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Allison 1000 & 2000 Gen 5 Fault Code:P0123 Pedal Position Sensor Circuit – High Voltage

The TCM receives input on throttle position from either a TPS or a signal transmitted by the engine electronic controls.

Vehicles not equipped with electronically-controlled engines have a TPS attached to the engine fuel control linkage. The TPS continuously sends the exact throttle position to the TCM.

The TPS is a sliding resistor sensor (potentiometer) actuated by a mechanical linkage. The TCM delivers a constant voltage to one terminal of the TPS resistive strip. The other TPS terminal connects to ground. The resistor contacts of the TPS are connected to provide a regulated voltage signal input to the TCM.

When actuated by the mechanical throttle cable, the contacts of the resistor move along the resistive strip. As the contacts slide along the resistive strip, a voltage is sent to the TCM. At each increment of 0.178 mm (0.007 inch) along the resistive strip, the contacts deliver a different voltage to the TCM. The different voltages are interpreted as throttle sensor movement. The TCM converts travel distance (mm) into throttle opening percentage.

1. DTC P0123 is stored in the TCM history.
2. The TCM uses the default throttle value, based on engine torque and speed.
3. The TCM forces Variable Modulated Main off.
4. The TCM freezes shift adapts (DNA).
5. The TCM inhibits Torque Converter Clutch (TCC) engagement.
6. The CHECK TRANS light Illuminates (non-OBD II strategy).

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.

DTC P0123 indicates the TCM has detected a voltage signal from the TPS higher than the calibrated supply voltage, that can be caused by the following:
1. A short to battery in W112 or W144 circuits.
2. Analog TPS source set in calibration 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.

When diagnosing for an intermittent short or open circuit condition, 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.