Allison Automatic Transmission

ALLISON AUTOMATIC TRANSMISSION

DTC P0708

DTC P0708 Step 1 - 5:

DTC P0708 Step 6 - 9:

Circuit Description for Allison Automatic Transmission
The installation of a transmission-mounted neutral start/reverse signal switch is required. This switch, commonly called a "Park/Neutral Position (PNP)" switch, mounts directly onto the transmission housing from the outside and detects the angular position of the shift selector shaft. This position is communicated to the Transmission Control Module (TCM) so that certain vehicle control functions can be coordinated with the position of the shift controls. The PNP switch has redundant circuitry to alert the TCM in the event of a single wire or switch failure.

The neutral signal output of the PNP switch is typically used as confirmation that the transmission is in neutral before the engine starter is engaged. The PNP switch is interfaced to the starter circuit with weatherproof electrical connectors. The reverse signal provision may be used to activate vehicle back-up lights and/or reverse warning devices.

DTC P0708 is a type A DTC.

Conditions for Running the DTC
^ The components are powered and ignition voltage is greater than 9 V 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 P0708 sets when the TCM detects an invalid condition and parity errors occurring over consecutive ignition cycles.

Action Taken When the DTC Sets
While diagnostic response is active, the following conditions can occur:
^ The TCM illuminates the Malfunction Indicator Lamp (MIL).
^ The transmission fails to hydraulic default.
^ DTC P0708 will be stored in the TCM history.
^ The TCM freezes shift adapts (DNA).
^ Illuminates the MIL when active.
^ Due to this failure and associated response, DTC P0722 and DTC P0845 also set.
^ 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 the DTC recurring.

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.
^ If a P0875, previously P1713, is shown in failure records as being set first on the failure list followed by a P0872, previously P1711, combined with P0708, this generally indicates a PNP internal switch failure. Often when the DTCs are cleared, they do not return. This may be due to moisture that was present at the time the DTC was set, causing a short internally. Inspect the PNP switch linkage carefully for proper adjustment. Also check the PNP switch for signs of damage.

Test Description
The numbers refer to the step numbers on the diagnostic table.
3. This step tests PNP switch status.
4. This step tests TCM input response.
5. This step tests the wiring harness for opens or shorts.