Allison 1000 Transmission • Problems & Solutions

Allison 1000 Transmission • Problems & Solutions

Next Gen Drivetrain Research & DevelopmentJune 26, 202114 comments

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

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.


     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 (14)

Tim Pritchard on May 06, 2022

I have been working on a 2007.5, 1000 series allison transmission in my sons chevy truck, I put a stage 5 PPE trans upgrade, the valve body operation is smooth and I done an upgrade to it also, the pump was upgraded, rhe valve body has new solenoids( allison solenoids, not aftrer market and a new TCM, when you put the gear selector on the column in what should be drive, the courser shows flashes under 1 on the instrument cluster and the transmission sometimes shifts and sometimes don’t do nothing, I have checked the wiring harness, there is no issues with it, I have even contacted the company the TCU came from, they assured me that there was no way it couild cause this problem and I must have a valve sticking, there is usually a laundry list of faults when you start it up and put the trans in gear, one that really caught my eye, both shift solenoids engaged at the same time, low pressure, pressure to high, you name it, it has probably logged it or is active, any suggestions would be greatly appreciated, I have started calling it a TCU on bad drugs.


Eugene on May 06, 2022

2010 duramax with allison tranny when its really cold out truck doesn’t move at all in reverse sometimes can go forward enough to get to the heated shop. Now it has developed losing the indicator under the cluster for drive on the dash and can happen while driving once that happens the tranny doesnt shift and starts to heat up. Low kms for the year never been used much in winter because trans issues

Ralph Boecker on March 22, 2022

04 chevy 2500hd 6.6 diesel duramax transmission line blow off I put it back on full the fluid and it still won’t go into gear what can I do to fix it

Angelo Bertoni on January 25, 2022

I have a 2003 Duramax with Allison 1000 trans built with PPE upgrades vehicle thru a p0747 code found the internal harness to be bad in trans replaced but now when vehicle is shifted from reverse to drive or park to drive there is a 3-5 sec delay before vehicle feels to be in drive only when selecting gears vehicle drives fine and shifts fine afterwords any help or thoughts would be helpful at a loss.

Ron Wolf on January 25, 2022

07.5 Chevy 2500 Allison 1000, transmission goes into 2nd gear only and gives me a code of PO847. I have replaced the TCM Module, Pressure Manifold switch. Any ideas what else could be the problem, would be greatly appreciated. Thank you

Lance Devaney on January 25, 2022

How much for a transmission for my 02 d max shipped to ak ?

Robert valdez on December 04, 2021

Why would fluid be blowing out of the vent line. I just replaced the whole entire valve body. There are no codes shown on the scanner.

Butch Boehmer on December 04, 2021

I have an 03 Duramax and developed a serious trans fluid leak coming out the bell housing. I pulled it out thinking there would be something obvious because of the amount of leakage, but there wasn’t. Every thing looked good including the torque converter shaft and seal. I replaced the seal and put it back in only to find it still leaking. I have it out again and am going to replace the O-ring on the pump even though it looks fine. Can you suggest anything else that I should do? I’m getting pretty good at removing and installing this allison but it’s a miserable job laying on your back in the driveway!

bob farmer on October 10, 2021

I have a new 2020 gmc 3500 serra with 10/l 1000 transmission less than 10,000 miles and when in low gear at slow speed then let up on the gas it’s makes a loud noise! are any of you having this problem?

larry churchwell on September 30, 2021

looking at an lbz with 1000 Alison with 277,000 miles will it last

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