Brake Lights and CHMSL
FIGURE 39-14
Center high mount stop light.

Brake lights, which may also be called stop lights, are red lights mounted to the rear of the vehicle. They are usually incorporated in the taillight cluster. Many vehicles by law now have a higher additional third brake light mounted on top of the trunk lid or on the rear window. This light is called a center high mount stop light (CHMSL), or “chimsul” Figure 39-14. The brake lights are activated whenever the driver operates the foot brake to slow or to stop the vehicle or when a control module automatically applies the brakes. The lighting circuit consists of the battery, fusible links and fuses, a brake light switch, brake light bulbs, wiring to connect the components, and the ground circuit to return current from the light to the battery. It may also include a BCM to command the lights on when the proper inputs are present.

On older vehicle models, when the operator of the vehicle depresses the brake pedal, a switch mounted on the pedal support closes. This allows the electrical current to flow from the battery through the fuse, through the switch, to the brake lamp and to return to the battery by the ground circuit. When the driver releases the pedal, it returns to the rest position and opens the brake switch. The flow of electrical current stops and the brake lamps are extinguished. Today, computer-controlled brake lights are activated by the body control module (BCM) when the computer sees an input from the brake pedal switch.

AS-33: Refraction: The technician can demonstrate an understanding of refraction as it occurs in systems that employ fiber optics.
A vehicle is in the shop for repairs to the lighting system. The repair order cites as the customer concern that the illumination for the console-mounted shifter indicator is not working. The driver is unable to determine which gear is selected for the automatic transmission at night.

Ethan is the apprentice technician at the dealership who was given the assignment of solving this concern. The first step is to verify the customer concern by covering the windows of the vehicle to simulate night conditions. Ethan looks up the manufacturer’s information on a shop computer. Expecting to find an illustration showing the location of a bulb for illumination, he finds the procedure for removal of the illumination control. He then begins the disassembly procedure to get to the components involved. Rather than a conventional light socket and bulb, Ethan finds a fiber-optic lighting assembly that provides illumination for the shifter indicator.

Fiber optics components have been used by Mercedes-Benz, BMW, Daimler, Audi, Porsche, Volvo, and Cadillac. Some of the applications include dashboard lights, interior lights, taillights, and entertainment and information systems.

Refraction is defined as the change in direction of light due to a change in speed as it passes from one medium to another. A reference for the path of light is described in respect to the normal path of light. If light enters a new medium and is slowed down, it bends to the normal path. If it speeds up, it bends away from the normal path.

Aftermarket lighting accessories that are available today include fiber-optic taillights with reverse lights for certain vehicles. These lights are advertised to be considerably brighter than stock units and to light up faster and run cooler. Due to the use of fiber rather than light bulbs, fiber-optic lights are said to be vibration-proof.