Brake Switch Testing
Brake light switches are normally open. When the pedal is depressed, the contacts close resulting in a complete circuit and brake light illumination.
A multimeter is used to test this switch. Make sure the switch is adjusted properly and receiving proper voltage. With the pedal released there should be voltage entering the switch, and no voltage out. After pressing the pedal in and completing the circuit, voltage should be coming in and out to the brake light circuit as well. Some switches have multiple wires for accessories like cruise control. The manufacturer's manual will provide a diagram to identify circuits and pins.
A brake switch can be tested with and ohmmeter. Remember, never use an ohmmeter on a powered circuit. Remove the plug and connect the tester leads to the switch. Bring the vehicle to a fast idle of around 2000 rpm and turn on accessories like the headlights, A/C blower motor, etc. If the switch fails either of these tests, replace the switch with a new one.