Allison 6sp: Trans goes into neutral

[MAXOVERMOTOR]

I have a 08 LMM with a new motor with twins,after motor and twins were installed pulled truck out of garage and truck went into neutral,if truck sets for a minute your hear a solenoid click an truck goes in gear an moves 20' then neutral again,truck is in 2nd or 3rd in forward gears. Oil is fine,wiring is good,truck drove in garage fine before change,Has Robs twin tunes.
Showing on dash " put in neutral to reset " & " service parking assist"
codes TCM PO-848
TCM PO-877
ECM PO-848
cleared codes,and they come rite back.
ANY IDEAS?

 

[JoshH]

You sure the transmission fluid level is good? I had the same thing happen to me once, and it was low on fluid. Those LMM transmission dipsticks suck! Did you do anything to the transmission while the motor was out?

 

[MAXOVERMOTOR]

Oil is good,checked alot....trans wasnt touched

 

[Mike L.]

Disconnect harness at the TCM and check pins for looseness. Check harness for rubbing or chafing.
P0848 is your concern and not P0877. That code indicates that either you have a valve stroked when it shouldn't be,( rare in this case )or the TCM is not getting the feedback from the PSM. Low fluid can cause this as Josh indicated, stuck valve, ( doubtfull ) or open wire ( probable ).

 

[MACKIN]

You have twins, built motor and a stock transmission?

 

[MAXOVERMOTOR]

just did the motor and was getting it running to bring to george to do the tranny...

 

[MAXOVERMOTOR]

Have fixed problem,tranny was in limp mode...kept on unhooking batteries an unplugging an reflashing an it came out....thanks