Allison 3000 & 4000 Gen 4 Fault Code: P2721 Pressure Control Solenoid 4 (PCS4) Control Circuit High

DIAGNOSTIC RESPONSE:
DNS, SOL OFF (hydraulic default)

CIRCUIT DESCRIPTION:
Pressure Control Solenoid 4 (PCS4) is a normally closed solenoid used to apply the C4 clutch in 2nd and 6th ranges. The Transmission Control Module (TCM) commands the solenoid ON to produce hydraulic pressure in the clutch apply circuit. When PCS4 is commanded OFF, the clutch pressure is released. The TCM sends control current to PCS4 from High Side Driver 1 (HSD1) via wire 111. HSD1 is continuously ON unless the TCM detects a fault condition. The TCM regulates the amount of current to PCS4 by switching PCS4 low side driver ON and OFF. Wire 155 completes the circuit between PCS4 and its low side driver. DTC P2721 indicates that the TCM has detected a failed high condition in the PCS4 electrical circuit.

CONDITIONS FOR RUNNING THE DTC:
TCM initialized normally and engine speed above 200 rpm.

CONDITIONS FOR SETTING THE DTC:
This DTC sets when the TCM detects PCS4 low side driver has been detected in a failed high state.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P2721 is active:
1. If failure occurs while in forward range, then shift to another forward range. If failure occurs while in Neutral (N), the transmission remains in Neutral (N). If failure occurs while in Reverse (R), the transmission remains in Reverse (R) or shifts to Neutral (N). Once range is attained, the transmission will be locked in range. There is no response to shift selector requests.
2. DTC is stored in the TCM history.
3. The TCM inhibits Torque Converter Clutch (TCC) engagement.
4. The CHECK TRANS light illuminates.
5. 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 P2721 can be caused by:
1. PCS4 issues such as:
– W155 short in chassis harness.
– W155 (terminal 2 of 20-way) short in internal transmission harness.
– PCS4 defective.
2. Broken or defective connector assemblies allowing pushed back pins to short together.
3. Unlocked connectors, missing seal plugs allowing moisture in connectors.
4. Defective TCM.

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