There's an interesting thing about early Chevrolet Silverado
and GMC Sierra
trucks equipped with the Duramax diesel engine and Allison transmission. From 2001-2005, these trucks were equipped with a five-speed Allison automatic transmission. In 2006, the trucks received a big update, a "new" six-speed Allison transmission. The interesting discovery came after the new transmission was torn down for the first time. It quickly became apparent that the internals were, for the most part, exactly the same.
That's right, the 2001-2005 Allison five-speed transmission was actually a six-speed all along. The difference was in the valve body and the transmission control module (TCM). Much like the legend of Big Foot, it's been told that the reason for not utilizing all six gears from the start was a lack of processing power in the TCM, taking until 2006 for the controller hardware technology to catch up to the hardware. Another story frequently told is that it took a disgruntled Allison employee to smuggle out the early TCM code and make it available to tuners.
Whatever the story might be, the Duramax and Allison experts at Duramax Tuner soon figured out how to convert an early five-speed to a six-speed easily. The process involves simply swapping the transmission's valve body for a new one and reprogramming the TCM.
| We chose to install the Duramax Tuner six-speed conversion as part of a larger transmission build. The techs at PPE Diesel built us a "Stage 6" Allison that will hold about 1,000 rear-wheel horsepower.
The benefits of swapping to a six-speed are pretty great and immediately noticeable. Adding a sixth gear essentially adds a second overdrive, which lowers engine RPM by 14 percent. Another great benefit is lower road noise when in sixth gear. Fuel economy is improved but only marginally. The Duramax Tuner conversion kit works with retrofit tap shifters and high-idle kits, and the gear display works accurately on Edge CTS and Insight monitors.
We had the folks at PPE Diesel in Montclair, California, install the Duramax Tuner six-speed conversion for us while they were building the Allison transmission for our 2002 Chevrolet Silverado
2500HD. The process is very simple and can be performed with the transmission in the truck, though ours was done on the bench. The only special tool needed is a good inch-pound torque wrench.
We wanted to wait until after we've put a bunch of miles on the conversion to report back, and we're pleased to say that the six-speed conversion works flawlessly. The truck happily shifts into sixth gear when cruising at highway speeds and will hold it as if it were factory. The TCM tuning from Duramax Tuner is spot on as well, with shifts being performed quick and crisp. There's no stutter or harsh shifting to be found. Overall, we are beyond happy with the performance of the six-speed conversion and would recommend it to anyone who owns an older five-speed truck.
| The first step to installing the conversion, no matter where or how you do it, is to drain the fluid. Because we had previously installed a PPE deep pan, that meant draining into a 5-gallon bucket. Always dispose of old fluid properly.
| The transmission's valve body is exposed after removing the oil pan. The valve body is responsible for controlling which gear the transmission is in by directing the flow of pressurized fluid through its many small passages.
| Care needs to be exercised when removing the valve body, as the small bolts that hold the valve body in place in the transmission case also look similar to the ones holding the two halves of the valve body together. Appropriate power tools can be used to remove a valve body but should never be used install one.
| Placed side-by-side on a workbench, it's easy to spot the differences between the early (left) five-speed valve body and the new (right) six-speed unit. You can also see the remaining bolts previously mentioned (and the empty holes where the mounting bolts were removed).
| Seen here is another angle (the backside) of the new versus old valve bodies. The new valve body appears to be out of a 2017-2019 Allison six-speed; however, it has been rewired to work with the older controller.
| Installing the valve body is somewhat of a delicate operation. The orifices on the valve body need to line up perfectly with those in the transmission case.
| When everything is lined up, a slight wiggle and pressure is all that it takes to secure the valve body into place. If you're working with the transmission in the truck, it's imperative to have a couple bolts ready to hold it in place (because gravity, of course).
| There are four bolts that are supplied the Duramax Tuner conversion kit. They are in a bag stapled to bright yellow paper, so you know that they are important. Do not miss this step!
| Seen here are the four bolts in the valve body. The factory bolts are longer, and thus, if used, will cause catastrophic (their words, not ours) damage to the transmission.
| The bolts that hold the valve body in place get torqued to 108 in-lbs. That's INCH pounds. Or, about 9 ft-lbs. Since ft-lb torque wrenches generally don't go that low, it's necessary to have a good -inch drive in-lb torque wrench for the job.
| Once the valve body is installed, the pan gasket and filter can go back as well. The Allison pan gasket is reusable, so be careful not to damage it during removal. It's a $50 mistake if you cut the gasket.
| With the pan back on and the transmission presumably back in the truck (if it was removed) it can then be filled with fluid. Allison transmission with a deep pan will take between 18 and 24 quarts of fluid for a full fill. Dropping the pan, if done without removing the truck, should require about 8 to 12 quarts, depending on the pan.
| With the valve body install wrapped up, we turned to the other half of the conversion, the transmission control module. On early five-speed trucks, the TCM is located under a black cover bolted to the driver side of the fan shroud.
| After removing the two upper bolts, the whole unit can be lifted up and out of the truck. Pay attention to the lower mounting tabs on the cover as you remove it, as these will be important to line up upon reinstallation.
| The TCM itself is just snapped inside the cover with four plastic tabs. Be careful not to break any of the tabs when removing the old TCM. Duramax Tuner charges a hefty core charge for the TCM, so be sure to ship the old one back ASAP.
| Speaking of the TCM, the "new" one from Duramax Tuner comes refurbished and tuned specifically for your truck. When ordering the six-speed conversion kit, Duramax Tuner will also take information such as the modifications to the engine and tire size into consideration for the new TCM tune. Unfortunately, TCM tuning can't be done with a handheld programmer for this generation truck, so it needs to be performed at their facility.
| To finish the install, plug the two connectors back into the color-coded ports on the TCM and reinstall the whole unit in the truck. The transmission learning procedure will take several days of stop-and-go driving to get the hang of your driving style. However, the more that it's driven, the better it'll get.