If you’re one of the many, many customers waiting on a replacement valve body for the 10-speed Allison transmission in your GM heavy duty truck, there may finally be some light at the end of the tunnel. The company announced in August that it has both a new set of replacement parts along with a new repair procedure for its HD line, which should should help ease the backlog of owners waiting on transmission fixes.
The bad news? It may only be a band-aid fix.
First, a little background: GM’s version of the 10-speed transmission it co-developed with rival Ford Motor is used in virtually every rear-wheel-drive model the company sells, from the Allison variant installed in its heavy-duty trucks on down to the lighter duty version found in the Cadillac CT4 and CT5 sedans. That’s a lot of gearboxes. Over the past two years, GM has recalled virtually all of them (at least through ’22) after valve body issues caused some customers’ rear axles to lock up while their vehicles were in motion.
But the recall wasn’t the end of the story. At the time GM initiated the campaign, it had not actually identified or addressed the mechanical root of the problem. The “repair “remedy” associated with the recall is actually a stopgap measure that puts the truck into limp mode when a failure occurs, preventing the lock-up, but still leaving customers with a compromised pickup.
In early August, GM finally opened a gap in the floodgates when it released a revised valve body and updated repair procedure for its gas-powered pickups, introducing new replacement parts to the pipeline and making life much easier for those customers experiencing the issue for the first time.
Right around the same time GM was pushing out the third-gen valve body redesign, a customer waiting on a new valve body for his 2023 Chevrolet Silverado 3500 HD reached out to alert us to an oft-overlooked population that was also dealing with this issue: heavy duty truck owners. In fact, if you look closely at GM’s updated procedure we linked above, you’ll see that it leaves out every truck with a diesel engine, light- and heavy-duty alike. The common thread there? Torque, and lots of it.
These heavier-duty transmissions utilize a different valve body, and at the time, GM didn’t have one available. That tipster was Robert Shafto, and barely a week after he reached out to us, GM threw a fresh wrench into the works by posting an updated service bulletin, which was sent to us by another reader.
Whether this repair actually solves the problem or not, its existence is good news, because GM held off on pushing the new procedure out to its dealerships until it actually had the parts to ship along with it. As we learned last month, the actual failure is rarely the biggest nightmare for GM’s owners; that honor goes to the lengthy waits many have endured for replacement parts. Shafto was without his truck for two months, and based on the emails we received in the wake of our first report, he wasn’t alone, nor was he the most egregious case.
“Ten-speed transmission valve body replacement parts availability for both light- and heavy-duty diesel engine pickups is improving,” said Bill Grotz, Global Public Policy and Regulatory Communications Senior Manager at General Motors. “In addition, we anticipate a recently launched valve body service repair process for 2021-2024 Silverado and Sierra 2500 / 3500 HDs will help customers and serve to improve availability of valve bodies.”
“We regret any delay for our customers and are working to deliver parts and make needed repairs as soon as possible,” Grotz said.
That same week, Shafto got his truck back. Whether it was repaired per the most recent service guidance remains unclear—and Shafto has gone back and forth with GM’s customer care agents trying to clarify that—but he is once again in possession of his truck.
As for the fix itself? Well, not everybody is convinced it will permanently rectify the issue. Shortly after our story ran, a handful of tipsters reached out suggesting that I contact Nate Valentin of NextGen Drivetrain to get his take on the situation.
NextGen is in the business of beefing up factory drivelines for serious power users. Valentin is a (fiercely) independent transmission engineer and the chief architect of everything NextGen builds. His transmissions are backed by a lifetime, no-questions-asked warranty and, as you might expect, NextGen charges a pretty penny for those upgrades.
Or, in his words, “Whatever they cost, they ****ing cost.”
“We have around 150 different inventions; all of them were designed by me,” Valentin told us. “And the reason I’m bringing this up is roughly a quarter of them are specifically for General Motors and the Ford/GM 10-speed,” he said. “I know more about this valve body than anyone at General Motors or Ford does,” he added, with absolute conviction.
“I reinvented every version of this valve body. The non-shift by wire, all of them, the diesel, the gas, I reinvented all of them myself. And [ours] is currently the most popular upgrade kit on the market for all of them,” he said. “So, I don’t mean to sound bloviating, but the reason I’m giving you this context is because there’s no more credible source on the topic, period.”
Valentin was happy—eager, even—to explain exactly what is causing these valve bodies to fail.
“I understand it’s getting attention now, but this is not a new problem,” he noted off the top. “I figured this out during COVID.”
And here I was just trying to make decent pizza.
“The problem is caused by what’s called a dual clutch engagement,” he said, referring to the two clutches in the transmission that engage and disengage synchronously as each gear is selected.
Imagine the transmission is a fork in the road; power can go left or it can go right. An axle lock-up happens when a failure within the gearbox essentially says, “Hey, why not both?”
“So, what takes place inside the transmission when the rear wheels are locking up is the output shaft is getting stuck in position trying to downshift very aggressively to a much lower gear ratio,” he continued.
“If you do this on a motorcycle, the same thing happens. So what’s happening here is you have a conflicting clutch pack turning on at an inappropriate time that triggers a very aggressive downshift far beyond the scope of what the transmission can safely do,” he said. “The reason this is happening is because oil pressure is leaking by a specific valve in the valve body called the feed limit valve.”
The feed limit valve moves a lot, wearing more rapidly than others, Valentin explained to us, but in this case the issue is that the bore around the valve (meaning the casting itself) is eroding. As this progresses, oil can begin to leak by the valve.
“The reason this is so dangerous is because it’s leaking around the inboard side of the nose of the valve,” he elaborated. “And that part of the valve is the part where oil pressure enters that pushes it against a spring. When that oil pressure leaks out of that circuit, it can now no longer push the valve into the open position. So, this valve becomes unable to move.”
When that valve sticks closed, it forces the oil pressure to the opposite end of the circuit, where it encounters a default solenoid.
“The default solenoid remains off unless the computer issues a command for that solenoid to be energized,” he continued. “So, if that is hydraulically forced open by pressure exiting the feed limit valve circuit, the vehicle is now going to try and get put in fifth gear—limp mode.”
As an added bonus, this almost always happens when the truck is moving at highway speeds, because they contribute to increased oil pressure within the valve body.
“There’s more engine load,” he explained. “It’s pushing into the air. The air is thicker because you’re traveling at a higher speed; it’s a vehicle with a lot of surface area on the front.”
“And so what then happens is if enough pressure enters this circuit and ‘drives down the wrong road,’ what ends up happening is the vehicle goes into this failsafe condition at what is almost always a super unsafe speed,” he said.
“Obviously you would skid down the road on a motorcycle, but in a big ass truck, it’s going to lock the hell up,” he said.
This is obviously not ideal, and the safety concern it prompted was the primary reason GM was pressured to initiate a recall in the first place. And because that happened before any updated parts or repair procedures could be spun up, GM instead implemented a band-aid fix that preemptively disables shifts into higher gears when the early signs of valve bore erosion are detected.
Despite the shared fundamental blueprint, Ford’s transmissions don’t exhibit this issue. Through 2019, the two gearboxes had a lot more in common, including a clutch engagement system fed by latch valves. GM moved away from this configuration for its second-generation gearbox, and that’s when its axle lock-up issues began.
Ford, still utilizing latch valves, does not have this particular problem, but practically speaking, its version of the gearbox has been just as problematic. The 2024-2025 F-150s were even recalled for—wait for it—bad valve bodies.
And even though GM has an updated procedure for its HD trucks, Valentin remains skeptical.
“The kit that they believe or so they claim is going to fix this is a separator plate, a valve, and a spring,” he said. “So let’s break them down one by one. So the plate, the only thing they did I’ve seen this separator plate before because we’ve built a lot of 2025 valve bodies and it is currently in use. This separator plate is not one ounce more reliable than the one that was in it before; all they did was change some of the hole dimensions.”
“So, it’s stuff that obviously doesn’t make a difference, but if you’re not a transmission engineer, you would have no idea,” he said. “So, the separator plate is General Motors giving themselves a high five, in my opinion.”
“The spring that they include is slightly heavier from what I have seen. That again means literally nothing. I think what they’re trying to do there is prevent any oil pressure that is leaking from getting into the default circuit by trying to make the valve stay closed all the time. That is my guess,” he said. “But the bottom line is it doesn’t do anything. It has zero impact on the problem.”
“And then the third thing they did is they replaced the aluminum valve with a low-grade steel valve that is the exact same shape.” he said.
“Now, this might slow down slightly the problem from taking place in a vehicle that is not currently experiencing maladaptive behavior,” he continued, “but the issue is that, if you are currently experiencing this problem because the valve has eroded the valve bore, then changing the valve doesn’t do anything.”
Best case? The upgraded hardware slows down the valve body wear in a truck that is not yet exhibiting any symptoms, Valentin said.
“Which obviously means nothing,” he added, “Because if you had a healthy valve body, you would not be going in to get this sort of labor performed in the first place.”
And if it does push failure down the road, that leaves customers to deal with the problem long after their warranties have expired. And if you ask Valentin, that’s as much a feature as it is a bug.
“What they’re doing here is they’re trying to get people out of warranty so that this design flaw becomes an asset rather than a liability,” he said. “The bottom line here is they’re playing finances. They’re playing financial chess.”
How is it possible for something like this to slip by GM’s quality control process? If you ask Valentin, it all comes down to the time-honored tradition of corporate bureaucracy.
“There’s so much demand to get items out with unreasonable timelines that a lot of things end up entering the market in their proof of concept state,” he said. “And that’s why you’re seeing a dramatic increase in both voluntary—which does not exist—and involuntary recalls.”
For many owners, just the prospect of being back in their trucks will make it worthwhile to roll the dice on GM’s new procedure. After all, you’re likely getting it for free either under warranty or as part of the recall campaign, so why not?
As for Valentin? Business is booming.
Got a tip? Let us know at tips@thedrive.com.
Byron is a contributing writer and auto reviewer with a keen eye for infrastructure, sales and regulatory stories.
