TRANS Codes (D14416, 4621, 4627) SOLVED

[jpinreno]

Hello all,

I wanted to post this as a reference thread for anyone dealing with Allison transmission issues. There isn’t much consolidated info on Steel Soldiers about how these problems actually get resolved, so this is less of a question and more of a full summary of my issue and final fix.

Last year I began experiencing chatter, downshifts to limp mode (3rd), and eventually a full lockout. The codes that consistently showed were:

• D1 4416
• D2 4621
• D3 4627

Like many here have suggested, wiring was the common suspected culprit. I did extensive testing at both the cab connector and the transmission connector using all applicable TMs for OHM values—everything tested normal, and I couldn’t identify a break. For context, my 2002 A1 wiring insulation still looked bright white and flexible, so nothing obvious stood out.

Below is the systematic approach I took:

1. Initial Wiring Tests & Solenoid Checks

• Verified wiring and solenoid OHM readings (all passed, though I didn’t fully trust the results).

2. Dropped the Pan & Inspected Internals

• Found the “O” solenoid questionable, and evidence the valve body had been opened previously by military maintenance.
• Solenoid screens were gummy, though no metal debris was present.
• Since dropping the pan is a major job, I replaced:
  • All solenoids
  • Internal harness (looked previously modified)
  • Speed sensor
• Roughly $2k in parts.

3. Improved Grounding

• Ran a new battery ground to frame, and a second ground from the frame through-bolt to the transmission case.

4. Tested a Known-Good TCM

• Purchased an upgraded TCM from Superman (wanted a spare anyway since I disperse camp in very remote Nevada).
• Tested against original.
• Symptoms improved slightly (intermittent recovery from limp mode), so something was helping—but the issue remained.

5. Finally: Rewired the Critical Harness Runs
I decided to run a new harness parallel to the original from transmission to TCM, splicing in the most failure-prone wires:

Priority	Wire	Reason
High-A Power	Shared by A & E solenoids—known point of failure
Solenoid A Control	Repeated 4627 code
Solenoid E Control	Repeated 4416 code & multiple replacements
Sensor Return / Ground	Affects current-sensing logic

6. The Actual Root Cause (Likely)
After all of this, the issue turned out to be a hidden short somewhere in the cab harness, extremely difficult to locate. Rewiring the core group solved everything.

Work Performed by Allison Shop
I ultimately had an Allison-certified shop:

• Run the new harness (using their approved connectors)
• Verify all pinouts with J-box diagnostics
• Provide a warranty

Between my parts and their labor, I’m into this for about $11k. Likely overkill for most people, but reliability was critical for me. I camp extremely remote—any tow would be $5–10k, if they could even get to me—and the nearest shop wouldn’t be local or familiar.

This truck has over 12k miles of real off-road use, so this was still within my “expensive hobby” budget.

Final Thoughts
I now feel I understand our truck transmission system end-to-end, but I can honestly say this is a bear of a job without the proper J-box test equipment for pinout and loopback testing of the Deutsch connectors.

Hopefully this helps someone facing these same codes. If you’re dealing with 4416, 4621, or 4627, feel free to reach out—happy to share everything I learned or provide any of the documentation I gathered along the way.

Josh

 

[NDT]

Typical harness problem areas are: contact with accelerator pedal linkage, wiper linkage, cab rotation wear point contact. You literally have to run your hands over the harness end to end to find problems, in addition to ohm checks.
Thanks for your analysis.

 

[Aviator4x4]

Typical harness problem areas are: contact with accelerator pedal linkage, wiper linkage, cab rotation wear point contact. You literally have to run your hands over the harness end to end to find problems, in addition to ohm checks.
Thanks for your analysis.

Can confirm.