Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisEasy
A technician is diagnosing a vehicle with a complaint of intermittent reverse lights not illuminating. The wiring diagram shows the reverse light switch is located on the transmission and supplies power to the reverse lights when reverse gear is selected. Using a multimeter, the technician tests the switch and finds 12 volts on the input side but 0 volts on the output side when reverse is selected. What is the MOST likely cause?
- AA faulty reverse light bulb.
- BA faulty reverse light switch.
- CA short to ground in the reverse light circuit.
- DAn open circuit in the reverse light wiring after the switch.
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty reverse light switch.
If 12 volts is present at the input side of the switch but 0 volts at the output side when the switch should be closed (reverse selected), it indicates that the switch itself is not completing the circuit. This points directly to a faulty reverse light switch.
Why the other options are wrong
- A. A faulty bulb would still allow 12 volts to reach the bulb socket, but the bulb wouldn't light.
- C. A short to ground would likely blow a fuse or cause continuous current flow, not 0 volts at the switch output.
- D. An open after the switch would still show 12 volts at the switch output, but the lights wouldn't illuminate.
Switch Continuity Test
Testing a switch involves verifying voltage at its input and then at its output when activated. If input is present but output is absent, the switch is faulty.
- Switch must be activated to test output.
- Input voltage confirms power supply to switch.
- Absence of output voltage indicates internal switch fault.
Memory trick: Input in, output out, if it's stuck, then there's no doubt.