensure the robustness,” the vehicle engineering manager told me.
Styling and off-road capability all meant significant compromises in fuel economy and on-road manners when compared with the Escape—compromises that Car and Driver enumerates in its comparison and that I mentioned in my own review. These compromises could make customers think: Does the styling matter that much to me? And am I getting enough capability to offset the drawbacks versus the Escape? Let’s get into the latter some more.
You’ll find plenty of media showing the Ford Bronco Sport undergoing off-road testing, and indeed, Khan said his team put the machine through the wringer. Khan said that thermal issues did not arise in testing and that the company takes the 1.5-liter base model everywhere it takes the 2.0-liter models other than on the “extreme hill climbs.”
The Bronco Sport, he told me, has been to off-road parks all around the country, including Borrego, Johnson Valley, Moab and of course, Ford’s own proving grounds. The Blue Oval’s team has done testing on various surfaces and different inclines, with Khan telling me about the latter: ”I get nervous in those hills sometimes.”
Search the web and you’ll find all sorts of literature on how Ford tested the Bronco Sport. “Ford Torture-Tested the 2021 Bronco Sport Off-Road Just Like the Big Bronco,” claims an article from The Drive. The story’s lede definitely makes the little Escape-based SUV seem impressive:
Possibly the only vehicle under more intense scrutiny than the newly revealed 2021 Ford Bronco is its diminutive doppelgänger, the Bronco Sport. For the first time in its history, the Bronco badge will be found on a unibody SUV—one which could easily sully the Bronco name if it didn’t meet the public’s expectations. Ford knows this, so to ensure the Sport exceeds expectations, it ran the junior model through many of the same extreme testing gauntlets as its larger, truck-based sibling.
Though he wouldn’t tell me the extreme use cases that drove the sizing of the vehicle’s four-wheel drive system clutches and cooling system, Khan said the Bronco Sport was put through much more extreme driving conditions than what the media saw on its press drives (“We were running much...harder than those journalists,” he said), and he also mentioned the Rebelle Rally as an example that proves the vehicle’s toughness. “All-New Bronco Sport Nabs Off-Road Win at Grueling Rebelle Rally,” Ford’s press release reads. Here’s an image of one of the Bronco Sports in action during that event:
Ford’s “Go Over Any Type of Terrain” modes, or GOAT modes, alter the way the vehicle puts power to the ground, because the modes adjust engine output and brake application. With four-wheel drive clutch temperatures highly contingent on wheel speed and engine operating point (which itself is a function of grip and overall vehicle demand/driver input), it’s clear that drive modes can have effects on a four-wheel drive system’s thermal performance.
“Maybe the user was not in the right mode, and that could have created different slip conditions and different thresholds,” Khan was sure to note. “When we tune these systems, we did make sure that you are in the right drive modes,” he told me, saying his team clearly sees significant differences in off-road performance in different GOAT modes. “If you tend to deviate from the intended operating conditions that we recommend you to use...then you can have that [derate] situation.”
OK, let’s talk about whether these overheating events constitute a problem. To answer that, we’ve got to talk about vehicle-level functional objectives. These define what the vehicle must be capable of in order to satisfy the customer.
Often, it’s product planners and members of a “brand team” who hand down objectives/directives to the engineering team about how the vehicle should be able to perform off-road. The engineering team translates those directives into engineering requirements like drive cycles and other scenarios that the vehicle must be able to handle. Engineers then develop the vehicle based on these engineering requirements, using simulation tools and physical testing for validation.
So that brings us back to the question of whether these Ford Broncos are failing. I think the answer to that can be looked at in three different ways.
1. Does the vehicle meet the vehicle level functional objectives that Ford set? FoMoCo says yes.
2. Does the Ford Bronco Sport compete favorably with the competitive set? Ford says yes, but independent off-road comparisons will be the judge of this. Those have the potential to play a big role in shaping the Bronco Sport’s reputation.
By the way, the vehicle’s “competitive set” (i.e. what Ford used as benchmarks) include: Jeep Compass and Cherokee (Sport up to Trailhawk trims) and some Subarus. Ford, Khan told me, also looked at the Toyota Rav4 and Range Rover Evoque.
It’s also worth showing the video below to drive home just how difficult “split mu” scenarios are for clutch-based all-wheel drive systems (you can read all about such systems in our all wheel-drive explainer). When one axle has significantly more traction than the other, and the grade is steep, getting the vehicle to ascend the grade will often cause lots of slip in the clutches in the PTU, which are trying to couple the front axle’s rotation with that of the rear axle. Slipping clutches means heat, and heat means, well, lots of warning messages on the dash and an overheat-protection mode that some might call “derate.” This is quite common for vehicles in this class:
3. Most important: Are customers satisfied with the vehicle’s capability? Answering this can be tricky, but a key to satisfying customers is making sure that their expectations are met. Do the few customers who actually go off-road think the Ford Bronco Sport should be able to tackle more than it’s actually capable of? Were customer expectations exceeded or were they not met? And if the latter, did Ford over-market the vehicle by using some of the phrasing I mentioned in my opening paragraph, and by calling this vehicle a Bronco in the first place?
Some of this stuff will take time to answer. More off-road comparisons will have to reveal how the Bronco Sport competes with the likes of the Jeep Compass and Cherokee on the sand dunes. And as more people buy Bronco Sports and take them off-road, we’ll learn more about how customer expectations for the vehicle’s off-road skills align with reality. From what I’ve seen just browsing forums, most folks seem fairly happy, as do most journalists who’ve had time behind the wheel.
So are these Bronco Sports — which are going into a self-protect “derate” mode to prevent component damage when temperatures get a bit too high — failing? The boring answer is that “It all depends on what the expectations are.”
I reached out to a bunch of engineers well-versed in vehicle drivetrains to get their opinions on these two Ford Bronco Sport overheating incidents. Before we get into that, I’ll point you all to a Society of Automotive Engineers article titled “Dana’s twin-clutch axle boosts Bronco Sport’s off-road cred.” The story shows the system’s basic layout (see above). You’ll notice that the PTU allows for a clutch to disconnect the rear driveshaft from the front axle. Two other clutches in the RDU apportion power left-to-right. One paragraph from that story addresses thermal capabilities of the four-wheel drive system. From SAE:
The development team, co-led by Kahn and program chief engineer Adrian Aguirre, put a premium on robustness. They ran duty cycles beyond Ford’s standard and tested in a variety of off-road environments and events, including Borrego Springs (Calif.) and Moab. Bronco Sport’s thermal reliability is ensured with a trans cooler, a PTU cooler and an engine cooler. There also is what Kahn calls “a very advanced” predictive thermal-management system that will intervene without causing a loss of 4wd capability.
The rest of the story is mostly about the rear drive unit/rear drive module, shown below:
Go ahead and check out that SAE story, but with that out of the way, let’s get to some input from other automotive engineers. This first bit of wisdom comes from an axle engineer working for a competing major automaker. The math-and-science whiz mentions that the vehicles I mentioned were clearly going through a derate, which was designed to protect components from damage. The engineer notes that the vehicle could likely have performed better had it had larger clutches that could better handle the speed disparity between the front and rear axles:
The thermal issues you described seems most likely a thermal protection algorithm. The fact that it “cooled down” so fast makes me think it never really got the bulk oil temperature very high. That takes a looong time to cool. But the algorithm (whether based on an actual temperature sensor or just an energy model looking at clutch pressure and slip speed) did what it was supposed to do, protect the parts from permanent damage. The fact that they added a liquid cooler to the systems suggests they didn’t size the parts to do this job on the regular. Seems like this is something that we will read about more in the near future. It’s too late for Ford to make the parts bigger and if you give someone a vehicle with a Bronco badge on it you better believe some of them are going to expect it to actually do stuff off road.
Here are some words from another engineer well-versed on all wheel-drive:
Simply put the coupling they are using for the all wheel drive is sized too small for the kind of abuse you put it through. Here’s what I experienced with Ford on a trip to Silver Lakes Sand Dunes sometime around 2010. They invited us out with our just then released Jeep Grand Cherokee two speed active t-case for some fun on the dunes. They brought along a bunch of vehicles for their execs to drive including the then pretty new front wheel drive based Explorer. They also had some other competitive vehicles, I can’t remember them all now but there was some flavor of GM full size SUV and probably something from the Asian OEMs.
The Explorer and the GM vehicle had the exact issue you are discussing. After a few runs up a dune, or just bombing around in the sand both vehicles would call it quits with sending torque to the rear axle and the vehicle would be stuck until you waited it out. Talking with the Ford Engineers they said that the rear coupling only had a small capacity, 900 Nm comes to mind but I can’t remember the exact number. Basically it didn’t have the capacity to fully lock the drivetrain front to rear in all conditions, and in conditions such as the sand dune there was a fair amount of slip across the clutches. Their software would monitor the slip and torque across the clutch and call it quits to save the clutch after a certain amount of heat was generated. The idea being that for 99% of their customers the reduced clutch capacity would be fine. Big cost savings when you consider how many vehicles they planned to produce. And your average soccer mom will never come close to overheating the coupling driving to the mall or Starbucks.
Jeep on the other hand, as you know, was and still probably is very concerned with their off road image and sized their couplings appropriately for all kinds of abuse. Also take into account the torque coming out of the low range planetary and you need a mighty big clutch. The Grand Cherokee never skipped a beat all day playing in the sand, it became a crowd favorite as there was no worry about having to wait for the coupling to cool so you could continue.
This driveline engineer goes into detail on why a loose hill climb like the one I witnessed leads to heat buildup in the PTU or RDU:
Low grip, forward hill ascents are one of the hardest scenarios for drivelines that are FWD primary/permanent drive with clutched couplings to the rear, because they’re inherently limited in how much torque they can divert to the rear axle. In this driving scenario, the weight distribution of the car is heavily biased to the rear because you’re pointing up the hill, and so you want to divert as much torque to the rear axle as possible because it has way more traction available than the unloaded front axle. However, the amount of torque you can bias rearward is limited by the driveline clutches - whichever is the weaker out of the one on the PTU at the front, or the one(s) on the rear drive unit (RDU).
When you’re driving up the hill, firstly you’d hope the control systems will be smart enough to request maximum locking torque from the clutches. This means that up to the torque threshold of the clutches, torque can move between the front and rear axles naturally, as if you had a locked centre diff on a “proper” 4x4. Where it gets difficult is once the front axle has no more grip available, and the driveline isn’t capable of sending more torque rearward. At this point, the front tyres can’t react enough torque to keep the car moving, and the powertrain torque exceeds the amount that the clutches can transmit to the rear. The power has to go somewhere, and so the front wheels slip, and at the same time so does the PTU and/or RDU clutch. That clutch slip is occurring at the same time that the clutch is at maximum torque transfer. Clutch energy is Torque * Slip so there’s now suddenly a load of energy to dissipate as heat.
You don’t want to damage friction plates:
Looking at the hill ascent video in your article, you can see what I’m describing above happen - the front wheels are scrabbling and spinning, especially after bouncing over the bump, but the rear wheels aren’t and so my guess is there’s a very unhappy clutch in the driveline somewhere. The driveline control modules have thermal models within them that are constantly estimating clutch temperature based on torque and slip across the clutch - once they realise shit’s getting hot enough to damage the friction plates, they’ll open to protect themselves, and in so doing leave you with FWD only.
It’s interesting that Ford refer to an upgrade to a “liquid cooled” PTU clutch on the Badlands model. I assume this means that the base spec driveline has a dry clutch on the PTU, and probably one with a low-ish torque capacity. This means that a) it won’t be able to send much torque to the rear axle before it slips, and b) when it does slip, it’ll overheat quickly because there’s no oil to dissipate that heat. My guess would be that “liquid cooled” specifically means that the Badlands model has a wet PTU clutch which will naturally be much better able to dissipate heat and therefore be more robust to high-slip, high-torque events.
If you’ve just read over 3,500 of my words, only to arrive at a conclusion that Bronco Sports may or may not have a problem, I’m sorry, but that’s just how automotive engineering works sometimes.
Whether a vehicle is a failure for derating during certain conditions can be highly subjective, though sometimes it’s not. The Ford Bronco Sport’s 2,200 pound towing capacity isn’t subjective, it’s based on SAE’s J2807 test, which lays out a specific test that the vehicle must be able to complete in order to be rated to tow a certain amount.
Unfortunately, there is no commonly-used equivalent test regimen when it comes to “certifying” that a vehicle offers something like “unrelenting Bronco capability.” These marketing terms are subjective, and that means Ford doesn’t owe any particular performance under any particular driving conditions. That hill climb that caused the journalist’s Outer Banks Bronco Sport to overheat? Ford never promised that the vehicle could handle that grade without derate. That sand dune that Hall was driving? Ford never promised that the vehicle can be hooned for 15 minutes in ~80 degree weather without derating.
Ultimately, what matters is what the customer thinks. So long as the vehicle’s capability meets or exceeds the customer’s expectation of what the Bronco Sport should be able to do, then there is no problem. We’ll have to keep an eye out on loose-terrain off-roading reviews from customers, and on comparison tests to show how the Bronco Sport stacks up against the competitive set. And we’ll have to keep in mind how drive modes and driver behavior factors into the vehicle’s performance.