P0708

ALLISON AUTOMATIC TRANSMISSION

DTC P0708
Circuit Description for Allison Automatic Transmission
The installation of a transmission mounted park/neutral position (PNP) switch is required. This 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.

DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0708 Transmission Range (TR) Switch Circuit High Voltage

Conditions for Running the DTC
^ 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.

Conditions for Setting the DTC
DTC P0708 sets when the TCM detects an invalid condition and parity error occurring over consecutive ignition cycles.

Action Taken When the DTC Sets
While diagnostic response is active, one of 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 TCM history.
^ The TCM freezes shift adapts.
^ TCM inhibits torque converter clutch (TCC) engagement.

Conditions for Clearing the DTC
A scan tool must be used to clear the code from TCM history. This DTC does not automatically clear from TCM history after 40 ignition cycles.

Diagnostic Aids
^ Inspect the wiring for poor electrical connections at the TCM and powertrain control module (PCM). 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.
^ If a P0875, previously a P1713, is shown in failure records as being set first on the failure list followed by a P0872, previously a 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, inspect the switch for signs of damage.

Test Description








The numbers below refer to step numbers on the diagnostic table.
4. This step tests TCM input response.
5. This step tests the wiring harness for opens or shorts.
6. This step tests the wiring harness for a short to voltage.