2007.5-2010 GMC Duramax LMM Transmissions & Parts

How GMC spec'd the Allison behind the LMM

Work Truck, SLE or SLT, it makes no difference: every LMM Sierra HD pickup got the Allison 1000 six-speed, the only transmission paired with that engine. The double-overdrive six-speed came in for 2006 with manual gear selection, and the family's input torque rating grew from 620 lb-ft back in 2001 to 765 lb-ft by 2011. On these trucks that means 650 lb-ft for 2007, 660 lb-ft for 2008-2009 and 765 lb-ft for 2010. Sixth gear is a 0.614 overdrive on top of a 0.711 fifth, which is why a loaded Sierra turns such low rpm at highway speed. Tow/Haul moves the shift points, auto grade braking holds speed on a downgrade, shift stabilization keeps it from hunting gears, and an external heavy-duty transmission oil cooler was standard HD equipment. Fluid is Dexron-VI. If your Sierra HD has the 6.0L gas V8, it is a Hydra-Matic 6L90 truck and none of these parts apply. The engine side lives on our Duramax page.

Known trouble spots on these Sierras

Start under the truck. The transmission cooler lines leak at the factory crimp where hose meets hardline, which has shown up on trucks under 25,000 miles and gets worse as the hose bakes near the exhaust. The tell is red ATF misted along the frame and crossmember, often with higher fluid temperature and flare shifts. Next is the torque converter, which is where added power lands first: the stator is thin enough to crack and the front cover is soft enough to balloon, and the metal it sheds then circulates and ruins everything downstream. Owners who add power or tow heavy report slipping in the C3 gears and limp mode locked in third, often north of 100,000 miles. Limp mode comes on with excessive slip, overheating or a heavy load, and a Sierra with an ECM calibration and no TCM calibration will find it fast. Age brings its own list: valve body valves seizing in worn bores, pump gears losing efficiency and babbitt bushings wearing, felt as mushy shifts, quick downshifts under load and a delay when you drop it in gear. Only at extreme power do the C2 hub and the P2 planetary snout break on hard three-to-four upshifts, with the P2 around 800 hp and the pre-2011 input shaft near 600 hp. Be careful reading the shift quality, because the adaptive logic covers up wear until something actually breaks.

Rebuild, replace or upgrade

The honest ceiling for a stock LBZ/LMM-era Allison is about 430 to 450 rwhp and 850 to 900 lb-ft, where the 2011-and-up version is good for 500 and more. If your Sierra lives under that, put the money into fluid and filters on schedule, 50,000 miles normal or 25,000 severe, plus cooler lines before they weep and a deep pan with the right filter if you pull a trailer most weekends; plenty of heavy towers shorten those intervals well beyond the book. Going past stock has a sequence: TCM calibration, then a billet-cover multi-disc converter, then the valve body, then a built unit from a shop that assembles them. The internals worth paying for are a billet C2 hub, a billet P2 planetary, a PTO cover that feeds oil to the C3 clutches, hardened pump gears, bronze bushings and factory Allison solenoids.

Parts we stock for the Sierra HD Allison

Hard parts come mostly from PPE: billet input, intermediate and output shafts, a billet C-2 clutch hub, C5 clutch upgrade kits, Stage 5 and Stage 6G transmission kits, valve body recalibration and pressure calibration kits, deep and low-profile aluminum pans, bar-and-plate and performance coolers, a transfer case brace and gasket, and a performance transmission mount. BD Diesel covers six-speed valve bodies and cooler and filter kits for 2007.5-2010, while ATS Diesel Performance and Fleece Performance Engineering add transmission hard parts. For a reseal or a fluid service, Merchant Automotive has the converter housing reseal kit and the pass-through connector seal, Allison Transmission supplies the deep pan, the spin-on filter and the PTO cover gasket, and Hot Shot's Secret covers fluids and additives. Cooling choices run from Mishimoto pans, universal fluid coolers and a heavy-duty cooler with a fan to XDP's direct-fit oil cooler and flex plates, with Mag-Hytec deep sump pans, plus Dorman converter bolts and an ACDelco transfer case output shaft seal.

Before you order

Three things trip people up on these. Valve bodies and shift kits are split at the 2011 model year, and the shift kit itself was only built for 2006-2008, so a 2009 or 2010 Sierra uses the valve body instead. A built transmission is either a 2WD or a 4WD assembly, never both, so confirm your drivetrain. And filter depth has to match the pan you run, deep filter with a deep sump, standard filter with the stock pan. Keep in mind that the 2011 Sierra HD moved to the LML with a new frame and revised suspension and driveline, so 2011-and-newer listings are not shortcuts for a 2010 truck. Bed length, cab style and single versus dual rear wheel on the 3500HD all change what clears under there. If you are unsure, use the fitment checker on the product page or call us with the VIN.

Frequently asked questions

Is the Allison in my 2010 Sierra 3500HD the same as the one in a 2008?

Mechanically it is the same 2006-2010 six-speed, but the input torque rating is not the same on paper: 660 lb-ft for 2008-2009 and 765 lb-ft for 2010. Fluid and filters are shared, Dexron-VI with the TF920 sump filter and the TF950 spin-on. Internal upgrade parts for the six-speed cover the whole 2006-2010 window.

There is red fluid sprayed along the frame rail of my Sierra. What is it?

That pattern is almost always the transmission cooler lines letting go at the crimped hose-to-hardline joint. It has happened to trucks with well under 25,000 miles, and heat from the nearby exhaust ages the hose. Check the fluid level before you drive it, because a low Allison will overheat and flare its shifts, and plan on updated OE lines or braided aftermarket lines with flared unions.

Do I need transmission tuning if I add power to the LMM?

Yes, and it goes first. Limp mode locks the Allison in third when it sees too much slip, too much heat or too much load, and an ECM calibration without matching TCM calibration gets there quickly. Do the TCM side before the converter and valve body, not after.

Will 2011 Sierra HD transmission parts fit my 2010?

Generally no. Valve bodies and shift kits for the 2006-2010 six-speed are cataloged separately from 2011-and-up parts, and 2011 is when the HD trucks changed over to the LML with a new frame, suspension and driveline. Always buy to the 2007.5-2010 listing rather than assuming the later part is the same.

Not sure what fits? Call the shop at the number in the header, or use the fitment checker on any product page. We install everything we sell.