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Thermostat, Electrical Test - 5.4L, 6.8L

WARNING: This page is about a different car, the 2005 Ford Pickup, 2005 Ford F550 Super Duty, 2005 Ford F450 Super Duty, and 2005 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
CAUTION: Always vent the exhaust to the outside when carrying out this test.
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. A diagnostic tool may be used to monitor the cylinder head temperature (CHT) sensor 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 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 engine coolant temperature (ECT) voltage. Record the ECT sensor voltage every 60 seconds. When the ECT sensor 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.
    VOLTAGE AND CORRESPONDING COOLANT TEMPERATURE

    Coolant Temperature °C (°F) ECT (Volts)
    22 (71) 3.00
    43 (109) 2.01
    71 (159) 1.01
    82 (180) 0.75
    91 (195) 0.59
    97 (206) 0.50
    105 (221) 0.40
  5. If the thermostat opening temperature is less than 88°C (190°F) on 5.4L engines or 82°C (180°F) on 6.8L engines, install a new coolant thermostat.
  6. If the thermostat opening temperature is greater than 82°C (180°F), the coolant thermostat is good, a new thermostat should not be installed. GO to SYMPTOM CHART  for further instructions.
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