Allison 6 spd shifts into gear and drops back out.

Pops Dually

New member
Jul 6, 2012
Tucson, Arizona
Did a quick haul for my sister and bro in law, they moved from Phoenix to Tulsa (1100 miles). Pulling a 20” car trailer loaded heavy, tool boxes etc. We drove hard, 85-90 mph running the top of the yellow on EGT’s almost the whole way. I had problems shifting in the morning cold that I attributed to the Transgo shift spools, more on that below. When I got back home, I had multiple problems the next morning. Thank God she waited to get me home!

Ran for shit, troubleshoot to the Fuel pressure regulator, replaced and fixed (that’s a pain in the ass to get to!). Also installed a PPE race Fuel pressure relief valve I had waiting.

Alternator died (actually the overrun pulley), replaced and fixed.

Have had the cold limp problem ever since I put the Transgo in, it would go into limp mode until it warmed up and I reset the ignition, would be fine after that. When I pulled the valve body, I cleaned up the Transgo spools, “A” had a noticeable ding in the side. That problem seams to have gone away but could be masked by my main problem:

The truck goes into gear and then drops back out after moving a couple feet or none. Does it in Reverse and drive, Shift inhibited comes on. Threw codes P0751, P0757, P0843, P0848, P0873, P0877 and P0700. All parts I ordered were Allison OEM. Ordered the shift solenoid kit to replace A and B and the others for good measure. Cleaned up the Transgo spools at this time and changed fluid, pan filter and spin-on filter. Well, that didn’t do it, same symptoms and similar codes. 751 went away and was replaced with 756. Yes, I am clearing codes between cycles. Next, I ordered the Pressure switch manifold and installed. Same issues. After boning up on using EFILive to read data, I recorded a data stream while attempting shifts. I saw that the gear range wasn’t correct so I ordered an IMS and installed, also replaced the internal wire harness as it was seeping fluid through the connector pins. Same issues, turns out I was looking at commanded gear not selected gear. Checked wiring harness, unplugged and removed the loom from the firewall down. No apparent damage to the wires or connectors, new loom and retape. Replaced the engine speed and turbine speed sensors while I was up there. No change. I am thinking the clutch packs are good as it pulled good and strong the duration of the trip and it seems to want to pull until it drops out. Data stream shows the commanded gear dropping into neutral when in rev or D. At this point I figured it has to be the TCM and ordered an A40 enhanced TCM from Dan’s Diesel. Nope, not it! So, I’m back to the wire problem theory. Unplugged and ran continuity at the TCM connector for the solenoid traces, getting 5 ohm on solenoids A and B and 22 ohm on shift C, D and E, those look good and confirm the harness is good at least for those traces. I’m running out of ideas. I’ve scoured the forums looking for ideas (for the last 5 weeks). A couple that came up were the external input devices. Hail Mary, I replaced the ignition switch, TPS (I did see “Fault” flicker on and off in the scan data) and Shift lever (which I had previously repaired a broken wire). Yup, Nope! Current codes: P0756, P0757, P0847, P0848, P0877 and P0700. I thought I was following a line of logic here but looking back, just call me a Dumb Ass now. I feel like it’s going to be something stupid I overlooked. Yes, I did check the fuse (TCMB) even checked that it had 12v on it, and fluid level is rechecked on every refill. Waiting on an ASTG manual as the one online that I found was for 5 speed and has different wiring. Want to do a full pin out on the TCM connector. Sorry for the book, just wanted to get all the facts down. I made a little spread sheet from the scan data trying to figure out what’s going on. I’ll attach it but I really don’t know what I’m looking for. Someone please, save me from my ignorance!

I’ve been waiting to upgrade my tranny until I kill this one, Have I? I see an Inglewood in my future. I should have already as now I’ve lost the opportunity to have Rob update his tunes for me ☹.

Anyone know where I can find a copy of the Allison SM4006EN Service manual?


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