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Thermostat - Electrical Test

WARNING: This page is about a different car, the 2005 Mercury Montego, 2005 Ford Freestyle, and 2005 Ford Five Hundred. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: The electrical thermostat test is most accurate if carried out indoors at less than 37.8°C (100°F) ambient air. This test may be carried out with or without the hood open and with the engine warm or cold.
  1. Check the engine coolant level. Fill as needed.
  2. With the ignition OFF, attach the 73III Automotive Meter to the engine coolant temperature (ECT) Sensor. Voltage values (0-5 V) may now be monitored while the sensor retains its connection to the wiring harness.

    A diagnostic tool or the Service Bay Diagnostic System (SBDS) may be used to monitor the ECT. The SBDS sequence to use for the screen is: Toolbox-Electronic Engine Control and DCL-Item.

    3. NOTE: Running this test with the vehicle in gear or with the A/C compressor clutch engaged (running) will cause incorrect diagnosis.
  3. Place the transmission in PARK (P) or NEUTRAL (N).
  4. Start the engine and allow the engine to idle throughout this test. Allow the engine to run for 2 minutes, then record the ECT voltage. Record the ECT voltage every 60 seconds. When the ECT voltage trend changes direction or only changes slightly (0.03 volt or less) from the previous reading, record this as the thermostat opening voltage. Use the voltage and corresponding coolant temperature chart listed below.
    DIAGNOSIS PROCEDURE

    Coolant Temperature °C (°F) ECT (Volts)
    20 (68) 3.14
    45 (113) 2.98
    70 (158) 1.08
    75 (167) 0.94
    82 (180) 0.75
    90 (194) 0.64
    95 (202) 0.53
    100 (212) 0.49
    105 (221) 0.43
  5. If the thermostat opening voltage is greater than 0.94 volt and less than 75°C (167°F), or less than 0.53 and greater than 95°C (202°F), install a new thermostat.
  6. If the thermostat opening voltage is less than 0.75 volt and greater than 82°C (180°F) or greater than 0.50 volt and less than 97°C (206°F), the thermostat is good and a new thermostat should not be installed. GO to SYMPTOM CHART  for further instructions.
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