CDL Combination Vehicles TestInspecting Combination VehiclesMedium
During an en route inspection, you notice that the trailer's brake lights are not illuminating when the service brake pedal is pressed in the tractor. The turn signals and running lights are working correctly. What is the most likely cause of this specific issue?
- AA faulty connection or break in the brake light circuit wire within the pigtail or trailer wiring.
- BA problem with the tractor's air compressor.
- CThe trailer's ABS system has a major fault.
- DA blown fuse in the tractor's lighting circuit for all trailer lights.
Show answer & explanationAnswer & explanation
Correct answer: A. A faulty connection or break in the brake light circuit wire within the pigtail or trailer wiring.
If only the brake lights are not working while other trailer lights (turn signals, running lights) are functional, it points to a problem specific to the brake light circuit. This is most likely a loose connection, a break in the wire, or a faulty bulb within that specific circuit, either in the pigtail or the trailer's internal wiring.
Why the other options are wrong
- B. The air compressor affects the air brake system, not the electrical brake lights directly.
- C. An ABS fault would affect the anti-lock braking function and typically has its own indicator light, not usually causing only brake lights to fail while others work.
- D. A blown fuse for 'all' trailer lights would affect turn signals and running lights as well, which are stated to be working.
Trailer Lighting Troubleshooting (Specific Faults)
Troubleshooting specific trailer light failures (e.g., only brake lights) involves isolating the fault to the particular circuit, wire, or bulb responsible, as opposed to general power issues.
- Each light function (running, turn, brake) has a dedicated circuit.
- A selective failure points to a localized issue in that circuit.
- Inspect bulbs, connections, and wiring for corrosion or breaks.
Memory trick: One light out, check its wire; all lights out, check higher!