Electrical System and Safety Devices
What the exam checks
This domain tests the seven-way electrical system, trailer lighting, ABS lamps, reflective devices, and the safety equipment that must work on every trailer in a double or triple. You must be able to trace a lighting failure from tractor to dolly to rear trailer.
Core rules
Inspect the electrical cord and receptacle for cracked insulation, corroded pins, missing strain relief, and poor contact. The cord must be long enough to turn without stretching, but routed so it cannot drag or burn on the catwalk. With lights on, check clearance lamps, marker lamps, identification lamps, tail lamps, brake lamps, turn signals, and four-way flashers on every trailer. Confirm the ABS lamp cycles and goes out, and check reflective tape and conspicuity markings. Safety devices include mud flaps, safety chains, emergency lines, warning triangles, and a fire extinguisher. On a converter dolly, check its lamps and connections because a rear trailer light failure may begin at the dolly.
Worked example
The rear trailer's right turn signal does not work, but the tractor's signal does. The driver should check the dolly connection first, then the rear trailer cord and bulb. Because the dolly is between the tractor and rear trailer, a corroded dolly receptacle can interrupt the circuit. The driver must repair it before driving; a hand signal does not substitute for required electric signals.
Traps that cost marks
Do not check lights only at the tractor. Do not ignore a dim lamp or a fast-flashing signal. Do not let the electrical cord drag on the dolly or fifth wheel. Do not treat ABS warning lamps as unimportant.