Electrical Thermostat Test

CAUTION: Always vent the exhaust to the outside when performing this test.

NOTE: Procedure 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.

1. Check coolant level in radiator and engine coolant recovery reservoir. Fill as needed.
2. With the key OFF, remove the Engine Coolant Temperature ECT sensor harness connector and attach the Rotunda Service Coolant Temperature Monitor Harness 007-00064 or equivalent as a jumper between the Powertrain Control Module PCM and the ECT. Attach a Rotunda 73 Digital Multimeter 1005-R0051 or equivalent to the thermostat monitor harness. Voltage values (0 to 5 V) may now be monitored while the sensor retains its connection to the wiring harness.
3. Place transmission in PARK (P) or NEUTRAL (N).

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

4. Start the engine and allow it to idle throughout this test. Allow engine to run for 2 minutes, then record the ECT voltage. Record ECT voltage every 60 seconds. When ECT voltage trend changes direction or changes only slightly (0.03 volts or less) from the previous reading, record this as the thermostat opening voltage. Use the voltage and temperature chart that follows:
Coolant Temperature ECT Volts
22° C (71° F) 3.00 volts
43° C (109° F) 2.01 volts
71° C (159° F) 1.01 volts
82° C (180° F) 0.75 volts
91° C (195° F) 0.59 volts
97° C (206° F) 0.50 volts
105° C (221° F) 0.40 volts
5. If the thermostat opening voltage is greater than 0.75 volts and less than 82° C (180° F), replace the water thermostat.
6. If the thermostat opening voltage is less than 0.75 volts and greater than 82° C (180° F), water thermostat is good and should not be replaced.