What is the brake light switch and how does it work?
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.
When to consider replacing the brake light switch?
- No brake lights. If depressing the brake pedal does not cause any of the brake lights to illuminate, the continuity of the switch will be tested. If only one brake light is out, then the issue is not the switch but rather the bulb, bulb socket or brake light circuit integrity.
- Unable to shift out of park. The brake light switch is also wired into the shift interlock mechanism. If you are unable to shift out of park, with your foot on the brake, check to see if the brake lights will illuminate as you push the pedal. If they do not illuminate, failure of the brake light switch may be the cause of the no shift.
- Intermittent operation of the brake lights or late operation. The brake light switch has to be in the proper position, and work consistently if the brake lights are to come on the instant you press the brake pedal. The switch can sometimes require adjustment, or the contacts may degrade, causing inconsistent operation of the switch.
How do mechanics replace the brake light switch?
- 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.
Is it safe to drive with a brake light switch problem?
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.
When replacing the brake light switch keep in mind:
- The mechanic will test the brake light switch directly to ensure that the issue is not elsewhere in the brake light circuit.
- Occasionally, the issue is a maladjusted switch or a loose contact, and the mechanic can resolve the problem without switch replacement.