Thermostat, Electrical Test-4.6L, 5.4L and 6.8L

WARNING: This page is about a different car, the 2005 Ford RV Cutaway, 2005 Ford Econoline, 2005 Ford E450 Super Duty, and 2005 Ford Cutaway. 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 the hood open or closed and with the engine warm or cold.

Check the engine coolant level. Fill as needed.
With the ignition OFF, voltage values (0-5 V) may now be monitored while the cylinder head temperature (CHT) sensor retains its connection to the wiring harness. A scan tool is used to monitor the CHT voltage.
Place the automatic transmission in PARK (P) or the manual transmission in NEUTRAL (N) and engage the parking brake.

Start the engine and allow the engine to idle throughout this test. Allow the engine to run for two minutes, then record the CHT voltage. Record the CHT voltage every 60 seconds. When the CHT 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 COOLANT TEMPERATURE CHART

Coolant Temperature °C (°F)	CHT (Volts)
22 (71)	3.00
43 (109)	2.01
71 (159)	1.01
82 (180)	0.75
92 (197)	0.58
97 (206)	0.50
105 (221)	0.40

If the coolant thermostat opening voltage is greater than 0.58 volt and less than 92°C (197°F), install a new coolant thermostat.
If the coolant thermostat opening voltage is less than 0.58 volt and greater than 92°C (197°F), the coolant thermostat is good and a new coolant thermostat should not be installed. GO to SYMPTOM CHART for further instructions.