Allison 3000 & 4000 Gen 5 — Engine Speed Sensor Circuit No Signal Solution

This guide explains how to diagnose and repair the Engine Speed Sensor Circuit No Signal fault (DTC P0727) on Allison 3000 and 4000 Gen 5 transmissions.

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Diagnostic Response

When the transmission detects a loss of engine speed signal, it defaults to turbine speed as a substitute reference.

Circuit Description

Speed sensors convert mechanical motion into an electrical signal. The transmission control module (TCM) monitors the signal to ensure engine speed data is within reasonable limits. DTC P0727 is triggered when the TCM detects that the engine speed signal is missing or abnormally low.

When the DTC P0727 Sets

When DTC P0727 becomes active, the following events occur:

1. DTC P0727 is stored in TCM history.
2. The CHECK TRANS light does not illuminate.
3. If ignition is OFF, the engine speed value defaults to 0.
4. If the turbine shaft speed exceeds the engine speed calibration default, the TCM substitutes turbine shaft speed as the engine speed.

Clearing the DTC

You can clear DTC P0727 in two ways:

• Manually: Use a diagnostic tool to clear the fault code from TCM history.
• Automatically: The TCM will clear the DTC after 40 warm-up cycles if the fault does not reoccur.

Diagnostic Aids

DTC P0727 can be caused by several wiring or component issues. Inspect and verify the following:

1. W139 and/or W159 wires shorted to another wire or to ground.
2. W139 and/or W159 open circuit or high resistance.
3. Defective or damaged connectors (pushed-back pins, loose fits, or corrosion).
4. Unlocked connectors or missing seal plugs allowing moisture intrusion.
5. Fatigued or broken wires at the speed sensor connectors.
6. Poor pin crimps or expanded terminals.
7. Defective engine speed sensor.
8. Faulty TCM (Transmission Control Module).
9. Improper wiring practices — wire pair not twisted or insufficiently twisted.

Important Wiring Note

Ensure the speed sensor wiring harness consists of twisted pairs — 12 to 16 twists per 300 mm — extending the full length of the harness to within 50 mm of the speed sensor connector. This reduces signal interference and improves accuracy.

Additional Reference

Consult the appropriate Allison control system and transmission specifications for:

• Circuit schematics
• Connector pinouts
• System diagrams
• Further troubleshooting instructions

For more detailed Allison repair cases, refer to: Allison Trouble Repair