Allison 6sp: Allison 6 speed no reverse

[Ddperformance]

I'm at a loss.... I have an 06 Lbz in the shop that has me beating my head against the wall. Truck came in with a weak or near no reverse and seem to have a weak 1st gear. If you were headed down hill and tried to back up it would not even try and go into gear, just range inhibited. However if you were backing down hill and could roll a foot or two it would fall into reverse. Only code I'm getting pulled from it with efilive besides p0700 is p0878. So my first move was to pull pan and check pressure switch. Pressure switch still had all seals on it so I replaced with known good switch... no change. No debris in pan either I replaced internal and spin on while i had it apart. After I had no luck I pulled valve body and replaced auto range switch, still no results. I then put pressure gauge on test port it holds 100-120 at idle in gear but, would go to 240 in limp mode in reverse and it would have reverse when it would throw Cel. I pulled trans and checked the c1,c2,c5 clutch toterances and all were within spec c3 were slightly burnt looking and at bottom of spec so they were replaced. all seals and the apply pistons were checked for defects. The c3 umbrella seal seemed to be a little loose so It was replaced. Trans was reinstalled with no change then was pulled and taken part again. Then was checked reassembed with nothing seen to be wrong inside trans. At that point I was going to try and change something so I had another known good valve body on shelf that had trans go jr kit installed in it. The vb was then installed along with different internal harness. Then trans was installed in truck. No change other then 1st gear seems to be stronger when you put it in gear. When you put it in drive input speed stays same and turbine speed drops to 0 like it should but, when you put it in reverse the turbine speed never slows down. I cannot currently get it to throw 878 code and I cannot even get it to go into reverse when I push it off lift now. Any help is appreciated at I am at a loss and tired of pulling trans out of truck.

 

[Bdirtymax]

You'll have to get a scan tool to read the state of the PS4 and IMS or setup the EFIlive Pids to monitor them. With Key On Engine Off, the PS4 should say ON. It's a normally closed switch. If it says off, you have an open in the circuit from the switch to the TCM or the switch is stuck. If that is ok, fire it up in Neutral. The switch should change to Off when it receives pressure in Neutral. Or if you have the pan off already, remove the switch from the valve body and ground the frame of the switch to a good ground. Monitor the PS4 PID while you depress and release the PS4 switch. It should go from On to Off. Could be a bad switch, wiring, loose pin on terminal K at the trans connector, or lack of fluid pressure to the switch. Linkage out of adjustment, loose Reverse transfer tube or possibly a TCM.