1991 Mercedes-Benz 420SEL Brake Light Switch Replacement Cost

The brake light switch is known to fail under all conditions, and at no particular age or mileage. Often they are placed where the drivers feet may contact them, and dust, dirt, water, and other contaminants are common. These factors can contribute to malfunctioning. Especially in stop and go traffic, the brake light switch is constantly in use, and this wear and tear will eventually lead to failure.

Diagnosis of the brake light switch is quick and easy for the trained professional. When the brake lights fail to illuminate, or the transmission cannot be shifted from park, the technician will look for a bad fuse first, then, the brake light switch electrical connector must be tested to confirm it is delivering sufficient electrical power to the switch. If there is sufficient power, the switch is tested with the button pressed and released. The result will determine if the switch has failed. When the brake lights will not turn off, the switch is inspected to ensure contact is being made with the brake pedal. If the brake light switch is mechanically functioning properly, it will be tested with the button pressed and released.

Replacement of a faulty switch requires removal of retaining hardware, and one electrical connector. Installation is the reverse, but the switch must be adjusted so that the brake lights will turn on when the brake pedal is pressed, and turn off when the brake pedal is in the resting position.

If the repair and diagnosis of your brake light issue is being trusted to a repair facility, we recommend having the vehicle towed, and taken out of service prior to the repair. In cases where the transmission cannot be shifted from park, the best course of action is to use a flatbed wrecker that raises all four wheels of the vehicle off the ground, preventing harm to the transmission as the drive wheels will not turn.

Failure of the transmission to shift from park is not always caused by the brake light switch. The shift interlock system may be faulty, or contaminants, like coffee and soda, may have spilled in the shift interlock release, causing it to stick.

This is an excellent DIY project that can be completed with minimal effort. We do not advise swapping parts as a testing method, as greater cost is normally incurred in this manner. Assuming a very basic knowledge of the path of electricity through the switch, testing the switch, fuse and circuit are very basic, and straightforward. Also, replacement requires only basic hand-tools. Persons who lack a basic knowledge of testing a series circuit should refer diagnosis to a certified professional.