P0845

ALLISON AUTOMATIC TRANSMISSION

DTC P0845
Circuit Description for Allison Automatic Transmission
The pressure switch manifold (PSM) is a multiple-switch assembly made up of three normally open (N/O) and one normally closed (N/C) pressure switches. The D pressure switch monitors D shift valve positioning and relays it to the transmission control module (TCM). When D pressure switch is in the open state, D shift valve should be in the unstroked position.

DTC P0845 is a type A DTC.

DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0845 Pressure Switch D Circuit Malfunction

Conditions for Running the DTC
^ The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts.
^ Engine speed is greater than 200 RPM and less than 7,500 RPM for 5 seconds.
^ D shift valve is commanded to the OFF, unstroked, position.

Conditions for Setting the DTC
DTC P0845 sets during steady state operation when pressure switch D indicates ON for 40 milliseconds after D solenoid has been commanded OFF.

Action Taken When the DTC Sets
^ The TCM illuminates the malfunction indicator lamp (MIL).
^ When in a forward range and shifting is completed, the transmission will fail to another forward range when engine speed is reasonable.
^ When in NEUTRAL or REVERSE, the transmission fails to NEUTRAL.
^ While diagnostic response is active, the shift selector is moved from a forward range to NEUTRAL or from a forward range to REVERSE, then the transmission will shift to NEUTRAL and a forward range cannot be attained again.
^ DTC P0845 will be stored in TCM history.
^ The TCM freezes shift adapts.
^ The TCM inhibits torque converter clutch (TCC) engagement.

Conditions for Clearing the DTC
A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.

Diagnostic Aids
^ Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions:
- A bent terminal
- A backed-out terminal
- A damaged terminal
- Poor terminal tension
- A chafed wire
- A broken wire inside the insulation
^ When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
^ You may have to drive the vehicle in order to experience a condition.
^ This DTC can be set by a loss of prime.
^ When a P0845 and P0847 are set in combination, this may indicate a short to ground is present at the pressure switch circuit or the shift valve is stuck in the stroked state.

Test Description








The numbers below refer to the step numbers on the diagnostic table.
5. This step tests the TCM response function.
6. This step tests the transmission wiring harness integrity.
7. This step tests and evaluates the internal wiring harness.
8. This step tests the PSM for a failed condition.