Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is troubleshooting a vehicle with dim headlights despite having a fully charged battery and good grounds. The voltage at the headlight bulb connector, with the headlight on, is measured at 10.5V. The specification for headlight voltage is 12.0V-12.6V. Where should the technician primarily focus their next diagnostic step?
- AReplacing the headlight bulbs with higher wattage units.
- BChecking the headlight switch for continuity
- CInspecting the battery cables for corrosion.
- DPerforming a voltage drop test on the headlight power and ground circuits.
Show answer & explanationAnswer & explanation
Correct answer: D. Performing a voltage drop test on the headlight power and ground circuits.
A significant voltage drop (12.6V - 10.5V = 2.1V) at the bulb connector indicates excessive resistance somewhere in the headlight circuit, either on the power side or the ground side. Performing a voltage drop test on both the power and ground circuits will pinpoint the exact location of this excessive resistance.
Why the other options are wrong
- A. Replacing bulbs with higher wattage units would draw more current and likely exacerbate the voltage drop, potentially damaging the circuit or components.
- B. While the headlight switch could be a source of resistance, a voltage drop test is a more comprehensive way to find resistance anywhere in the circuit, not just the switch.
- C. While battery cable corrosion can cause voltage drops, the problem is localized to the headlight circuit. A general battery cable inspection is less specific than a headlight circuit voltage drop test for this symptom.
Voltage Drop Test (Load)
A diagnostic test performed with the circuit operating under its normal electrical load, measuring the difference in voltage between two points to identify excessive resistance.
- Performed under load.
- Measures unwanted resistance.
- Identifies circuit section with high resistance.
Memory trick: Dim lights mean voltage is shy; find its hiding spot with a drop test.