Allison code 6600

brainsboy said:

If this is the case then can I program the TCM for softer shifts or different RPMS ?

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Well with Allison doc you can program different target and max RPMs, but if you have 2:1 hubs/7.8:1 axle ratio, it probably won’t shift any smoother until you put 2.5+ tons in the back…

The Allison is constantly learning/refining its shifts, its base program looks at the engagement speeds and amount of slip and remembers this for the next time to try and improve it. That base programming was made to accommodate a wide range of real world gear and load configurations, and not our product of the mythical land of MILSPEC. In the end it can only engage a clutch so slowly under light load, so the clutch discs lockup almost immediately and the shifts are harsh. Our 2-3 shift has a double wammy, in that as soon as it meets a low enough converter slip, it brings on converter lockup. Low final gear = low slippage and almost immediate TC lockup upon the shift into 3rd…

Add weight or change the final gear ratio to something more common and it then can adjust the clutch engagement to allow for a more comfortable amount of slip And more comfortable shifting…

you really want a ton or two in the back to balance axle weights anyway…

Maybe this allison is different but dont usually torque converters that lock up are only for final gear ratio. My trans has no issues what so ever on up shifts and down shifts are all good except the 3rd to 2nd downshift. I would have to assume that TQ lock ups would not only be for a 3rd to 2nd downshift. If it was a lock up issue wouldnt all the gears involving a lock up release be harsh? If anything it would seem that on a downshift the TQ would release not engage.

The TC in the allison locks in 3rd and above 100% of the time and does all shifts with TC locked. Somewhere in 2nd when downshifting it unlocks the TC.

have one person report maintaining ex brake and TC lock when manually shifting down to 2nd until ~10MPH.

3-2 is a large ratio step, and the oncoming gear clutch can only engage so slowly so that clutch is probably grabbing/locking faster with less slip…

brainsboy said:

TCM tested on black plug #13 and #29 shows 60.32 OHMS .. going to take a guess and say check engine ECU now?

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As mentioned in post 639 of the below thread, on my truck, the TPS comes from the J1587( J1708 ) Databus, so fiddling with J1939 is not going to help you.

I would recommend reviewing the Schematics for the truck, and exhaustively testing the J1587 data link wiring.

Good Luck

Problem FIXED / SOLVED


This truck has been a fun build but its been mostly correcting all the issues from the last owner. Although the truck always had hard downshifts even after I rebuilt the transmission, it just started having a hard time starting. Last owner had 4 of the wrong batteries in it and were too tall he had a bungee cord holding the fiberglass top on. I switched to two commercial batteries 4DLT, they are about 21" wide by 8" tall and fit perfectly. During the swap I noticed one of the four old batteries was not even tight the connections were full of corrosion and loose pulled right off the battery. One of the other batteries had a bad cell. After switching the batteries the hard down shifts went away, I have been driving it everyday to work for over a week now 450+ miles and not a single hard shift. Silky smooth up and downshifts.

Yep. Number one issue with FMTV's is always the batteries, and the battery and ground connections. Address that and 90% of the "problems" these trucks have vanish.