Thermostat - Electrical Test

WARNING: This page is about a different car, the 2004 Lincoln LS. 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 can 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, attach the Rotunda 73 Digital Multimeter to the cylinder head temperature (CHT) sensor. It may be used to monitor sensor voltage values between 0-5 volts.

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 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. See COOLANT TEMPERATURE SPECIFICATION .

COOLANT TEMPERATURE SPECIFICATION

Coolant Temp °C (°F)	CHT Min (Volts)	CHT Nom (Volts)	CHT Max (Volts)
0 (32)	4.7714	4.8032	4.8723
25 (77)	4.3428	4.4148	4.4727
50 (122)	3.5354	3.6495	3.7473
80 (176)	2.3049	2.4201	2.5260
110 (230)	1.3021	1.3808	1.4562
140 (284)	0.6960	0.7414	0.7857
170 (338)	0.3784	0.4024	0.4261

If the thermostat opening temperatures are not between 89°C (192°F) and 104°C (219°F) or the corresponding voltages shown on the chart, install a new thermostat.
If the thermostat opening temperatures and voltages are within the specified range, the thermostat is good and a new thermostat is not required. GO to the SYMPTOM CHART .