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Thermostat-Electrical Test

WARNING: This page is about a different car, the 2003 Ford RV Cutaway, 2003 Ford Econoline, 2003 Ford E550 Super Duty, 2003 Ford E450 Super Duty, and 2003 Ford Cutaway. However, it is still accessible from the selected car via links, so may be relevant.
CAUTION: Always vent the exhaust to the outside when performing this test.
NOTE: The electrical thermostat test is most accurate if performed indoors at less than 37.8°C (100°F) ambient air. This test may be performed 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, remove the engine coolant temperature (ECT) sensor harness connector and attach ECT Sensor "T" Cable as a jumper between the powertrain control module (PCM) (12A650) and the ECT Sensor. Attach the 73 Digital Multimeter to the ECT Sensor "T" Cable. Voltage values (0-5 V) may now be monitored while the sensor retains its connection to the wiring harness.

    The scan tool can be used to monitor the ECT on vehicles equipped with data link connector (DLC).

    3. NOTE: Running this test with the vehicle in gear or with the A/C compressor clutch engaged (running) will cause improper 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 voltage or less) from the previous reading, record this as the thermostat opening voltage. Use the voltage and corresponding coolant temperature chart listed below.
    Fig 1: Coolant Temperature And ECT Volts Chart
    G01537988Courtesy of FORD MOTOR CO.
  5. If the thermostat opening voltage is greater than 0.75 volts and less than 82°C (180°F), install a new water thermostat.
  6. If the thermostat opening voltage is less than 0.75 volts and greater than 82°C (180°F), the water thermostat is good and should not be replaced. GO to SYMPTOM CHART  for further instructions.
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