Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairMedium
A technician is diagnosing a complaint of dim interior dome lights on a vehicle. The battery voltage measures 12.6V. When the dome lights are on, a voltage reading taken at the dome light bulb socket shows 9.5V. A voltage drop test performed across the dome light's ground wire to chassis ground shows 0.2V. Which of the following is the MOST likely cause for the dim lights?
- AInsufficient voltage drop in the ground circuit.
- BA faulty dome light switch.
- CExcessive resistance in the power side wiring to the dome light.
- DA worn-out dome light bulb.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive resistance in the power side wiring to the dome light.
The battery voltage is 12.6V, but only 9.5V reaches the bulb, indicating a 3.1V drop on the power side (12.6V - 9.5V). The ground side voltage drop is only 0.2V, which is acceptable. This significant voltage drop on the power side suggests excessive resistance in the power wiring.
Why the other options are wrong
- A. 0.2V is an acceptable voltage drop for a ground circuit. Insufficient voltage drop would mean the circuit is working efficiently, not causing dim lights.
- B. A faulty switch would likely prevent the lights from working at all, or cause intermittent operation, rather than just dimness with proper voltage at the switch output.
- D. While a worn-out bulb could be dim, the voltage measurement at the socket (9.5V) is significantly lower than battery voltage, indicating a circuit problem, not just a bulb issue.
Power Side Voltage Drop
Power side voltage drop occurs when there is excessive resistance in the positive (+) circuit path, reducing the voltage available to the component and potentially causing dim operation or component malfunction.
- Measured by comparing battery voltage to voltage at component positive terminal.
- Indicates resistance in wiring, connections, or switches on the power side.
- Excessive drop (typically >0.5V) can lead to dim lights, slow motors, etc.
Memory trick: Dim light? Voltage drop is the fight!