How to Tell if a Brake Light Switch Is Faulty

Place the vehicle in park or neutral with the emergency brake set. Raise the hood and disconnect the negative battery cable with a socket and wrench. Have an assistant step on the brakes. Determine how many brake lights have no illumination, including a top-mounted or roof brake light. Use a screwdriver to remove the brake light lens cases to the taillight assembly. Refer to your owner's manual for the proper access points of the mounting bolts. Some rear taillight assemblies must be accessed through the rear compartment kick panel.

Twist the bulb locking ring on the back of the taillight housing and pull the bulb socket out of the case housing. Remember that some dual-element bulbs function as the taillight and brake light. Twist the bulb from its socket with a rag on your fingers and examine the filament or filaments. A melted or missing filament means the bulb has blown and you must replace it before continuing on. Replace all blown bulbs.

Unclasp the wire jack connectors to the brake lights. Use electrical contact spray to clean the inside contacts of all the connector jacks. Reconnect the wire jacks, then insert any new bulbs. Replace the taillight lenses back into their housings and tighten the mounting screws or bolts with a screwdriver or socket.

Refer to your owner's manual for the location of your main fuse box. Remove the fuse box lid and read the schematic on the inside cover. Use a fuse puller to pull out the taillight and brake light fuses. Replace any fuse that has a blown filament. Swap the taillight relay for another similar relay that has the same amperage and pin configuration. This will rule out a bad relay. After checking the fuses and relay, reconnect the negative battery cable with a socket and test the brake light operation.

Slide under the dashboard on the driver's side and follow the brake pedal up to the brake light switch. The switch looks like a small case, with a pin that extends from the case and rests on the back of the brake pedal. Unhook the brake switch wire jack. Applying no pressure on the brake pedal, touch the probe of a test light to both connectors inside the wire jack. Make sure to ground the test light alligator clip to a metal surface.

Look for either wire contact to illuminate the test light. If both wires light up, it means the brake light switch has stuck in the on position. Press the pin actuator on the switch back and forth manually with your fingers. Spray contact cleaner inside the pin and work the pin in and out. If both wire connectors still illuminate, it means the brake light switch has broken internally and must be replaced.

Touch the wire jacks with the test light probe, with no pressure on the brake pedal. If the test light does not illuminate on either connections, it indicates a wire short from the switch to the fuse box. Trace the wire down and look for breaks or damaged insulation in the wire loom leading from the switch to the fuse box.

Push the brake pedal down. Place the test light probe on each connection. If both connections do not illuminate the test light, it indicates a defective switch. Push the brake pedal down. If one of the connections illuminates and the other does not, it means the brake light switch has proper voltage going to it and works correctly.