P0848

ALLISON AUTOMATIC TRANSMISSION

DTC P0848

DTC P0848 Step 1 - 10:

DTC P0848 Step 11 - 13:

Circuit Description for Allison Automatic Transmission
The Pressure Switch Manifold (PSM) is a multiple-switch assembly made up of three normally open and one normally closed pressure switches. Fluid pressure is fed from Shift Valve D to the switch based on the position of the valve. The shift valve fluid pressure reflects the logic condition at the corresponding solenoid. This logic indicates the current transmission operating range to the Transmission Control Module (TCM).

The fluid pressure switch corresponding to the shift valve is normally open, contacts not touching, when no fluid pressure is present, so that electrical current is stopped at the switch. When fluid pressure is routed to the switch, it moves the diaphragm and upper contact so that the contact element touches both the positive and ground contacts. This closes the circuit and allows current to flow from the positive contact and through the switch.

DTC P0848 is a type A DTC.

Conditions for Running the DTC
^ The components are powered and ignition voltage is greater than 9V and less than 18 V.
^ Engine speed is greater than 200 RPM and less than 7,500 RPM for 5 seconds.

Conditions for Setting the DTC
DTC P0848 sets during steady state operation when pressure switch D fails to switch ON after 0.1 second of Solenoid D being commanded ON.

Action Taken When the DTC Sets
^ 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 re-attained.
^ DTC P0848 will be stored in the TCM history.
^ The TCM freezes shift adapts (DNA).
^ Illuminates the MIL when active.
^ TCM inhibits TCC engagement.

Conditions for Clearing the MIL/DTC
A scan tool can clear the code 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
^ Inspect the wiring for poor electrical connections at the TCM. Look 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 fault.
^ Solenoids are sensitive to battery voltage spikes and poor ground connections. This condition might not be indicated with your Scan Tool, or even with a DVOM. The ideal way to test for these conditions would be to attach an oscilloscope.
^ Multiple inactive pressure switch DTC's may be due to a plugged Control Main Filter. Ensure that the initial 8,000 km (5,000 mile) filter change was performed.

Test Description
The numbers 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 and checks the Pressure Switch Manifold (PSM) for a failed condition.
8. This step tests and evaluates the internal wiring harness.