What is the working principle of a 4-wire brake switch?
2 Answers
A 4-wire brake switch essentially consists of two brake switches—one normally open and one normally closed. When the brake is pressed, the normally open switch closes, and the normally closed switch opens. Each of the four wires serves a different purpose: one is a 12-volt power supply, one is the brake signal wire for the ABS system ECU, one is the brake signal wire for the automatic transmission ECU, and one connects to the rear brake indicator light. 2-wire brake switch: A 2-wire brake switch operates on a break-and-make principle—when the brake is pressed, the switch closes; when the brake is not pressed, the switch remains open. 4-wire brake switch: The two wires on the side of the brake light switch control the brake lights, while the middle two wires provide the brake signal to the vehicle's computer (ECU). Based on this signal and other conditions, the ECU stops fuel supply to the engine, achieving fuel-saving effects.
I've been rolling around in auto repair shops for over a decade, and I've seen plenty of those 4-wire brake switches. It's just a small device that detects brake pedal position, and those four wires aren't there for nothing. When you press the brake, the pedal pushes a little slider inside the switch, making the contacts close or open to change the circuit. Specifically for the four wires: one connects to the positive power input for 12V current, one is the ground wire completing the circuit, and the other two are output signal wires - one normally closed to power the brake lights and make them illuminate, and one possibly normally open to trigger system responses for ABS or the body control module. Simply put, the working principle is that pressing the pedal turns mechanical action into electrical signals, telling the car to light up red warnings for vehicles behind. In daily work, most failures I encounter come from oxidized wire ends or worn contacts, causing lights to flicker or go out completely. That's when you need to check conductivity with a multimeter, clean the contacts or replace the switch during repair - don't delay to avoid increasing rear-end collision risks. During inspection, always check the fuse isn't blown before testing the switch.
