Allison - Automatic Transmission

DTC P0756

Circuit Description

The pressure switch manifold (PSM) is a multiple-switch assembly made up of 3 normally open (N/O) and 1 normally closed (N/C) pressure switches. The pressure switch 2 (PS2) monitors shift solenoid valve 2 (SS2) positioning and relays it to the transmission control module (TCM). When PS2 is in the open state, shift solenoid valve 2 (SS2) should be in the destroked position.


DTC P0756 is a type A DTC.


DTC Descriptor

This diagnostic procedure supports the following DTC:


DTC P0756 Shift Solenoid (SS) 2 Valve Performance - Stuck Off


Conditions for Running the DTC

* The components are powered and the 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.
* SS2 is commanded to the ON, stroked, position.



Conditions for Setting the DTC

DTC P0756 sets when SS2 is commanded ON and PS2 status remains OFF for 5 seconds, depending on fluid temperature.


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, unless the transmission is compromised by overspeeding or direction change, then the transmission will shift to NEUTRAL.
* When in a forward range and the shift is in process, the transmission will return to a previous range, except for 1-2 shifts and 2-1 shifts where the shifts will be completed.
* When in NEUTRAL or REVERSE, the transmission will fail 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, the transmission will shift to NEUTRAL.
* DTC P0756 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 the 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 may indicate that SS2 is mechanically defective or the shift valve is stuck in the destroked state.
* When DTC P0756 and P0848 are set in combination, this may indicate an open circuit condition is present at the pressure switch circuit or the shift valve is stuck in the destroked position.



Test Description

The numbers below refer to step numbers on the diagnostic table.
5. This step tests 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.