Re: Transmission / Drive Train Thunk / Lurch / Bang When Downshifting
Reply #5 –
Allison ECM 'learns' driver throttle style of driving to control shift points. Sounds like you would benefit from an ECM reset so it can start over learning how to shift:
"Re-Learn is required when replacing the transmission, working on the transmission, installing a Shift Kit or adding horsepower.
The computer that controls the Allison transmission has adaptive strategy that constantly adjusts shift clutch pressures to match engine torque and vehicle load. It stores a database and records the relationships between rate of acceleration and throttle opening. It also calculates engine torque output based on inputs such as fuel consumption, boost pressure, air density, temperature, throttle position and many other factors.
To insure an ideal shift with maximum speed and minimal strain on the drive train the computer must 're-learn' when replacing the transmission, working on the transmission, installing a Shift Kit or adding horsepower.
Start the re-learn by making at least six sets of light throttle upshifts through all the gears, then make another six sets at 1/3, 1/2 and 3/4 throttle. Downshift the same way by starting at light throttle and working up to full throttle.
When the shifts are quick and smooth press the 'tow haul; button and repeat the previous re-learn process in 'tow haul' mode. Re-learn normally takes up to two hours (this time can be significantly reduced by using Tech2 or PC based Allison software to clear the memory and place the TCM in the 'fast learn' mode).
During the relearn there will probably be some clunks, bumps and/or short flares, especially during the 3-4 shift. This is normal."
Where is the tow-haul button? My duramax had that, but not my Foretravel.