This step shows how to test the belt switch will require a multimeter. If you aren't familiar with how to use one, here is a guide that will help you.

Check the continuity between the switch wires while the idler pulley arm is down.

The multimeter should show an open circuit (OL).

Next, test to see that the belt switch shows continuity when the idler pulley arm is raised.

The multimeter should read less than 1 Ω.

If the switch fails either of these tests, replace it. Go on to the next step.