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Failures and Diagnosis of the Pressure Sensor in Automotive Air Conditioning

In this article, we will explore what happens when the pressure sensor in automotive air conditioning fails, how to identify the symptoms, and the proper process for diagnosing it.

Difference Between a Pressure Sensor and a Pressure Switch

Before starting, it’s essential to distinguish between a pressure sensor and a pressure switch:

  • Pressure Switch: A mechanical device that activates or deactivates the compressor when a specific pressure is reached.
  • Electronic Pressure Sensor: Continuously sends signals to the system controller, allowing precise strategies to ensure user comfort and optimal system performance.

Common Symptoms of a Faulty Pressure Sensor

  1. Air conditioning does not turn on:
    • When pressing the power button, the system fails to activate due to incorrect signals from the sensor.
    • A “Check Engine” light may illuminate on the dashboard.
    • This issue can often be diagnosed using a scanner to read error codes related to the sensor circuit.
  2. Intermittent compressor operation:
    • The compressor cycles on and off rapidly, creating a start-stop pattern that compromises system performance.
  3. Insufficient cooling:
    • The air conditioning cannot reach the desired temperature because the controller shuts off the system prematurely due to incorrect sensor readings.
  4. Fluctuations in engine RPMs:
    • Erratic sensor signals (too high or too low) cause engine RPMs to rise and fall intermittently.

Common Causes of Pressure Sensor Failures

  1. Natural wear and tear:
    • Continuous use and exposure to high temperatures degrade the sensor over time.
  2. Voltage fluctuations:
    • Electrical issues in the vehicle can disrupt the sensor’s operation.
  3. Refrigerant contamination:
    • Impurities in the refrigerant can damage the sensor and alter its readings.

Diagnosing the Pressure Sensor

Before replacing the sensor, it is essential to check the voltage, signals, and wiring to ensure the issue does not persist after replacement.

1. Checking the Power Supply Voltage

  • With the system turned on, use a multimeter set to DC voltage.
  • Place the multimeter’s black probe on one of the sensor pins and the red probe on the other pins until a reading of 5V or 12V is found.
  • The pin showing positive voltage indicates the power supply, while the corresponding negative pin is the sensor’s ground.

2. Identifying the Signal Wire

  • The remaining pin is the signal wire, which should display variable voltage when the air conditioning is active.
  • If the sensor is on the high-pressure side, the voltage should increase when the vehicle accelerates.
  • If the sensor is on the low-pressure side, the voltage should decrease when the vehicle accelerates.
  • If there are no variations in the signal, the sensor is likely faulty.

3. Checking for Power Supply Issues

  • If no power supply pin is identified, the issue may lie in the vehicle’s electrical circuit rather than the sensor itself.

By carefully diagnosing and addressing pressure sensor issues, you can restore your automotive air conditioning system’s performance and reliability.

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