How To Test An Thermal Vacuum Switch

You want to know how to test your thermal vacuum switch to see if it needs to be replaced. If a thermal vacuum switch needs to be replaced it can affect how the engine cools your car and therefore can affect the temperature. The vacuum may not get cool enough to cool the car or warm enough to heat it up properly. The thermal vacuum switch is used to supply or block the vacuum the use of the vacuum to the engine. With the right tools and some knowledge about cars you can test a thermal vacuum switch yourself to see if needs to be replaced.

To test a thermal vacuum switch you need: 

  • vacuum pump
  • vacuum gauge
  1. Perform a visual inspection. Before you test the switch, look around the thermal vacuum switch for cracks or broken back imports. If you see these, you know that your thermal vacuum needs to be replaced.
  2. Connect the vacuum gauge. First, remove one of the vacuum supply lines from the switch, and then connect the hose to the vacuum pump. Remove another of the vacuum lines to from the switch, and now connect the vacuum gauge.
  3. Start the engine. Apply about twenty inches of vacuum. Start the engine to see if the thermal vacuum switch is closed while the engine is cold. Once the engine is started the switch should purge back in from the vacuum pump, and the pump should transfer it to the vacuum gauge. If the switch is not holding vacuum, it may be defective or may just need to be opened when normally cold.
  4. Check if the engine holds vacuum. If normally open when it's cold, after the engine is warm the thermal vacuum switch should not allow the passage of vacuum. If the switch doesn't transfer vacuum as the engine warms, if it never holds vacuum, or if both gauges remain at the same position and don't move, then you need to replace the switch.


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