Thermostat - Electrical Test

WARNING: This page is about a different car, the 2002 Ford Ranger. 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.

Check the engine coolant level. Fill as needed.
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.
A scan tool may be used to monitor the ECT on vehicles equipped with Data Link Connector (DLC).

NOTE: Running this test with the vehicle in gear or with the A/C compressor clutch engaged (running) will cause incorrect diagnosis.

Place the transmission in PARK (P) or NEUTRAL (N).

Start the engine and allow the engine to idle throughout this test. Allow the engine to run for two 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.

VOLTAGE & 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)	0.059
97 (206)	0.050
105 (221)	0.040

If the thermostat opening voltage is greater than 0.58 volt and less than 90°C (194°F), install a new water thermostat.
If the thermostat opening voltage is less than 0.58 volt and greater than 90°C (194°F), the water thermostat is good and a new thermostat should not be installed. GO to SYMPTOM CHART for further instructions.