Allison 3000 & 4000 Gen 4 Fault Code: P2686 Actuator Supply Voltage 3 (HSD3) High

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 tuned 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. 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. This test detects if the voltage measured at the HSD3 detection circuit indicates high during initialization when the circuit is off.

CONDITIONS FOR RUNNING THE DTC:
During TCM initialization while high side driver is commanded off.

CONDITIONS FOR SETTING THE DTC:
This DTC sets during TCM initialization, if HSD3 voltage detected to be greater than 6V while HSD3 circuit is OFF.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2686 is active:
1. Transmission fails to Neutral (N).
2. DTC is stored in the TCM history.
3. The CHECK TRANS light illuminates.
4. 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 P2686 can be caused by:
1. HSD3 circuit issues such as:
– W131 shorted to another wire in chassis harness.
– W131 (terminal 11 of 20-way) shorted to another wire in the internal transmission harness.
2. TCC issues such as:
– W137 shorted to another wire in chassis harness.
– W137 shorted (terminal 12 of 20-way) to another wire in the internal transmission harness.
3. PCS5 (7-speed Differential Lock, retarder) issues such as:
– W115 shorted to another wire in chassis harness.
4. SS2 issues such as:
– W119 shorted to another wire in chassis harness.
– W119 shorted (terminal 17 of 20-way) to another wire in the internal transmission harness.
5. Air solenoid (SS2) issues such as:
– W119 shorted to another wire in chassis harness.
6. Defective solenoids TCC, PCS5 (retarder solenoid), SS2, 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.