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Call or Text (352) 722-5295The Allison's computer in the 2007.5-2010 GMC Sierra LMM
GM paired the LMM with a single transmission in Sierra pickups: the Allison 1000 six-speed, a double-overdrive automatic with 0.711 and 0.614 top gears and a 4.49 reverse. Tow/Haul mode, auto grade braking, shift stabilization and Tap-Up/Tap-Down manual mode are all TCM functions. That module sits apart from the engine computer, so a tune on the Duramax side leaves the TCM's own calibration untouched unless it gets a file of its own.
Why the transmission goes first
When an ECM tune goes on and the TCM stays stock, the Allison is the part that pays. It starts slipping and drops into limp mode soon after the tune. The engine has a limit too, since overly aggressive tunes and timing crack LMM pistons around 600 hp, but a stock transmission draws the line well below that.
How far to take it
For a Sierra that tows, keep combined power under roughly 430 to 450 rwhp on a stock Allison, and tune the TCM in the same visit as the ECM. Any engine file you pair with it should be emissions-compliant, which is the same advice intake-kit makers give for this engine. The CARB Executive Order to know is D-802-13. It covers the Edge Evolution CTS3, part number 85400-200 with a California edition 85401-201, on emissions-equipped 2001-2016 Sierra and Silverado 2500/3500 6.6L Duramax trucks. It is not in this collection.
In this collection
The EFI Live AutoCal V3 - Blank is the piece here: a handheld that stores calibration files and writes them to the truck's modules through the OBD-II port. Because it comes blank, the TCM file is built by your tuner for your Sierra. Other programmers and handhelds sit in our tuners category.
Check before you flash
Sierra WT, SLE and SLT pickups from mid-2007 through 2010 all pair the LMM with the Allison 1000. A 2007 Sierra Classic does not; it is the GMT800 body with the LBZ. Chassis cabs over 10,000 lb GVWR start from a derated engine rating, and Savana vans and TopKick trucks ride on different chassis that these pages do not cover. Run the VIN through the fitment checker first.
Frequently asked questions
My Sierra slips after an engine tune. Is the TCM the reason?
It is a known pattern. ECM tuning without TCM tuning is tied to Allison slipping and limp mode soon after the tune. Have the truck looked at before more miles go on it, and pair any engine file with a transmission file.
Is a tuned TCM worth it on a Sierra that mostly tows?
Yes, once the engine has been tuned. Staging TCM tuning with any ECM tune is the recommended path, and staying under roughly 430 to 450 rwhp keeps a stock Allison inside its limits. On a fully stock Sierra, the factory TCM calibration was written for the factory power level.
Which Sierra trims does this page cover?
Every LMM Sierra HD pickup from mid-2007 through 2010: WT, SLE and SLT, in 2500HD and 3500HD form, with single or dual rear wheels on the 3500HD. The Denali name did not reach the heavy-duty Sierra until 2011, so there is no LMM Denali HD.
Not sure which piece fits your Sierra? Call the shop at the number in the header, or run the fitment checker on any product page. We install everything we sell.