Brake Switch Testing
Brake light switches are simple (NO) normally open switches. When the brake pedal is depressed, a set of contacts are closed, resulting in a complete circuit and brake light illumination. In time these contacts wear, resulting in no signal. This problem can affect the brake lights, the cruise control system, and other systems that depend on a signal from this switch.
A multimeter can be used to test a brake switch. With the brake 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 present at both ends of the brake light switch. Some switches have multiple wires for accessories like cruise control. The manufacturer's manual will provide a diagram to identify these pins. Always confirm where a wire leads before testing it. ECM's are sensitive, and one wrong move could spell disaster.