Thermostat-Electrical Test

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. 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) and the ECT Sensor. Attach the 73111 Automotive Meter 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 may 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 incorrect diagnosis.

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 volts or less) from the previous reading, record this as the thermostat opening voltage. Use the voltage and corresponding coolant temperature chart listed below.





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 a new water thermostat should not be installed. Refer to the Symptom Chart for further instructions.