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How to Test a Car Fan Thermostat Switch

by William Kinsey

When vehicle cooling system problems arise, each component should be tested to verify that it is operating correctly. The first components that should be tested are the cooling fan, the cooling fan relay and the thermostat switch, which turns the fan on and off depending on the temperature of the engine. The thermostat switch can be tested easily after it has been removed from the vehicle. Testing requires only a few minutes and a few tools.

Step 1

Place a small sauce pan on the stove and fill it with one cup of water. Place the thermostat switch in the sauce with the terminals turned up.

Step 2

Place the thermometer in the sauce pan so you can read it as the temperature increases. Connect one of the ohmmeter probes to one of the thermostat terminals and connect the other probe to the second thermostat terminal.

Turn on the burner and slowly increase the water temperature. The ohmeter should be turned on and have a reading of high resistance. When the water reaches between 180 to 220 degrees Fahrenheit, the switch should turn on and the resistance seen by the ohmmeter should change to zero. If the resistance does not change to zero, then the switch is bad.

Items you will need

  • High temperature thermometer
  • Pan
  • Water
  • Wrench
  • Ohmmeter with alligator clip probes

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