P0870

ALLISON AUTOMATIC TRANSMISSION

DTC P0870

DTC P0870 Step 1 - 8:

DTC P0870 Step 9 - 13:

Circuit Description for Allison Automatic Transmission

Important: DTC P0870 was previous P1709.

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 E 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. The pressure switch corresponding to reverse is normally closed, since fluid pressure is always present unless the selector valve is moved to Reverse.

DTC P0870 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 P0870 sets during steady state operation when pressure switch D indicates ON for 0.1 second after solenoid D has been commanded OFF.

Action Taken When the DTC Sets
^ The TCM illuminates the Malfunction Indicator Lamp (MIL).
^ When in a forward range, the transmission will fail to another forward range, unless the transmission is compromised by overspending or direction change, then the transmission will shift to NEUTRAL while diagnostic response is active.
^ If the shift selector is moved from a forward range to NEUTRAL or from a forward range to REVERSE, the transmission will shift to NEUTRAL.
^ If the shift selector is moved to REVERSE, the transmission will shift to REVERSE if it is not compromised by overspending or direction change. If compromised, the transmission will shift to NEUTRAL.
^ DTC P0870 will be stored in the TCM history.
^ The TCM freezes shift adapts (DNA).
^ 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.
^ This DTC may indicate that solenoid E is mechanically defective or the shift valve is stuck in the stroked state.

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.
8. This step tests the [Pressure Switch Manifold (PSM) for a failed condition.