Allison 3000 & 4000 Gen 4 Fault Code: P0123 Pedal Position Sensor Circuit High Voltage

DIAGNOSTIC RESPONSE:
Use default throttle values. Freezes shift adapts.

CIRCUIT DESCRIPTION:
The TCM may be calibrated to receive an analog Throttle Position Sensor (TPS) pedal position signal from a TPS. Throttle position information comes from a resistive sensor. The resistive throttle sensor is attached to throttle linkage. This device produces a voltage proportional to throttle position.

NOTE: Refer to Throttle Position Sensor (TPS) Adjustment for the TPS adjustment procedure and signal voltage specifications.

NOTE: The transmission TPS is not the same accelerator pedal sensor that the driver actuates to the accelerate the vehicle. The transmission TPS is not used to fuel, accelerate, or decelerate the engine or vehicle. The transmission TPS is only wired to the transmission TCM and is only used for shift scheduling purposes based on the TPS signal amount (shown as TPS voltage in data monitor).

NOTE: A throttle source can be specified by:
1. Autodetect of the throttle source (prior to MY 09 new release calibrations). Calibrations that have rolled forward from a previous software release will continue to have the autodetect source available in later CIN versions.
2. Use of CMC Override Throttle Sensor with the diagnostic tool to set throttle source (Analog, SAE J1939, SAE J1587, PWM, or Autodetect). Choose “Autodetect” from the CMC list only if the calibration is prior to MY 09 or the autodetect feature has rolled into a later CIN version.
3. Communication link type picked during calibration request process for MY 09 and later calibrations.

NOTE: New MY 09 and later calibrations that did not maintain autodetect, i.e. did not roll forward from earlier software, should not be re-programmed by the CMC Override Throttle Source. This disables some datalink features for the calibration’s specified communication link type. If a different throttle source is needed, calibrate with a new CIN that enables the desired throttle source.

The TCM is connected to the TPS by:
1. A 5V reference voltage (W112),
2. TPS Signal (W144), and
3. Analog ground (W1458).

The TCM provides a 5V reference voltage to the TPS. A voltage divider network in the sensor produces a TPS signal in response to the pedal position. The TCM interprets this signal as TPS voltage. At each incremental distance of 0.78 mm (0.007 inch) along the resistive strip, the sensor produces a proportional voltage to the TCM on W144. The varying voltage at W144 is interpreted as pedal position and the driver’s demand for engine torque and vehicle speed. The TCM uses this TPS information for transmission shift scheduling and modulation.

CONDITIONS FOR RUNNING THE DTC:
1. An analog TPS must be the throttle source.
2. After TCM initializes normally and engine speed is above 200 rpm for more than 5 seconds, this DTC test will run at 0.10 second intervals.
3. DTC P0123 not active.

CONDITIONS FOR SETTING THE DTC:
DTC P0123 sets if the TCM has autodetected, forced, or been calibrated to receive an analog TPS signal, and the signal voltage is more than 4.75V for 5 seconds.

ACTION TAKEN WHEN THE DTC SETS:
When DTC P0123 is active, the following conditions occur:
1. The TCM does not illuminate the CHECK TRANS light, unless January 2010 software or later.
2. DTC is stored in TCM history.
3. Default value used for % throttle based on engine torque and engine speed.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
Use the diagnostic tool to clear the DTC 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:
DTC P0123 indicates the TCM has detected a voltage signal from the TPS in the high error zone and may be caused by the following:
1. A short to battery in W112 or W144 circuits.

NOTE: Shorts to battery in W112 typically will log other sensor failed low DTCs that are using W112 (5V), such as the Oil Level Sensor (OLS), or Retarder Modulation Request (RMR) Sensor. Shorts to battery in W144 (TPS signal) only causes DTC P0123.

2. Analog TPS source set in calibration or has been autodetected in non-analog TPS applications.
3. Analog TPS source set by CMC Override Throttle Source using the diagnostic tool in non-analog TPS application.
4. Defective sensor sharing 5V supply shorted to battery.
5. 5V wire shorted to battery in transmission internal harness.
6. Broken or defective connector assemblies allowing pushed back pins to short together.
7. Unlocked connectors, missing seal plugs allowing moisture in connectors.
8. TCM issue.
9. TPS not adjusted. Refer to Throttle Position Sensor (TPS) Adjustment.

Review applicable information in Control System and Transmission Specifications to find additional circuit specifications, system and connector diagrams, and troubleshooting tips.