Allison 1000 & 2000 Gen 4 Fault Codes: P0708 Transmission Range Sensor Circuit High Input

CIRCUIT DESCRIPTION:
The installation of a transmission-mounted Neutral Start/Back-Up (NSBU) 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 NSBU switch has redundant circuitry to alert the TCM in the event of a single wire or switch failure.

The neutral signal output of the NSBU switch is typically used as confirmation that the transmission is in N (Neutral) before the engine starter is engaged. The NSBU 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.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V.
2. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.

CONDITIONS FOR SETTING THE DTC – SOFTWARE LEVELS PRIOR TO N11:
DTC P0708 sets when the TCM detects an invalid condition or parity errors occurring over consecutive ignition cycles.

ACTION TAKEN WHEN THE DTC SETS – SOFTWARE LEVELS PRIOR TO N11:
1. The diagnostic response is to shut all solenoids off and allow the transmission to operate in hydraulic default mode or “limp home”. Shift selector and hydraulic state of logic valves determines range attained.
2. DTC P0708 is stored in the TCM history.
3. The CHECK TRANS light illuminates.
4. The TCM inhibits TCC engagement.
5. Due to this failure and associated responses, P0722 and P0845 may also set.

ACTION TAKEN WHEN THE DTC SETS – SOFTWARE LEVEL N11 AND LATER:
1. The CHECK TRANS light illuminates but no diagnostic response is implemented until after 50 warm-up cycles occur when a NSBU switch parity error is active. A warm-up cycle is defined as a 20 degree change in transmission temperature.
2. The diagnostic response is to shut all solenoids off and allow the transmission to operate in hydraulic default mode or “limp home.” Shift selector and hydraulic state of logic valves determines range attained.
3. DTC P0708 is stored in the TCM history.
4. The TCM inhibits TCC engagement.
5. Due to this failure and associated responses, P0722 and P0845 may also set.
6. A shift to R (Reverse) will allow reverse range if a P0876 (previously P1714) is not active.

CONDITIONS FOR CLEARING THE MIL/DTC
A Scan Tool may be used to 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:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. 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.

When a P0708 is shown in failure records, this indicates that conditions to set a P0708 were present at one time. If the condition was due to moisture intrusion, the moisture may no longer be present, and thus you may not be able to reproduce the DTC. In such case, variables such as vehicle mileage, external physical condition of the switch should be factored in when making a decision to replace the NSBU switch.

NOTE: Due to the TCM logic used to detect and set a P0708, this code can remain active even after a NSBU switch replacement is complete and the ignition has been cycled. It is important to clear all active DTCs from the TCM after servicing the NSBU switch.