* Low Air Pressure Switch: A crucial component is a low air pressure switch. This switch is typically set to activate around a certain low air pressure threshold (usually around 60 PSI, though it varies by manufacturer and model). When the air pressure drops below this threshold, the switch closes, completing a circuit and activating the brake lights. This is a safety feature – if the air pressure is low, it means the brakes may not function properly, so the brake lights warn other drivers.
* Brake Pedal Switch (Secondary): While the low air pressure switch is the primary method, many trucks also incorporate a secondary brake light switch activated by the brake pedal itself. This works similarly to a car's brake light switch: when you depress the brake pedal, this switch closes and activates the brake lights, regardless of the air pressure (assuming the air pressure is sufficient to operate the brakes). This switch serves as a backup to ensure the brake lights work even with normal air pressure if the low air pressure switch fails.
* Electrical Circuit: The switches (low air pressure and brake pedal) are part of an electrical circuit that leads to the brake lights. When a switch closes, it completes the circuit, allowing electricity to flow and illuminate the lights.
In summary: The brake lights on a truck with air brakes are activated primarily by a low air pressure switch as a safety measure, and secondarily by a brake pedal switch for normal braking operation. Both work to complete an electrical circuit, turning on the brake lights. If one system fails, the other provides redundancy to ensure the lights operate.
