The brake light switch is a mechanical switch activated when you press the brake pedal. The switch has a plunger in it that contacts a lever in the brake pedal assembly. As that plunger is depressed when you push on the brake pedal, the contacts in the brake light switch closed and voltage flows to the brake lights — turning them on. The brake light switch is also in the circuit that controls the cruise control disengagement and the shift interlock safety feature installed on all newer cars. Failure of the brake light switch occurs due to wear on the internal electrical contacts in the switch, poor switch terminal connections, maladjustment of the switch, or damage to the brake pedal assembly.
The switch is first tested using a multimeter, including the terminal contacts to the switch, to determine if it is faulty. If the switch is functioning, the entire circuit must be tested to determine the cause of no brake lights. Should the switch be judged defective, the switch is unthreaded from the mounting boss near the brake pedal assembly. The new switch is threaded in and locked with an appropriate locknut once the new switch is properly adjusted. The operation of the switch is tested to ensure instant illumination of the brake lights upon pressing the brake pedal.
No. Should the brake light switch have failed, it is unsafe to drive the vehicle because drivers behind you will have no indication that you are slowing, intend to slow, or are stopped. Always address a failure of this type promptly to ensure your safety while driving.