Allison 1000 Transmission • Problems & Solutions

Allison 1000 Transmission • Problems & Solutions

Next Gen Drivetrain Research & DevelopmentJune 09, 202325 comments

Table of Contents:

- Introduction to the Allison 1000 Transmission
- Allison LCT 1000/2000 Product Resources
- Popular Transmission Information Resources
- In this Drivetrain 101, we will be addressing
- A Brief History of the Allison in DuraMax
- How the Allison LCT 1000/2000 Clutch Packs Work
- What is Allison Adaptivity Protocol?
- Problem #1 - The Torque Converter
- Problem #2 - The Valve Body
- Problem #3 - The Oil Pump
- Problem #4 - The Bushings
- Problem #5 - The Electronics
- Problem #6 - The C2 Hub
- Problem #7 - The P2 Planetary
- Problem #8 - The C2 Clutches
- Problem #9 - Oil Deprivation to C3
- Problem #10 - Input Shaft
- Conclusion to the Allison 1000 Transmission

Introduction to the Allison 1000 Transmission:

     When we build our coveted and durable Allison transmissions and rebuild kits, each one is designed to never return broken. In order to ensure this over the years, we’ve gone to great lengths to understand and compute how these units fail, and how to prevent it.

     Hence, we wanted to provide a technical publication serving to aid our DuraMax clientele in understanding their own unique transmission needs and how to ensure maximum reliability for their Allison 1000 Transmission. Without further ado, we’ll jump right in!

     In this Installment of our popular Drivetrain 101 Series, we will be analyzing all of the knowledge we have compiled here at Next Gen Drivetrain over our years of developing unique and innovative Allison 1000 Transmissions. Additionally, you can see our proprietary Allison Transmission Products here!

Allison LCT 1000/2000 Product Resources:

- LCT 1000/2000 Transmissions
- LCT 1000/2000 Torque Converters
- LCT 1000/2000 Rebuild Kits
- LCT 1000/2000 Valve Bodies
- LCT 1000/2000 Parts

Popular Transmission Information Resources:

Top 5 Benefits of an Upgraded or Built Transmission
Calculating Torque Multiplication Factor, Why is it So Important?
- Top 5 Ways to Extend the Life of a Factory Transmission

In this Drivetrain 101, we will be addressing:

- The most popular failures we witness on the Allison LCT 1000 and 2000 transmissions

- Its capabilities (or lack there of) in reference to TCM tuning and shift/lockup protocol modification

- The strengths (believe it or not, there are a few!) of this transmission

- Footnotes for GM AND Heavy Duty Commercial Vehicle owners

And much, much more!

A Brief History of the Allison in DuraMax:

     In 2001, General Motors began the use of the Isuzu 6.6L DuraMax series motors in their 3/4 ton and above trucks. Tandem with this powerful new motor was the need for a transmission that could capacitate the newfound torque, as GM historically used slightly armored versions of their 4 Speed lineup in their diesel trucks. This proved to be deeply unreliable over time.

     At the time, and currently, General Motors owns a considerable portion of Allison, and was able to use the coveted Allison transmission in their trucks at a market-competitive cost. The unit was introduced as a 5 Speed, despite having the ability to employ a 6th gear. Later, in 2006.5 when the LBZ DuraMax was introduced, came the 6th gear accessible Allison 1000 Transmission.

     In 2011, the LML DuraMax was introduced, yielding another revision to the Allison 1000 Transmission. The difference was mostly hydraulic and electronic, with a modification to the input shaft giving it a flat sealing surface at the end for oiling the Torque Converter Clutch (TCC). This will be elaborated upon later in the article!

     Until the 2020 model year’s introduction of the Ford manufactured 10L1000, the Allison 6 Speed platform continued. Our Transmissions 101 on the Allison 10L1000 can be found here. The operational tenure of the Allison 1000 Series transmission was ultimately from Model Year 2001 to Model Year 2019.

 How the Allison LCT 1000/2000 Clutch Packs Work:

     The Allison 1000 clutch application chart can be found below, with a clutch identification key above! As you can see, the final drive ratios are also included. The Allison 1000, like most 6 speed transmissions, is a “double overdrive”. This means that there are 2 overdriven and 2 underdriven gears. We will explain what this means below.


     The term “overdrive” simply means the input shaft to output shaft speed ratio is lower than 1:1, and underdrive is higher than 1:1. An input shaft to output shaft speed ratio of 1:1 (often 4th gear) is known as direct drive. This is a common platform in 6 speed transmissions.

     When the input shaft to output shaft speed ratio is less than 1:1, the tires will rotate more times for each one revolution of the crankshaft, equating to higher top speed. When the input shaft to output shaft speed ratio is below 1:1, the tires will rotate less times for each revolution of the crankshaft, multiplying available torque at the expense of top speed.

     Allison transmissions use a variety of algorithmic factors to calculate shift behavior electronically, rationalizing factors such as engine load, variables modified by TCM Tuning and in certain years, accelerometer data. In short, it is a very intelligent transmission that makes well calculated decisions about shifting.

What is Allison Adaptivity Protocol?

     Many late model electronic transmissions have an “Adaptivity Protocol” system. Adaptivity Protocol is a system where the transmission “learns” how the user drives, and adjusts its shifting behavior and shift tables accordingly. 

     For example, someone who has a “lead foot” will see later upshifts across normal driving conditions than someone who does not. This is because the truck’s onboard computer will believe you want to use the torque band of each gear more than you want to conserve fuel economy. A large part of Allison’s adaptivity protocol is complying with stringent EPA regulations regarding efficiency, and maintaining lower RPM’s and engine loads has a positive correlation on emissions control.

      Adaptivity Protocol also detects aggressive shifts and slows down the activation of the oncoming solenoid to minimize the aggression of the shift. This is because aggressive shifts are often caused by clutch tie-over, a condition where two clutches that activate competing final drive ratios attempt to apply on top of each other.

     Alternatively, “flaring” shifts derive from the opposite. This is generally a neutralization condition where NO clutch pack is in a synchronous state, which replicates the sensation of being out of gear. Allison combats this by doing the opposite, activating the oncoming clutch sooner to tighten up the inferior or poorly timed shift.

     This concludes as a consistently balanced and concise shift, without any aggression. It is for this that many Allison 1000 Transmission owners report that it will often shift just fine up until it’s final moments. This causes many failed Allison Transmissions to “sneak up” on DuraMax owners with actively functioning Allisons. As such, we will progress to analyzing the problems of this transmission, and how to understand, prevent and resolve them.

Problem #1 - The Torque Converter

     Despite the Allison’s formidable reputation, it actually has the weakest torque converter of the “Big 3”. The Allison torque converter makes primary problem is the Stator; the part that controls stall speed. It’s notoriously thin and flimsy, often cracking during anything that generates high internal converter pressure, such as towing, hills or high performance.

     The converter’s cover, facing the flexplate, is also made of very soft metal. In fact, when we cut the converter apart to begin manufacturing our popular Allison Torque Converters, we often see 6 distinct burn spots over the pads of the cover where the bolts thread in. This is because the cover flexes badly and those portions can’t flex, causing them to be the only ones to burn. Billet covers do not flex, allowing you to take advantage of the entire clutch mating surface.

     The Allison also only features one incredibly thin clutch from the factory. All of our torque converters for the Allison feature an upgraded clutch, or multiple upgraded clutches, billet stators, billet covers and billet lockup pistons, 100% standard. These are important factors to consider when upgrading the biggest failure point in the OEM Allison 1000 Transmission.

Problem #2 - The Valve Body

     Allison valve bodies are notorious for one problem in specific, valve seizure. Allison shift valves are very long with many different lands along them. The issue with this is that it presents many places where the valve can hang up on a valve bore.

     To determine if you have this problem, you can put the valve body in a freezer overnight and see if the valves still move freely. If they do, you are free of this problem, if they don’t, it’s time for one of our Allison 1000 Valve Bodies.

     The other prolific issue with the Allison Valve Body is internal valve bore wear. This plagues any low quality aluminum/magnesium valve transmission, for which the Allison is. As the valves travel axially over time, they slowly damage the bore similar to a piston traveling in an oscillatory fashion through a combustion motor.

     This problem can only be detected through vacuum testing, and can be permanently prevented with our precision machined Steel NeverLeak valves. Once this problem is present, the valve body cannot be safely employed without substantial machining. Symptoms of a bad valve body present as sloppier shifts than when the truck was new, or being quick to downshift under load.

Problem #3 - The Oil Pump

     The Allison 1000 Transmission Oil Pump is generally reliable, but begins to lose efficiency very early in life and it’s struggle to produce pressure quickly begins to scale as it ages. The Allison’s central pump gears work similar to how most engine oil pumps work; two gears, one residing inside the other.

     Much like old hemi and Ford motors, the outer gear can get compressed to one side opposite the side where pressure vents, causing disproportionate centrifugal to wear. This has a similar effect on fluid flow to damaged impeller fins in a turbo, they can no longer adequately compress fluid.

      The solution here is to use hardened steel pump gears that do not enable the coarse gear to pump relationship that generates this problem in the OEM. Our Allison 1000 pumps are designed to maintain day one performance for their entire lifespan, making them uniquely attractive for daily drivers and towing applications.

     Further, we install shims/heavier springs in various parts of the pump to enhance oil flow and endorse slightly firmer shifts. Our goal is to facilitate superior shift feel without conflicting with Allison’s adaptivity protocol.

Problem #4 - The Bushings

     The Allison 1000’s bushings are made from a weak babbitt material from the OEM. The babbitt material used is notoriously of poor longevity, and was only designed in the on-road transmissions to survive the warranty period of the truck, like most transmissions.

     In all of our built Allison 1000 Transmissions, bronze bushings are a standard. The reason for this is because they are designed to never need replaced. Much of the aerospace world uses bronze for bushing applications for this specific reason, making it an imperative part of our remanufacturing process.

Problem #5 - The Electronics

     Although the electronics are not inherently poor in quality, it is important to emphasize their potential for complication in the Allison 1000 Transmission uniquely. The Allison 1000 is a densely electronic transmission, relying on many solenoids to open and close at the appropriate times for it to operate.

     Many companies do not replace the electronics of the Allison during rebuilds as a method of cutting cost but rather just reseal them. Some companies do replace the electronics, but only with cheaper aftermarket solutions that are inherently less reliable.

     We have tested over time that the OEM Allison solenoids are the most reliable on the market, and they are the only Allison electronics we stock and use. In fact, all of our hand-picked Allison 1000 High Performance Rebuild Kits include a complete set of OEM Solenoids, OEM NSBU and OEM Preasure Manifold Switch.

Problem #6 - The C2 Hub

     The OEM Allison 1000 Transmission’s C2 hub is a large failure point in this specific unit. It is responsible for holding the C2 (Overdrive) clutches and synchronizes in speed to the input shaft on the 3-4 upshift. This is a very stressful shift for this specific component and can cause it to fail.

     The solution is a CNC machined Billet alternative. Our Billet C2 hub’s also have an oversized “fat” shaft to ensure the complete and total obsolescence of this problem. Billet C2 hubs are imperative for anyone planning to tow heavy OR produce substantial power.

Problem #7 - The P2 Planetary

    For many of the same reasons the C2 hub fails, the P2 Planetary also fails. The P2 Planetary operates synchronously to the C2 hub and also synchronizes in speed with the input shaft during the 3-4 upshift. The “snout” portion of the planetary can snap off during this shift.

     The alternative that resolves this is a billet steel P2 carrier, it is at about 800HP where these begin to fail, but we have also witnessed these failures in ultra-heavy towing applications. This is common in built transmissions because many companies do not know to upgrade this part when building a high performance transmission, sending certain high power trucks out with a unique inclination for transmission failure.

     Fortunately, our 4140 Billet P2 Planetary prevents these concerns totally. These have been tested in applications in excess of 1000HP, and are a must for any serious Allison 1000 Transmission Build.

Problem #8 - The C2 Clutches

     Coinciding with the previous 2 problems, is the clutch pack associated with them. The C2 clutches function as the “Overdrive” clutch pack of the Allison 1000, hence why they are so prone to wanton stress. The C2 clutch pack is insufficient in clutch volume to safely capacitate the power of a deleted or even tuned DuraMax.

     Further, it is notorious for poor longevity in towing applications. The friction lining on the Allison clutches is rather thin, so when a clutch drags frequently or a tremendous amount of load is disproportionately obligated to one clutch pack, it will likely isolate itself as most problematic.

     In the Allison 1000 Transmission, this is C2. The solution to C2 and it’s various complications is to use higher quality clutches in place of the weak OE clutch material. For those seeking to safely capacitate added power, we can add multiple clutches to this portion of the transmission to increase torque capacity; enhancing reliability and towing capability dramatically.

Problem #9 - Oil Deprivation to C3

     Many Allison transmissions have a common problem, and the 1000 series is no different; oil does not adequately reach the C3 clutch pack of the transmission. Fortunately, there is a rather easy workaround for this problem.

     On all of our built Allison transmissions, you’ll see that we have billet aluminum CNC machined PTO covers. This is because they have a fin inside that directs oil specifically to the C3 clutches, keeping them considerably cooler and dramatically enhancing their lifespan.

     Our goal is for all of our built Allison transmissions to stay on the road until the end of their tenure. In order to do this, we must take cooling the transmission seriously. Directly oiling the C3 clutch pack is a critical improvement to this transmission that allows us to encounter this transmissions maximum potential.

Problem #10 - Input Shaft

     The Allison 1000 Input Shaft is large and solid, but does incorporate an oil circuit in the middle in 2011. Adding an oil feed to the converter inside this shaft detracts from some of the steel volume to the shaft. This made LML and L5P DuraMax’s slightly more likely to suffer from input shaft failure, but not by any huge margin.

      For heavy towing, or added power, a CNC Machined Billet Steel input shaft is imperative. It is at about 550HP that the LML and above input shaft begins to die, but approaches 600HP safely in the earlier years of DuraMax. 

     Although it is not unheard of for the intermediate and output shaft to fail, it is exceedingly more common for the input shaft to be the problematic shaft. Although there are no acute design flaws in the Allison input shaft, it is still a prevalent failure point in this transmission and an easily justifiable upgrade for almost any application.

Conclusion to the Allison 1000 Transmission:

     The Allison 1000 Transmission may have a strong reputation, but it is not without its array of flaws and inferiorities. At Next Gen, we have spent years developing Allison transmissions that offer incredible longevity, stunning performance and staunch capability. If you have more questions about your Allison, feel free to call in and speak to one of our experts!

Comments (25)

Ken Fraser on November 24, 2023

I have a Silverado 2500 2007.5 Duramax with Allison 6 speed. When I have the RV on the back and in tow haul and coasting down hill it will feel like the hold back drops off and quickly comes back in. This glitch doesn’t always happen but enough I am worried about damaging something and I am sure this isn’t normal. Has anyone had this happen or would you know what is going on, thanks for any input.

Steve D Parks on November 24, 2023

I just bought a 2019 GMC 2500 DIESEL with 81,000 miles. My Allison transmission surprised me when coming to a stop at low speed by jerking and making a loud noise. I’m concerned. Please advise your thoughts on this. Thanks!

Jacob Campbell on November 24, 2023

Did minor over haul on my 03 allison 1000 boost pressure valve shift kit new clutches new stock convertor (temporary) fluid is fresh new filter new nsbu switch new pressure mani fold switch worked great for 5000 miles then one day it was hard to take off I let off gas and truck comes to a stop like something is binding everytime it shifts to first it does this what could be causing this?

Al S. on May 29, 2023

I had my transmission serviced recently on my 2003 GMC Duramax. I went on a long trip and it started to heat up around 800 miles into our trip. I found that the oil level was overfilled by at least three quarts. Would having it a gallon overfull damage internal parts? The oil is now the correct level. The oil I drained out was clean and no sign of being hot. I’m stuck in Baja any help will be appreciated! Thanks

Mike on April 17, 2023

What do you do when your LMM transmission control module goes bad and they don’t make them anymore or you have to buy one on eBay for 10k dollars? All 2009-2015 GMC/Chevys are junk when the TCM goes out.

justin distefano on February 08, 2023

i have a 2001 seirra 8.1 with an allison, it runs fine 90% of the time. occasionally will peg the trans temp guage to 300 . you feel the cooler lines and the pan with you hand it is not over 180. replaced PSM flushed coolers, no change to issue. will be changing internal harness next. is a sticking valve in the vb deadheading pressure and causing the fluid to overheat in that fluid circuit, it will come out of limp mode, shift into fifth gear and the temp will drop back to normal range.

Alick Munro on December 31, 2022

I have a 2018 GM duramax 6 spd.trans that appears to shift in to too high of a gear [like trying to starts a standard trans in 4th]. Severe “bucking/jerking” after coasting a hill or going up an onramp. occurs, but not every time. Using tow haul mode fixes that problem. dealer refuses to address. can it be fixed properly?

Allen Webster on December 31, 2022

I have a 2016 Chevy 3500 that will not go into D. It started acting up a little about 6 months ago. When I would pull a trailer when it would shift up to next gear it would kind of slip and check engine light would come on and go into a limp mode. I would shift to manual and it was like I was stuck in 6 gear. It wouldn’t let me manually shift. If I shut truck off for a few minutes and start it back up and the shifting would be fine. It would take a day or two then the check engine light would go away. Now it happened again last week and it won’t move at all and the D just flashes. Still have reverse but when I went to back it on trailer I lost reverse shut truck off started it had reverse again. Replaced TCM and checked wiring back to trans still nothing.

Mompati Gababonwe on October 15, 2022

I have a problem with international when I engage pto sometimes it locks on gear 3and then fails to disengage

Lyle Beard on September 18, 2022

my 2005 2500 hd 4×4 Alison. I ran over a stick that pushed the electric connector into the trans. the trans would not shift into rev and tried to move in drive. replaced wiring harness in trany, new filters and oil. still will not shift into rev (no reaction at all) and some, but not enough to move truck.

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