Brake Light Circuits
Taillight assemblies usually include circuits for the parking, brake, turn and hazard lights. Some vehicles have a bulb dedicated to each taillight function. These are called three bulb taillight systems. Each side of the vehicle has a bulb dedicated for the turning circuit, one for the brake, and one for the running lights. Most taillight assemblies use multipurpose dual filament bulbs. These are called two bulb systems. These bulbs have a high and a low intensity filament. The high intensity filament is shared by the hazard, turning, and brake light circuits.
It’s important for a technician to understand that on these systems the turn signal and hazard switch share the same circuit as the brake lights. These turn signal switches are often a complex series of contacts. When these contacts get old and burn the end result can be brake light failure.
The brake light switch sends power to the turn signal switch. If there’s power coming in from the brake switch and no power coming out to the brake light circuit the turn signal switch has an open and should be replaced.
There are two types of brake switches found on today’s vehicles. The most common is operated by the brake peddle. As the brake peddle is moved toward the floor the switch is closed and power is sent to the brake circuit. A pressure controlled switch is located on the master cylinder. As the pressure inside the master cylinder increases a set of contacts is closed sending power to the brake circuit.
