Allison 3000 & 4000 Gen 4 Fault Code: P2684 Actuator Supply Voltage 3 (HSD3) Open

DIAGNOSTIC RESPONSE:
SOL OFF, Inhibit TCC operation, Inhibit main modulation, DNA

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 TCM initialization and remain on during normal operation. High side drivers are turned off during cranking and for certain diagnostic responses. The open circuit diagnostic is determined by monitoring the status of the low side drivers. If two or more of the low side drivers circuits are detected by the TCM in a failed open/low state that are supplied by a particular high side driver circuit, the problem is assumed to be on the high side of the load and a 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 Control Solenoid 5 (PCS5) (retarder or speed models), and Shift Solenoid 2 (SS2) via wire 131. 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 if longer than 4 seconds, cranking while 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, SS2, air solenoid (SS2) low side drivers, have been detected in a failed low/open state while HSD3 voltage is greater than 6 volts.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2684 is active:
1. For 7-speed models, 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 P2684 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.
– W131 high resistance or an open circuit in chassis harness.
– W131 (terminal 11 of 20-way) high resistance or an open circuit in 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.
– W137 high resistance or an open circuit in chassis harness.
– W137 (terminal 12 of 20-way) high resistance or an open circuit in internal transmission harness.
3. PCS5 (7-speed Differential Lock, retarder) issues such as:
– W115 shorted to another wire in chassis harness.
– W115 high resistance or an open circuit 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.
– W119 high resistance or an open circuit in chassis harness.
– W119 (terminal 17 of 20-way) high resistance or an open circuit in internal transmission harness.
5. Air solenoid (SS2) issues such as:
– W119 shorted to another wire in chassis harness.
– W119 high resistance or an open circuit in chassis harness.
6. Defective solenoids TCC, PCS5 (retarder or diff lock), SS2 (air solenoid or C6 solenoid for 7-speed models).
7. Unlocked connectors, expanded terminals, poor pin crimps or unlocked pins causing high resistance or open circuit.
8. Defective TCM.

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