A vehicle's interior dome light and map lights are inoperative. The technician checks the fuse for the interior lighting circuit, and it is intact. Using a multimeter, the technician measures 12.6V at the power side of the dome light bulb socket when the door is open. When a test light is connected to the ground side of the bulb socket and its other end is connected to a known good ground, the dome light illuminates. What is the MOST likely cause of the inoperative interior lights?
- AA faulty body control module (BCM) controlling the interior lights.
- BA short to ground in the interior lighting circuit.
- CAn open circuit in the power wire to the dome light.
- DA high resistance in the ground circuit for the interior lights.
Show answer & explanationAnswer & explanation
Correct answer: D. A high resistance in the ground circuit for the interior lights.
The technician confirmed power (12.6V) is present at the bulb socket. The crucial diagnostic step is that the light illuminates when an external, known good ground is supplied to the ground side of the bulb. This indicates that the bulb itself is good, and the power supply to the bulb is good. Therefore, the problem must lie in the vehicle's original ground path for the dome and map lights. High resistance or an open in this ground circuit would prevent the lights from illuminating under normal conditions.
Why the other options are wrong
- A. While a BCM might control these lights, the test light diagnostic specifically points to a ground issue, not a BCM output signal issue, as the BCM's output might be fine but the ground path from the light is not.
- B. A short to ground in the power circuit would likely blow the fuse, which was found to be intact.
- C. An open in the power wire would result in no voltage at the bulb socket, contradicting the 12.6V reading.
Ground Side Diagnosis
When a circuit has power to the component but the component does not operate, and it operates when a known good external ground is supplied, the fault lies in the component's original ground circuit, typically an open or high resistance.
- Component has power but doesn't work.
- Works when external ground is applied.
- Indicates original ground path is faulty.
- Common issues: loose connection, corrosion, broken wire.
Memory trick: Dome light has power, but needs a friend (ground) to shine. If it lights up with a new friend, the old one's gone.