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Allison 3000 & 4000 Gen 4 Fault Code: P2685 Actuator Supply Voltage 3 (HSD3) Low

DIAGNOSTIC RESPONSE:
DNS, SOL OFF (hydraulic default)

CIRCUIT DESCRIPTION:
The high side drivers are connected to the high side circuits of the solenoids and commanded on during normal operation. The high side drivers serve as a master switch to supply battery voltage to the low side drivers. High side drivers are turned on during Transmission Control Module (TCM) initialization, and remain on during normal operation. High side drivers are turned off during cranking and certain diagnostic responses. The low circuit diagnostic is determined by monitoring the status of the low and high side drivers voltage. If two or more of the low side driver circuits are detected by the TCM in a failed low state that are supplied by a particular high side driver circuit, and the high side drivers voltage is below 6V, the problem is assumed to be on the high side of the load and high side driver fault becomes active instead of two or more low side driver faults becoming active. There may or may not be low side driver faults present with this DTC. High Side Driver 3 (HSD3) supplies battery voltage to the Torque Converter Clutch (TCC) solenoid, Pressure Clutch Solenoid 5 (PCS5), and Shift Solenoid 2 (SS2) via W131. The TCM regulates control current to the solenoids by switching the appropriate low side driver on and off.

CONDITIONS FOR RUNNING THE DTC:
1. DTC enabled with ignition ON, engine OFF, and battery voltage greater than 9 volts.
2. HSD3 is commanded ON.
3. DTC disabled while engine is cranking longer than 4 seconds, and the battery voltage is less than 10 volts.
4. DTC re-enabled once engine is running and TCM is initialized normally.

CONDITIONS FOR SETTING THE DTC:
This DTC sets when the TCM detects two or more low side drivers connected to HSD3 (e.g., TCC, PCS5, air solenoid (SS2) low side drivers) have been detected in a failed low state and HSD3 voltage is less than 6 volts.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2685 is active:
1. For 7-speed, limit ranges to Reverse (R), Neutral (N), and ranges 2-6 operation.
2. TCM commands retarder OFF.
3. The TCM inhibits TCC engagement.
4. DTC is stored in the TCM history.
5. The CHECK TRANS light illuminates.
6. The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without failure.

DIAGNOSTIC AIDS:
DTC P2685 can be caused by:
1. HSD3 circuit issues such as:
– W131 shorted to another wire in chassis harness or to chassis ground.
– W131 (terminal 11 of 20-way) shorted to another wire in the internal transmission harness or to the
control module.
2. TCC issues such as:
– W137 shorted to another wire in chassis harness or to chassis ground.
– W137 shorted (terminal 12 of 20-way) to another wire in the internal transmission harness or to the
control module.
3. PCS5 (7-speed Differential Lock, retarder) issues such as:
– W115 shorted to another wire in chassis harness or to chassis ground.
4. SS2 issues such as:
– W119 shorted to another wire in chassis harness or to chassis ground.
– W119 shorted (terminal 17 of 20-way) to another wire in the internal transmission harness or to the
control module.
5. Air solenoid (SS2) issues such as:
– W119 shorted to another wire in chassis harness or to chassis ground.
6. Defective solenoids TCC, retarder solenoid (PCS5), air solenoid (SS2), or differential lock solenoid (PCS5).
7. Broken or defective connector assemblies allowing pushed back pins to short together.
8. Unlocked connectors, missing seal plugs allowing moisture in connectors.
9. Defective TCM.

Review applicable information in Control System and Transmission Specifications to find additional circuit specifications, system and connector diagrams, and troubleshooting tips.