Allison transmission


Junior Member
I have a 2005 monaco with a allison 2500MH driven by a cummins ISB 300. The trans would not go into 5th. gear. The only code it showed on the trans side was a fail code p2773. this means the engine is ignoring the trans request for a torque derate. There was never a fault on the engine side and the only light that came on on the dash was the trans light.
I had it in a cummins shop for 21 days . they could never find the problem and walked away from a $3500.00 bill(new ecm/tech support/allison support /labor etc.)
I took it to a cumminc Dealer and a mechanic had a similar problem a year ago. i t took him two days at $120.
an hour to figure it out.
When I brought my coach in he changed the exhaust pressure sensor and found it full of water,removed sensor tube and blew it out. He replaced the exhaust pressure sensor and data shows 29.4 INHG
Trans works fine
For you tech savy guys the answer is this (if I have it right)
he hooked up insite and monitored data with key on but engine not running. barometric pressure is correct but exhaust pressure reads 23 INHG when it should read 29.4 INHG. The INHG is at the low end of the approved rang so it did not send a code to the ECM. The engine thought the coach was in a high altitude so it did not derate the torque. the mechanics last comment on the invoice was---the exhaust gas pressure sensor reading incorrectly was causing the ECM to make incorrect calculations and ignore the trans request for torque derate so it could shift properly!!!!!!!!!!!!!!!!!!!

i hope you all pass this along. This could save us all a lot of down time and money!!!!!!!!!!
the sensor swith was $112.00 plus 1.5 hours labor!!!!!!!!!!!!!!!
Good luck out there, fixit


Senior Member
Re: Allison transmission

Sorry about your $$$ loss but your heads up may save the next guy. Thanks for the post.



Senior Member
Re: Allison transmission

Thanks Dave for the info, sorry for your loss and very glad you found a honest shop to take it to. I have mine in the shop now for the dash air condition and I feel that they are doing a part replacement till they find the problem and it is fixed. :( :( :( :( , just running up the bill :laugh:


Junior Member
Thanks for the post.....possibly saved me $$$$$ 2005 Holiday Rambler with the same problem....took to Allison dealer for trouble shoot & repair....left with a $550 bill for inconclusive diagnosis, told to take it to cummins....spent time researching the problem and code and stumbled upon your post. I did an inspection of exhaust pressure switch which was ok, but found a cracked exhaust pressure pipe...temporarily patch the pipe to take for quick road test.....and problem was solved. Called local truck dealer here in town and will have part tomorrow for a cost of $75! Again thanks for the info....literally saved me thousands and lots of time!