Tranny problems on a 01 Allison

[Lenl]

It has happened 3 times to me now, tranny hangs shifting from 4th(I think) when I am under hard acceleration just shy of 3000 RPM. Code that is showing is P0700 Transmission Control System (MIL Request)and P0778 Pressure Control Solenoid B. While it is easy enough to replace the solenoid just wondering if there would be anything short of that I should consider.
BTW volatage seems stable so that should not be a reason for the code.

[DmaxMaverick]

Before chasing that demon, Have you serviced the tranny lately? The control main filter (spin-on) is small, and pretty efficient. If overdue for service, it can cause pressure issues, which the TCM blames on the tranny components. The fluid level is also very sensitive. The Allison TCM is "smart", and won't command a shift until certain conditions are met. Your 4th gear delay may be an indication of a restriction, namely the filter.

I'd suggest giving it a good service (even if you've done one recently), then start again with troubleshooting. Which filter brand do you have installed? Delco/Allison, or other? Was the magnet replaced with the new filter?

[Lenl]

Funny! My wife accuses of paying more attention to my truck than I do her! :)
First thing I did after the first event was check fluid levels, after second event ( thought that the first was possible fluke) was to flush tranny, cooler, clean pan, and replace filter (Delco from stealer). While it had been about 20k since the last change fluid looked pretty good. However I can replicate the fault at will now, all I need is to punch it hard and right on cue it will fault at 3000 rpm.
I have not pulled the solenoid so I don

[DmaxMaverick]

Looks like you covered your bases. Next thing I'd do is confirm all electrical connections are good. Pull them, clean, and reconnect with some dielectric grease (tune-up grease, but only on external connections). If it's still there, I'd replace the solenoid. They are easy enough, and not too expensive. Be sure to get it from an Allison dealer, not GM (big $$ diference).

[Lenl]

No kidding about staying away from Stealers! the local Chevy dealer wanted $160.00 and had to order it while the Allison dealer had three in stock and wanted $70 go figure!

[Big Dooley]

I too am having the same problem, the transmission refuses to shift from 4th to 5th under hard acceleration. I have also notice it when kicking it into passing gear on the interstate, it downshifts but drops the RPMs below 3000 (~2700). I have changed the filter and fluid and still no improvement. Where is and how hard is to to replace the Pressure Control Solenoid B valve? I have 90,000 miles on a 2002, mostly highway. Have been using AMSOIL synthetic transmission fluid with Allison filters. :(

[Biodiesel6.6]

How many miles on the current oil change and did you by chance change the internal filter at the last oil change? Not changing that could add to some of the problems. Drop and clean the pan change the internal filter, pull the solenoids and clean, change the external filter, flush lines and converter to be safe. Hope this solves the problem.
Just a thought :rolleyes:

[Big Dooley]

I have about 300 miles on the current oil change, internal filter was changed previous time. 50,000 miles on the previous oil change. I will change the internal filter and flush the transmission. Where are the solenoids and how would I go about cleaning them? Any special instructions? Thanks for any help.