allison trans problem

allisonman, I have an 02 chevy 2500HD 4x4 crewcab w/ dmax/allison. I had a problem with the allison, downshifting on uphill grade, in tow/haul mode with cruise control on. I pull a 33' 5th wheel . the problems start when the truck is in overdrive ,about 50-55 mph the trans will downshift to fourth i believe locks in (dealer says it the default mode) and will not upshift again until you stop shut of the key for 30 seconds and restart. it will not downshift to lower gears started doing this with cruise on but did it without cruise on also. had in dealer in orlando,fl they replaced trans with reman. GM goodwilled the trans while I paid the labor . truck had 53k on it. 350 miles north on the way home with new trans. it did it again any ideas thanks LEE
allison trans problem

I don't think Allisonman is posting anymore. If he is, great. Did the dealer check the trucks computer? Every action your truck makes is controled by it. Real hard for me to belive you have had 2 trans. doing the same thing and it be the transmission.

C Nash

Senior Member
allison trans problem

Did the dealer pull a code ck? There should have been a code stored. I agree with Grandview. Sounds like a computer, sensor related problem.
allison trans problem

I have the truck at my local dealer now.I took the tech for a ride with the trailer hooked up and his port. computer hooked up .I'm not sure what codewas stored but the trans downshifted and into default with him with me . scanner said it was in 5th gear but stated wrong gear ratio. they will have the truck inside in the morning for further diagnostics.
allison trans problem

well today now they are telling me that after more testing and driving that GM tech assistants(detroit) saym they think its in the transfer case. possability that it could be too much play in a shift fork which is where a senser is locatated and reads final drive information and relays it to the computer. could be causing a false reading and making trans go into default mode. could be the senser for $170 or shift forks, sincros,bearings, for$1300. I guess since we are diagnosticing like we are back in the '60 s (computerazation isn't shi!!!!) replace by process of elemination I'll just bend over good luck with your two LEE
allison trans problem

I talked to allisonman by phone yesterday in washington state. explained the whole problem and the last thing the dealer told me could be the problem . on 4x4 models GM puts the output speed senser in the transfer case, an it sends a signal to the trans to up/down shift based on final output speed they changed the (oss) and still does it so its probably internal with the parts that the senseris reading inside the case . hope this helps anyone that has had this problem . now its in the $1500 range to fix this I hope . but beware my truck only has 54k on it so there will be lots of others with this problem in the near furture.
allison trans problem

If anyone has been checking this ,I think chevy has finally fixed my truck . After 2 weeks of testing and me testing it with my 5er they have replaced the (ecm) engine control module, I hooked up friday and could not get the trans to drop in to default mode. even tried to force the downshift by tromping on throttle and it downsifts one gear and then up shifts as needed . now after $3600 out of pocket and GM goodwilling a reman allison I can fight with GM to get some reimbursement on unneeded repairs and there cost . good luck with your duramax/allison combos TEAM3360