Re: Allison transmissions and broken flexplates
I smoked a good cigar tonight and thought on this one at length. A nice big 60 ring cigar. I don't normally get out the big fat ones unless the occasion is something special. This post is a one fat cigar answer, meaning I have chewed (meaning I thought about it. I don't waste good cigars by chewing on them) on this one for about an hour.
I would as one suggestion at this point, involve Cummins. Engine irregularities are a possiblity as well as wrong parts. Gotta make sure it's 100% right according to the installation drawing. I don't know about this engine, but Cummins does supply the flexplates on some engines.
I am not aware of any engine modification (like horsepower upgrades), or an operating condition that can cause this failure as the primary mode of failure and not show any other evidence of its presence.
The only operational thing I can think of as even a remote possibility, is the vehicle sitting still with the brakes on, the engine at a high RPM and the transmission going into gear. But the transmission has a neutral to range engine speed inhibit built into its calibration to prevent this. Henry may have experienced something along those lines I haven't.
Another remote possibility is to make sure the transmission calibration that's in the TCM is for a motorhome. Fire truck cals, for example, do not have some of the "consumer" safety inhibits in them to allow an emergency vehicle to move whenever the driver wants it to, in spite of things like high engine RPM.
Operational stuff aside, I tend to agree with Len. It's gotta be something. Those flexplates can't handle a load that's being put on them. I'm sure I don't know everything, but my experience says the load those plates are experiencing has to be an excessive axial (front to back) movement, or they're trying to run elliptically as the vehicle is going down the road. Maybe the torque converter should be replaced in case the flexplate holes in the front cover are out of round in relation to the converter pilot on the front cover (the little nose that pilots the converter in the engine crank or crank adapter), or the converter pilot is off center, or the converter's rear hub that engages the pump is off center somehow.
Has there ever been any evidence of a front transmission seal leak? That may, but not always, accompany an adaptation running elliptically. I think I'd replace all the adaptation parts and not reuse anything, too.
I don't think it's the pilot hole in the engine crank because the adaptation measurements should have caught that. I also don't think it's the torque converter sitting too far inside the transmission case, because that's one of the adaptation measurements, too.
Getting Cummins' input and replacing the converter and all adaptation parts are my best suggestions at this point, but I'm not 100% sure any of those things would be the primary cause. I'd hate to see it go back together at this point having not changed anything.
When it does go back together, the tech should make sure the transmission slides easily into the engine flywheel house and doesn't have to be drawn up against the engine by tightening the mounting bolts. Also, the flexplate bolts should start easily by hand and not have to be forced through the flexplate holes into the torque converter.
Please come back and post the outcome and the action taken. This is a stumper.
My cigar is now burned down to where it's gonna burn my lips soon. I'm gonna hit the submit button and be checking back to see what they decide to do for this one. Please let us know what's done next.