How could you tell if hot and cold switch is bad?

Answer 1

Temperature switches usually provide ground to the device they run at the temperatures required. For instance, a temperature switch used to run an electric cooling fan will provide ground when the coolant level reaches a specified temp. If your cooling fan temp switch has three leads to it, it is designed to run the motor faster at a higher temperature. To test it unplug it and use an ohm meter to test between the power lead and the other lead(s) on the switch. It should have infinite ( closed circuit ) when cold. Plug it back in and let the engine idle until its warm; the fan should start up. If not, turn off the motor unplug the switch again and test the leads. They should show continuity. If you are testing a thermo time switch ( for a cold start injector ) with the engine cold have a friend start the motor while you use a volt meter ( preferrably with back tester attachments ) to the leads to the switch. When first started ( engine cold ) the volt meter should read voltage for a specified amount of time ( normally 7 or 8 seconds ) then shut off. If started with the engine warmed up it should show no voltage.