Allison - Automatic Transmission

DTC P0872

Circuit Description

The pressure switch manifold (PSM) is a multiple-switch assembly made up of 3 normally open (N/O) pressure switches and 1 normally closed (N/C) pressure switch. Pressure switch 3 (PS3) monitors shift valve 3 positioning and relays it to the transmission control module (TCM).


DTC P0872 is a type A DTC.


DTC Descriptor

This diagnostic procedure supports the following DTC:


DTC P0872 Transmission Pressure Switch 3 Circuit Low


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.
* Shift valve 3 is commanded to the OFF, destroked, position.



Conditions for Setting the DTC

DTC P0872 sets when PS3 indicates ON for 20 milliseconds after shift solenoid 3 (SS3) 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 overspeeding 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, then 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 overspeeding or a direction change. If compromised, the transmission will shift to NEUTRAL.
* DTC P0872 is 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.
* When DTC P0762 and P0872 are set in combination, this may indicate a short to ground is present at the pressure switch circuit.
* This DTC may indicate that SS3 is mechanically defective.
* This DTC can be set by a loss of prime.



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.