And The Aftermarket Parts That Address Them

For more than two decades, the Allison name has been synonymous with the Duramax, the common-rail, aluminum headed V-8 that put GM back at the forefront of the diesel pickup segment. The industrial-strength Allison 1000, with its 1.26-inch diameter input shaft, massive 1.75-inch output shaft, 12.2-inch diameter torque converter, and 330-pound heft was far superior to the 4R100 and 47RE available in the Ford and Dodge competition when it debuted in 2001. Since then, the Allison has gone from handling an input torque rating of 520 lb-ft to a mind-boggling 910 lb-ft at present, and chances are the current 10-speed rendition will be asked to grapple with even more in the future.

Of course, it’s also no big secret that the commercial-grade Allison has its fair share of shortcomings, namely when it’s tasked with harnessing large increases in horsepower and torque. Clutch pack failures and slipping factory torque converters have perpetually been a weak link when the Duramax ahead of it has been turned up. And at higher horsepower levels many have found out the hard way that the factory C2 hub and P2 carrier can fail catastrophically. We’ll cover each of those failure points plus a few more this time, along with their appropriate aftermarket solution(s). Whether you’re looking to preserve the life of your stock Allison or build it right the first time you pull it, you’ve come to the right place.

The Allison 1000 was dreamt up in the late 1990s and it brought a medium-duty work ethic to the light duty truck segment when it debuted behind the LB7 Duramax in ’01 model year Chevrolet Silverado and GMC Sierra HD’s. The five-speed version, which was offered from ’01 through ’05, featured 3.094: 1 (First), 1.809:1 (Second), 1.406:1 (Third), 1.00:1 (Fourth), and 0.711:1 (Fifth) gear ratios. In ’06 the Allison became a six-speed auto when it gained a second Overdrive (0.614:1).
Beginning with ’20 model year HD trucks, the Allison boasted 10 forward speeds. The Allison 10L1000 as it’s called, was developed by GM but with assistance from Allison engineers (and we’re told Allison actually performed much of the validation for the 10-speed). The 10L1000 offers converter lockup capability in first gear and triple Overdrive. The 10-speed Allison’s gear ratios are as follows: 4.54:1 (First), 2.86:1 (Second), 2.06:1 (Third), 1.72:1 (Fourth), 1.48:1 (Fifth), 1.26:1 (Sixth), 1.00:1 (Seventh), 0.85:1 (Eighth), 0.68:1 (Ninth), 0.63:1 (10th).
Looking at the Allison 1000, its structural makeup is modular in design. Its three primary sections consist of the front cover (bell housing), main case or gearbox module (where you find the geartrain, PTO if applicable, and valve body), and rear cover (tail housing). Built with plenty of brawn, the Allison 1000 utilizes a 1.26-inch diameter input shaft, a 1.75-inch output shaft, a 12.2-inch diameter converter, and possesses a dry weight of 330 pounds.
As a clutch-to-clutch transmission, the Allison 1000 is quite different from other automatics such as the aforementioned 4R100 and 47RE. There are no bands, sprags, or overrunning clutches. Rather, the clutch packs are engaged (and disengaged) in precise intervals determined and commanded by the TCM. During upshifts, full power from the engine cannot be applied (which is often mistaken as defueling), which is necessary to avoid shift flare, shift binding, or burnt up clutches.
If the Duramax in front of it is left at the factory horsepower and torque rating, an Allison will last indefinitely. Provided you properly maintain an Allison by changing its external filter (replacement of the internal filter is typically only required at overhaul) along with the fluid at the appropriate interval, the transmission can outlive the truck it’s in. However, it’s what happens after you add a programmer, usually the first mod a Duramax owner indulges in, that jeopardizes long-term longevity.
As for the O.G. Allison 1000, the five-speed found behind the LB7 and LLY Duramax (’01-‘05), it obviously has the lowest capacity for handling added torque. Specifically, its vertical limit is generally accepted as roughly 90 to 100 hp and 180 to 200 lb-ft over stock. While those numbers may seem dismal, if you can limit your power-adders to those gains you can enjoy the extra performance and daily drive, tow, or haul anything you need to without sacrificing long-term reliability. In rear-wheel horsepower speak (rwhp), an ’01-’05 Duramax-powered truck sporting the factory five-speed Allison should be limited to 400-rwhp (or 800 lb-ft of torque).
With added horsepower and torque in the mix, C3 clutch failure is one of the first weak points to surface in a five-speed Allison. The problem, in addition to being one of the hardest working clutch packs within the transmission, is that they will slip with too much additional torque applied to them. This typically triggers a P0700 on top of the Allison going into limp mode, where your only forward available gear is third. A textbook C3 clutch failure often entails burn marks on the inside edges of the friction plates.
Beyond the factory clutch packs’ inability to handle considerable torque increases, many believe the notorious C3 clutch problem stems from a lubrication issue. Because the C3 clutch pack only sees fluid when it’s being used, this lack of fluid lends itself to increased heat and (ultimately) wear, with added torque only compounding the problem.
To solve the C3’s factory lubrication issue, Goerend Transmission designed these PTO covers, which directly feed the C3 clutch pack. Specifically, the PTO covers funnel transmission fluid through a passage in the transmission case—ATF that would’ve otherwise been returned to the pan unused. By supplying fluid to the C3’s at all times, their operating temperature is greatly reduced.
Because installation of Goerend’s PTO covers requires drilling of the Allison’s main case, Goerend developed a jig tool for the job. The jig ensures that you properly locate the holes that will serve to channel ATF directly to the C3’s. Once bolted to the main case, it requires the installer to use a ¼-inch bit and drill the holes at the precise angles present on the jig tool.
Another clutch pack that’s notorious for failure within the five-speed Allison is the C2. C2 failure is similar to C1 clutch failure in that when the C2’s fail they appear to only be half worn. However, instead of bowing inward the C2’s have a tendency to bow outward.
After C3 clutch failure comes the C1’s. Thanks to being engaged by an off-center apply piston, they only see uneven apply pressure when in use. This also means that roughly just 50-percent of the clutch’s holding capacity can be employed. On top of that, the apply piston often snags the outside lip, bending it inward.
When it came to the six-speed Allison 1000, which debuted behind the LBZ Duramax in 2006, both of the C1 and C2 clutch issues were addressed. Most important here is that the redesigned Allison provided these clutch packs more torque holding capacity. Unfortunately nothing was done to address the C3 clutch pack’s lack of fluid supply.
The six-speed Allison 1000’s threshold for pain is a bit mixed. Factory updates made to keep pace with the increased torque ratings of the LMM, LML, and L5P engines (namely converter clutch holding capacity) all effectively raised the amount of power the Allison could handle. As for the version parked behind the LBZ and LMM mills (’06-‘10), 430 to 450-rwhp (or 850 to 900 lb-ft) is where most truck owners should draw the line.
Courtesy of a stronger one-way clutch within the dual friction face converter beginning in ’11, the factory Allison 1000 behind the LML Duramax is capable of handling in excess of 500-rwhp and roughly 1,000 lb-ft of torque. Upgraded C3 and C4 clutch packs also contribute to this version’s added ability to cope with additional torque.
Especially in five-speed and early six-speed versions of the Allison 1000, the OEM torque converter is arguably the biggest weak link. The problem with this is that a torque converter failure can take the rest of the transmission out with it. The internal breakdown of a torque converter contaminates all of the fluid circulating throughout the entire unit. Although it’s not the best performance option, many “budget built” Allisons have been assembled for ’01-’10 trucks over the years with an upgraded triple-disc converter, a shift kit, and reputable tuning and lived at the 500-rwhp mark.
Within the factory torque converter, the biggest area of concern is the stator. Its thin structural makeup is prone to cracking under the kind of high internal pressure experienced when towing heavy, ascending hills, drag racing, or sled pulling. As the means of redirecting transmission fluid returning from the turbine toward the impeller, the stator plays a very important role in both stall speed and coupling efficiency. It’s no wonder billet, optimized-design stators are so prevalent in the Allison aftermarket.
Another downside of the factory torque converter is its soft steel construction, which allows the front cover to balloon and flex under high pressure and heat. When the front cover grows, it can actually press against the flex plate. In fact, when an Allison fails it’s highly common to find burn marks on the six bolt pads present on the front cover. The burn marks occur because the pads themselves don’t flex while the front cover they’re welded to does.
Aftermarket torque converters feature billet front covers specifically because their stronger material keeps the converter from growing (i.e. ballooning) under pressure. The front cover material of choice for most reputable multi-disc performance converters is often forged billet-steel.
Pulled from a five-speed Allison converter, you can see that only one very thin clutch resides within it. It’s easy to see why this isn’t going to withstand torque inputs much greater than stock—at least not for long. Saddled with the workload of harnessing an additional 150 to 200 lb-ft of torque over stock, excessive slippage and transmission-killing heat (if not outright converter failure followed by total transmission mayhem) is not only possible, but likely inevitable.
The C2 clutch hub houses the C2 overdrive clutches and synchronizes in speed to the input shaft during the 3-4 shift. A hard upshift here with added horsepower in the mix is extremely stressful on this assembly. Unfortunately, many Duramax/Allison owners find out the hard way that the C2 hub is a notorious weak link in their transmission.
Given the previously-discussed, C2 clutch hub failure you can probably guess what the solution is: a billet C2 hub. This piece is a must-have item for high-horsepower or even mildly-modified trucks that tow heavy. Various versions are available in the aftermarket and are either made from 300M billet steel or 4140. Sun Coast’s unit even incorporates an added lubrication hole for the P3 sun gear and pilot. The Goerend units shown here, made from forged 4140 steel, feature more thickness in the snout area to prevent twisting under high horsepower and/or shift bind-ups.
Like the C2 hub, the P2 planetary is also often taken out during a stressful 3-4 upshift. Failure of the P2 either entails the splines stripping out or the snout breaking off in catastrophic fashion. While it usually takes considerable horsepower to break a P2 (somewhere in the neighborhood of 750 to 800-rwhp), trucks that tow heavy can also experience the failure.
Superior strength P2 planetaries are offered by several heavy hitters in the Allison game such as Goerend, Sun Coast, and Xcaliber to name a few. Usually made from 4140 billet steel that’s been induction heat-treated, aftermarket billet P2 carriers won’t strip out or twist with big horsepower and torque in the equation.
Beginning on LML Duramax trucks (‘11), the transmission control module (TCM) was electronically in control of main line pressure for the first time. During steady-state driving, main line pressure was reduced to as low as 80-psi to help reduce the Allison’s internal temperature. While this is fine for the factory 397 hp, 765 lb-ft rating, it’s not OK for higher horsepower and even tune-only trucks. The lack of main line pressure often led to slippage in modified ’11-newer trucks—and the problem persisted until the owner either swapped in an earlier model TCM or waited for TCM tuning to be released for the ’11 TCM. Luckily, the latter did eventually end up happening.
Now to the latest in the Allison lineage, the 10L1000, and the shortcomings it leaves the assembly line with. First and foremost, converter issues are common due to the lockup mechanism demonstrating a tendency to fail when ’20-newer L5P engines see additional horsepower or the truck is fitted with larger wheels and tires. After that, converter stator issues have been noted by Allison builders, along with the fact that the clutch packs will hardly withstand added torque (of any degree) for very long. Last but not least, the factory aluminum E clutch hub, which is used in First, Third, Fifth, Sixth, Seventh, Eighth, and Ninth gears, is prone to excessive wear anytime torque is increased beyond stock. With all of that said however, we have seen fresh 10L1000’s behind several 600-rwhp dyno graphs—although these were horsepower pulls made in a single gear and it’s unclear how long the Allison hung in there afterward.

Bonus Round: Transfer Case Failure

For 4×4-equipped ’01-’07 Silverado and Sierra 2500 and 3500 HD’s, the transfer case may need to be addressed before you ever have to dig into your Allison. Both the NP261XHD (manual shift) and NP263XHD (electric shift) that came on these trucks are notorious for a stealthy yet inevitable failure called “Pump Rub.” On one of the indexing tabs on the gear pump housing (the tab being use to locate the gear pump within the t-case) an anti-rattle clip is present and begins slowly wearing away at the magnesium case from the inside-out. Eventually, the transfer case begins losing fluid. But the catch here is that fluid only escapes the transfer case when the truck is being driven. Needless to say it’s a hard issue to detect—and once the problem does surface it’s way too late. The area of the NP263XD pictured here is where the pinhole develops.
Solving the pump rub problem entails pulling the transfer case, splitting it, addressing the pump housing, and resealing it. Transfer case pump upgrade kits from Merchant Automotive and PPE are great solutions for this across-the-board problem, and for just $75 their comprehensive kits rule out the failure for good. Similar to the way a set of tie-rod sleeves provides peace of mind for Duramax owners (which ironically go for about $75 as well), a pump upgrade kit does the same. Although there is quite a bit of labor involved, the average ’01-’07 HD owner will sleep better at night after having installed it.


Goerend Transmission

Inglewood Transmission

Merchant Automotive

Sun Coast



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