Allison 3000 & 4000 Gen 4 Fault Code: P0848 Transmission Pressure Switch 2 (PS2) Circuit High

DIAGNOSTIC RESPONSE:
None

CIRCUIT DESCRIPTION:
The difference between main pressure (before the filter) and Torque Converter Clutch (TCC) lockup pressure 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 P0848 indicates that the PS2 circuit is detected pressurized (open) when it should be exhausted (closed) with ignition ON, engine speed less than 250 rpm and output speed less than 25 rpm.

CONDITIONS FOR RUNNING THE DTC:
1. Prognostics package enabled by calibration.
2. TCM initialized normally and engine speed below 250 rpm and output speed below 25 rpm.
3. TCC solenoid not energized.
4. 40°C (104 °F) ≤ transmission fluid temperature ≤ 150°C (302°F).

CONDITIONS FOR SETTING THE DTC:
If PS2 indicates pressurized in the diagnostic tool at ignition ON and engine below 250 rpm, DTC P0848 sets.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0848 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 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 P0848 may be caused by:
1. Prognostics enabled calibration and no W118 in chassis harness or non-prognostics control valve module.
2. Open or high resistance in W118 or W158 in chassis harness.
3. Open or high resistance from terminal 17 to PS2 or from PS2 to terminal 19 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. Issue with TCM.

NOTE: This DTC sets only when there is an electrical circuit issue in the PS2 circuit (e.g. (missing W118). Hydraulic circuit troubleshooting is not required.