Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairHard
A technician is performing a diagnostic on a vehicle where the left front headlight is dim, but the right front headlight is bright. A voltage drop test is conducted on the power side of the left headlight circuit, measuring from the positive battery post to the headlight bulb's positive terminal, showing 2.5V. The ground side voltage drop for the left headlight measures 0.1V. What does this indicate?
- AA partially shorted left headlight bulb.
- BExcessive resistance in the ground circuit of the left headlight.
- CExcessive resistance in the power feed wiring or connections to the left headlight.
- DA faulty headlight switch affecting only the left side.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive resistance in the power feed wiring or connections to the left headlight.
A 2.5V voltage drop on the power side from the battery to the bulb's positive terminal is significantly higher than the acceptable range (typically less than 0.5V). This indicates excessive resistance in the power feed wiring, connectors, or possibly a faulty relay/switch contact on the power side, leading to dim operation of the headlight. The ground side drop is acceptable.
Why the other options are wrong
- A. A partially shorted bulb would typically draw more current, potentially blowing a fuse or causing the wiring to overheat, rather than just dimming due to voltage drop before the bulb.
- B. The ground side voltage drop is 0.1V, which is well within acceptable limits (typically <0.2V), so excessive resistance in the ground circuit is not the issue.
- D. While a faulty headlight switch could cause dimming, the voltage drop test pinpointing 2.5V lost in the 'power side' circuit (which includes the switch, wiring, and connectors) is the direct observation. The option 'excessive resistance in the power feed wiring or connections' is a more accurate description of the underlying electrical condition identified by the test.
Power Side Voltage Drop Interpretation
A high voltage drop on the power side of a circuit indicates excessive resistance in the path from the power source to the component, reducing the voltage available to operate the component.
- Measured between the power source (+) and the component's (+) terminal.
- Acceptable drop is typically <0.5V.
- Causes include corroded wires, loose connections, faulty switches/relays.
Memory trick: High drop on power, resistance is sour!