A vehicle's backup lights are inoperative, but all other lights on the vehicle work correctly. The technician checks the fuse for the backup light circuit and finds it to be good. When the transmission is shifted into reverse, a voltmeter connected to the backup light switch output wire shows 12V. What is the MOST likely cause?
- AA faulty backup light switch.
- BAn open circuit in the ground wire for the backup lights.
- CAn open circuit in the power wire from the backup light switch to the backup lights.
- DBoth backup light bulbs are burnt out.
Show answer & explanationAnswer & explanation
Correct answer: C. An open circuit in the power wire from the backup light switch to the backup lights.
Since the backup light switch is outputting 12V when in reverse, the switch and its input power are functioning correctly. If the lights are still inoperative, the problem lies between the switch and the bulbs. An open circuit in the power wire after the switch would prevent voltage from reaching the bulbs. If the bulbs were bad, voltage would still be present at the sockets; if the ground was bad, voltage would be present but the circuit incomplete.
Why the other options are wrong
- A. The backup light switch is confirmed to be working as it outputs 12V when in reverse.
- B. If the ground wire was open, a voltmeter connected from the power wire at the bulb socket to chassis ground would show 12V (assuming power reaches the bulb). The question implies no light, suggesting power isn't reaching the bulb at all or the circuit isn't complete. An open power wire is more direct if the light is completely off and no voltage is at the bulb.
- D. If both bulbs were burnt out, there would still be 12V present at the bulb sockets when the transmission is in reverse. The question implies no light, and the subsequent step would be to check for voltage at the bulb, which would be 0V if the power wire is open.
Backup Light Circuit Troubleshooting
Troubleshooting backup lights involves verifying power at the switch, then at the bulbs, and finally checking grounds and bulbs if power is present throughout the circuit.
- Typically activated by a switch on the transmission when in reverse.
- Power flows from fuse to switch, then to bulbs.
- Common issues include faulty switch, open wiring, or burnt bulbs.
Memory trick: Backup's dark? Power's lost its spark!