Allison 3000 & 4000 Gen 4 Fault Code: P0847 Transmission Pressure Switch 2 (PS2) Circuit Low

DIAGNOSTIC RESPONSE:
None

CIRCUIT DESCRIPTION:
The difference in pressure between main (before the filter) and Torque Converter Clutch (TCC) lockup determines whether to stroke or destroke the Filter Life Indicator (FLI) pressure valve in the prognostics valve body within the control valve module. When this pressure differential exceeds 175-200 kPa (25-30 psi), the FLI pressure valve moves against the return spring and allows control main pressure to open the Pressure Switch 2 (PS2). When the pressure differential is less, the hydraulic circuit to PS2 switch exhausts and closes the PS2 switch.

An active DTC P0847 indicates that the PS2 circuit is detected exhausted (closed) when it should be pressurized (open) while in converter mode.

CONDITIONS FOR RUNNING THE DTC:
1. Prognostics package enabled by calibration.
2. TCM initialized normally and output speed above 25 rpm.
3. TCC solenoid not energized.
4. 40°C (104 °F) ≤ transmission fluid temperature ≤ 150°C (302°F).

CONDITIONS FOR SETTING THE DTC:
1. If PS2 indicates Exhausted in the diagnostic tool while the transmission is in converter mode, then P0847 sets to indicate a PS2 hydraulic or electrical circuit issue.
2. After converter mode cycling, if PS2 state oscillates between pressurized and exhausted more than the switch would be expected in that given period of time based on fluid temperature, and then settles on an unexpected switch state value for the mode of operation, then P0847 sets.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0847 is active, the following conditions will occur:
1. The CHECK TRANS light illuminates.
2. DTC is stored in TCM history.

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

DIAGNOSTIC AIDS:
Refer to the OEM 4th Generation Control System Operation and Prognostics package and features for important information about Oil Life Monitor (OM), Filter Life Monitor (FM), and Transmission Health Monitor (TM). These sections discuss prognostics principles of operation, feature requirements, maintenance alerts, reports, and processes to reset features or change fluid type (January 2010 and later TCM software) using either the diagnostic tool or an OEM-supplied (MY09 and later) shift selector.

DTC P0847 may be caused by:
1. Pin/terminal damage in a connector populated with wire 118.
2. Shorted wire 118 to chassis or another circuit in vehicle harness.
3. Shorted wire 118 to chassis or another circuit in control module wire harness.
4. Broken or defective connector assemblies allowing pushed back pins to short together.
5. Unlocked connectors, missing seal plugs allowing moisture in connectors.
6. Issue with PS2.
7. Sticky FLI (PS2) pressure valve.
8. Issues with PS2 valve spring.
9. Hydraulic pressure issue e.g., low fluid, low main pressure, low control main pressure, incorrect or damaged control module gasket, etc.
10. Issue with TCM.