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P codepowertrainMedium severity

P07BFInput/Turbine Speed Sensor "A" Circuit Low

The transmission input/turbine speed sensor "A" signal circuit is reading below its expected voltage range, indicating a short to ground or a failed sensor.

Definition

The transmission control module (TCM) uses the input/turbine speed sensor to measure the rotational speed of the transmission input shaft, which it compares against engine RPM and output shaft speed to control shift timing, torque converter clutch apply, and to calculate gear ratio. P07BF sets when the voltage on the sensor "A" circuit drops below the calibrated minimum threshold, typically from a short to ground or an internally failed sensor. Expect harsh or erratic shifting, no torque converter lockup, an inaccurate or dead tachometer reading on some models, and the transmission defaulting to a limp-home gear with the MIL or transmission warning lamp illuminated.

Common causes

  • Input/turbine speed sensor internal failure
  • Sensor signal wire chafed and shorted to ground
  • Corroded or spread terminals at the sensor connector
  • Damaged harness section between the transmission case connector and TCM
  • Metallic debris on the sensor tip from internal transmission wear
  • Faulty TCM sensor input circuit (rare)

Common misdiagnoses

  • Replacing the valve body or shift solenoids for the resulting harsh shifts
  • Replacing the output speed sensor instead of the input/turbine sensor
  • Rebuilding or replacing the transmission for a wiring short
  • Replacing the TCM before checking the harness

Troubleshooting steps

  1. 1.Verify the fault with live data

    Watch input shaft speed on a scan tool while driving; a reading stuck at 0 RPM or dropping out while engine and output speeds look normal points to the sensor circuit.

  2. 2.Inspect the sensor connector

    Unplug the input speed sensor and check for transmission fluid intrusion, bent pins, corrosion, or green terminals; fluid wicking into the harness is a common cause of shorts.

  3. 3.Test the sensor itself

    For a two-wire magnetic sensor, measure resistance (typically 300-2000 ohms per the service manual) and check for AC voltage output while spinning; a Hall-effect sensor should toggle its signal between low and about 5V.

  4. 4.Check the circuit for a short to ground

    With the sensor and TCM disconnected, measure resistance from the signal wire to chassis ground; anything less than infinite (OL) indicates chafed insulation, often where the harness passes near the bellhousing or exhaust.

  5. 5.Verify reference voltage and ground at the connector

    Key on, confirm the supply pin shows the specified 5V or 8-12V reference and the ground pin shows less than 0.1V drop to battery negative.

  6. 6.Clear the code and road test

    After repair, confirm input speed tracks engine RPM in gear (roughly equal with the converter locked) and that the code does not reset through several warm shift cycles.

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