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The Perfect Allison Transmissions

The Perfect Allison Transmissions

WHY DO YOU NEED A "BUILT" ALLISON TRANSMISSION FOR YOUR HARD WORKING DURAMAX DIESEL PICKUP?In 2001 GM 2500 HD and 3500 HD pickups began offering the venerable 5-speed Allison 1000 automatic transmission backing the newly released LB7 and LLY Duramax turbo-diesel engines and optional with the 8.1L gas engines. The smooth-shifting automatic proved to be a much more robust option to the automatic transmissions found in lighter duty trucks or even competitive brand heavy-duty offerings. Additionally, the overdrive gear and lock-up torque converter worked together to deliver low RPM highway speeds while returning improved fuel efficiency. In 2006 the Allison backing the LBZ Duramax was enhanced with an additional overdrive gear, giving the now six-speed automatic even better fuel mileage along with TapShift gear selection driver controls. In 2007 GM also dropped the ZF6 manual transmission from the lineup offering only the Allison automatic trans in Duramax powered trucks.

Additional refinements have been made along the production years through the LMM and LML Duramax era of 2007.5 through 2016 but they were basically all built on the same architecture. Starting with the 2017 L5P further changes to the torque converter and input shaft design reflected a slightly different 6-speed Allison configuration that was still in the Allison 1000 family. But a radical change happened with the 2020 model year Duramax trucks—the L5P diesel engine is now backed by a 10-speed Allison branded transmission built by GM.

In general use the Allison transmissions are great units with many satisfied owners out there using them happily. But for owners that have made performance upgrades to their Duramax engines, work their trucks hard with heavy loads and strenuous towing demands or owners that compete with their trucks the stock Allison trans may not handle all that is demanded of it. This can lead to “limp mode” issues where the truck detects slippage and reduces performance to save itself from transmission self-destruction or even burnt clutches that can ultimately leave the truck stranded on the side of the road.


The Allison 1000 transmission works well at stock performance levels for owners that do not demand a lot from their trucks, and let’s face it there are a lot of HD truck owners that do not tow, haul or use their trucks to the limits. But owners that push the performance envelope or work their truck hard may find that the transmission will not hold up to their expectations. While the GM engineers designed their “limp mode” to protect the truck we have found that by the time it engages in many cases that the damage has already been done and the truck will fall into “limp mode” more and more until it becomes completely undrivable.Unlike other diesel transmissions on the market the Allison has strong input, intermediate, and output shafts that generally hold up well even in some performance applications. The most common problem areas for stock Allison transmissions are the clutch packs and the torque converter that do not hold up well under high loads especially with performance upgraded engines.

The stamped steel front cover on the torque converter connects to the flexplate on the back of the Duramax engine with welded tabs that can twist and flex under load and distort the front cover. When the front cover distorts it reduces the contact area for the internal lock-up clutch disk which then slips, causing increased heat and debris to be sent through the transmission fluid into the rest of the transmission. This can have a cascading effect that will eventually cause the torque converter lock-up to fail entirely. The internal clutch packs suffer a similar fate with slippage induced overheating and eventually burning the clutches to the point where they will not hold well enough to drive the truck. Additionally, with any of these burnt clutches or overheating transmission scenarios there will be a lot of associated debris in the form of both metal and clutch material that will make matters even worse leading to a total failure.

THE MERCHANT AUTOMOTIVE ALLISON SOLUTIONRather than throwing out the baby with the bathwater the team at Merchant Automotive worked on eliminating the weak points in the Allison trans and building on the strong suits when developing the MAximum Transmission series of rebuilt Allison transmissions. MAximum Transmissions are available in two configurations with the Work Series designed for Duramax diesel truck owners that want a smooth shifting economical replacement that is an upgrade over a factory Allison transmission and the Pro Series for owners that have upgraded the performance of their Duramax and/or operates with a “heavy foot”. Currently available for 2001-2016 Duramax diesel trucks we will soon be offering MAximum Transmissions for 2017-2019 L5P Duramax truck owners as well.When building both the Work Series and Pro Series Allison transmissions the MA team starts with Allison transmission cores that are completely disassembled and thoroughly inspected by skilled transmission specialists. Cases are inspected for internal wear damage as well as cracks or breaks before being painted orange and inserted into the MAximum Transmission assembly process. All seals, gaskets and wear components are replaced during the rebuild process to deliver a leak free transmission that will deliver years of performance behind the Duramax engine in your truck. Both series also include hydraulic improvements in the valve body and stator as well as reconditioning of wear areas.Friction surfaces (clutches, steels, and bands) are upgraded throughout each transmission build using a mix of OEM Allison, Exedy, Raybestos, and Alto frictions depending upon the component and application. In addition to using high-quality replacement friction surfaces the surface area is increased in nearly every clutch pack to improve the holding capacity of each MAximum Transmission allowing owners to tow more, work more, do more, and play more without worrying that their trans will let them down.While some transmission builders like to max out hydraulic line pressure attempting to hold more power, through our experience we know that excessive line pressure creates heat and causes premature wear on internal parts. Modifications to the transmission pump and valve body performed by Merchant Automotive transmission specialists ensure that the proper pressure is directed where it needs to go for optimal operation. The pressure applied to the clutch packs is critical and we make sure it is just right for every application. All MAximum Transmissions include changes to the overall line pressure that is kept in check with the overall needs of the transmission to ensure longevity and cooler operation.The valve body of an automatic transmission can be likened to the hydraulic brain or controller with its maze of passages, solenoids, controls, and valves working in concert to apply hydraulic pressure to the right areas to control the transmission’s operation. Each MAximum Transmission receives a custom-built MAximum Valve Body optimized for the performance need of that transmission using components from renowned transmission component suppliers RatioTek, Sonnax, and TransGo.Each MA valve body is tested in-house with simulation runs on our SuperFlow Axiline Test Stand. All solenoid operations are verified as well as upshift, downshift, and torque converter lockup. To ensure that the dyno testing mimics real world operation the fluid is heated to address simulated transmission operating temperature.

A critical link between your Duramax engine and the Allison transmission is the torque converter. Stock Allison torque converters use a single-disk lock-up clutch design encased in a stamped steel cover that can distort and even balloon under high-load operation which reduces the contact surface for the lock-up clutch causing slippage and additional heat. To combat the weaknesses of the stock torque converters all MAximum Transmissions come with a billet steel cover single-disk or five-disk lock-up torque converter to pass the power from the Duramax engine to the transmission without loss or excessive heat buildup. The billet front cover provides a solid surface for lock-up clutch engagement that resists flexing while reducing slippage and internal heat buildup as well as a solid machined mounting location for the flexplate bolts.Rather than confuse our customers with “Stages, Levels or Horsepower Ratings” that can be confusing in the best case and downright misleading in the worse case we simply offer two series of high-performance Allison rebuilt transmissions—the Work Series and the Pro Series. These performance designations and the components used within the transmissions target the suited application. In most cases it is not “horsepower” that breaks a transmission anyhow—driving habits (cut out those neutral drops, and tug-of-war battles…—CT), usage (or over usage like overloading then climbing a steep grade at 90 mph in the middle of summer—CT), and several other factors come into play with the typical broken or damaged transmission.To improve fluid capacity each MAximum Transmission is shipped with a stamped steel Allison deep transmission pan along with a deep filter and CNC-machined billet aluminum MA filter lock. While the additional fluid will not directly make the trans run cooler it does spread the thermal load across a greater volume of fluid to help the system maintain proper temperature control. We also ship 24 quarts of high quality fully synthetic CAM2 Dexron VI automatic transmission fluid to fill the transmission once it is installed. A new spin-on Allison transmission filter is installed along with a filter magnet to keep the fluid clean and make sure there are not extra parts needed for your installation.Each MAximum Transmission is hand built from start to finish by a single Merchant Automotive transmission specialist to ensure that every single unit meets the stringent MA standards for performance. We stand behind each MAximum Transmission with a 2-year unlimited-mileage limited-warranty for peace-of-mind knowing that you are installing a quality product that is backed by more than just words! They are shipped fully assembled and ready to install in your truck. Every MAximuim Transmission receives individual attention and is built to fit the requirements for each series with many popular models and options available and ready to ship while others may need to be built specifically for a customer’s order. Call our team of customer service specialists for up-to-date inventory levels and lead times at (616) 772-9551.


Merchant Automotive’s Work Series Allison transmissions are designed and built to exceed the performance of a factory Allison transmission in every way. As the name implies the design is optimized for truck owners that use their trucks for work or work-like play on a regular basis. The typical Work Series owner would be one that regularly tows and hauls heavy loads that asks more than the stock Allison can handle yet still wants a smooth shifting transmission that does not overheat when loaded down and worked hard. The Work Series is a great option for truck owners that use their trucks for work during the week then hook up to the family RV, boat, or toy hauler for weekend adventures. We built the Work Series for owners that demand a lot of their trucks and want to make memories at their destination not on the roadside!The billet single-disk torque converter included with the Work Series transmissions is optimized for hard working trucks to deliver great holding capacity without breaking the bank. The stall speed of the torque converter is set to work well with hard working everyday users who regularly tow and run heavy loads without being peaky. Likewise the MAximum Valve Body contained in the Work Series is detailed, upgraded, and dyno tested, but it is not the same as the Pro Series valve body, it is optimized for the Work Series and designed to cater to the work truck mentality and needs.


The Pro Series MAximum Transmission is the next step up the food chain to competition level performance. All Pro Series rebuilt Allison transmissions build on the fine foundation of the Work series and turn it up to “11” with additional performance enhancements to handle most competition use. Higher hydraulic pressure works in concert with additional clutch apply area and heavy-duty friction materials to handle high-performance applications that would utterly destroy a stock transmission. Further enhancements to the MAximum Valve Body deliver optimum operational control even under high stress applications like drag racing and sled pulling while still maintaining the drivability and street manners that Allison transmissions are known for.

Merchant Automotive’s Pro Series transmissions can be daily driven without ill effects like some “competition” transmissions yet will provide the holding power and capability to go to work when the hammer is dropped and the light turns green! Each Pro Series trans comes with a 5-disk billet torque converter that includes a billet aluminum stator to handle the high-stress internal loads without deflection like the cast aluminum stator used in the single disk torque converter. The standard 1,500-1,700 RPM stall speed of the 5-disk is well suited for multipurpose trucks that are still street driven but also used for competition like drag racing or sled pulling. Optional stall speeds (1,300-1,500 RPM, 1,700-1,900 RPM, and 1,900-2,100 RPM) are available for owners that have specific needs and uses for their truck and want to maximize that single area of performance, contact our customer service team for details and options.If you are planning to push the extremes of competition with your Duramax powered truck using an Allison based transmission your needs will go beyond a standard Pro Series transmission. But that does not mean that the Merchant Automotive team doesn’t have a solution for you. We can customize a Pro Series transmission with Billet shafts and other modifications, upgrades, and tweaks to make it handle virtually any performance needs. The transmission in Ol’ Blue (the Merchant Automotive 1,700 horsepower Ultimate Callout Challenge competition truck—CT) is a custom Pro Series transmission. Call our customer service team to discuss your needs and the options we have to make your wildest Duramax and Allison performance fantasies a reality.

DO I NEED BILLET SHAFTS???Our Pro Series MAximum Transmissions do not come standard with billet shafts, but they are available as an option for owners that will be pushing the limits with their trucks in competition or similar activities. Unlike some other transmissions backing diesel engines the Allison transmissions backing the Duramax engines in our GM trucks feature very robust shafts from the factory. For competition level performance it is wise to upgrade the input, output, and intermediate shafts as well as the C2 Hub with stronger Billet units.

A general rule of thumb to consider when thinking about Billet shafts is that trucks pushing around 800 horsepower to the rear wheels operated on sticky surfaces like drag strips and pull tracks are more likely to break stock shafts in our experience. While stock power levels can break shafts in the right (or wrong, depending on how you look at it—CT) situation we do recommend upgrading to full Billet shafts in high-power trucks intended for competition use.

If replacing stock shafts with stronger billet shafts is more than your current budget can handle, we recommend upgrading the C2 Hub first followed by the input shaft, then the output shaft and finally the intermediate shaft. The C2 Hub and input shaft are the most common points to twist and break a shaft that we have seen. Keep in mind when upgrading one shaft at a time that the complete transmission is only as strong as the weakest link so take it a little easy until you get all the factory shafts replaced with stronger billet shafts.

SUMMARYIf you want the best for your truck you have likely found that the stock Allison transmission was not up to your demanding standards. Replacing the factory Allison with a Work Series MAximum Transmission from Merchant Automotive will deliver more holding power than stock without sacrificing drivability. For Duramax owners that want to compete or just push the limits of their truck on a regular basis the Pro Series will suit your needs with options that can be ratcheted up to full blown competition units capable of handling virtually anything your Duramax engine can throw at it.NOTE: Whenever replacing a transmission be sure to thoroughly flush the transmission lines and cooler with a transmission flush machine or debris and contaminants from the previous transmission will get sent into the new transmission damaging it from the start and significantly reducing its life expectancy. Most DIY solutions found online are not capable of completely removing all debris from the stand-alone transmission cooler as well as the transmission cooler integrated into the radiator which will find its way into the new transmission when the lines are pressurized by operating the transmission. If you cannot have the lines flushed by a local transmission shop before installing your new MAximum Transmission, we highly recommend you replace both the transmission cooler and radiator to prevent a premature death from contamination.

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