Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairEasy
A technician is diagnosing a complaint of dim headlights on a vehicle. The battery voltage measures 12.6V with the engine off. When the headlights are turned on, the voltage at the headlight bulb connector measures 10.5V. Which of the following is the most likely cause?
- AExcessive resistance in the headlight circuit
- BA discharged battery
- CA faulty headlight switch
- DA short circuit in the headlight wiring
Show answer & explanationAnswer & explanation
Correct answer: A. Excessive resistance in the headlight circuit
A voltage drop of 2.1V (12.6V - 10.5V) indicates excessive resistance in the circuit, which would cause dim headlights. This resistance could be due to corroded connections, a frayed wire, or a faulty component.
Why the other options are wrong
- B. The battery voltage of 12.6V indicates a fully charged battery, so a discharged battery is not the cause.
- C. While a faulty switch can cause issues, a voltage drop at the bulb connector points more directly to resistance in the circuit, not necessarily the switch itself failing to pass current.
- D. A short circuit would typically blow a fuse or prevent the lights from working entirely, not just cause them to be dim with a voltage drop.
Voltage Drop Diagnosis
Measuring the difference in voltage between two points in a circuit to identify excessive resistance.
- Excessive voltage drop indicates high resistance.
- High resistance reduces current flow and component function.
- Common causes include corrosion, loose connections, or damaged wires.
Memory trick: Voltage drops, light goes flop.