Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairHard
A technician is performing a voltage drop test on a headlight circuit. With the headlights on, the voltage drop across the positive battery terminal to the headlight bulb's power terminal measures 2.5V. The specification for maximum allowable voltage drop is 0.2V. What does this measurement indicate?
- AA discharged battery requiring replacement
- BA short circuit in the headlight wiring
- CExcessive resistance in the positive side of the headlight circuit
- DA normal operating circuit with proper resistance
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive resistance in the positive side of the headlight circuit
A voltage drop of 2.5V, significantly higher than the 0.2V specification, indicates excessive resistance in the positive side of the circuit between the battery and the headlight bulb. This resistance consumes voltage, reducing the power available to the bulb and causing dim operation or failure.
Why the other options are wrong
- A. While a discharged battery might contribute to dim lights, the voltage drop measurement specifically isolates resistance within the circuit, assuming the battery itself is capable of supplying voltage.
- B. A short circuit would typically blow a fuse or prevent the circuit from working at all, not just cause a high voltage drop while still operating (albeit poorly).
- D. 2.5V is far beyond the 0.2V specification, indicating an abnormal condition, not normal operation.
Voltage Drop Test Interpretation
A diagnostic test measuring voltage loss across a circuit segment, indicating unwanted resistance that impedes current flow.
- Higher voltage drop means higher resistance.
- Performed with the circuit under load.
- Specifications for maximum allowable drop are critical for diagnosis.
Memory trick: Drop too high, circuit's cry!