Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisHard
A client complains that their vehicle's interior dome light does not turn on when any door is opened, but it works correctly when manually switched on. A technician tests the door jamb switches and finds they all show continuity to ground when the door is open. The fuse for the dome light is good, and the bulb is confirmed working. What is the MOST probable cause?
- AAn open in the ground-side control circuit from the door switches to the dome light module/relay
- BAn open in the power supply to the dome light
- CA faulty dome light assembly
- DA short to voltage in the dome light circuit
Show answer & explanationAnswer & explanation
Correct answer: A. An open in the ground-side control circuit from the door switches to the dome light module/relay
The dome light works manually, indicating power and the bulb are good. Door switches provide a ground signal when open. If the light doesn't activate with open doors, the ground signal from the switches is not reaching the dome light control module or relay, suggesting an open in that specific ground-side control circuit.
Why the other options are wrong
- B. An open in the power supply would prevent the light from working at all, even when manually switched on.
- C. A faulty dome light assembly is unlikely if it works manually, as the manual switch often bypasses or directly controls the same internal components.
- D. A short to voltage would likely cause the light to be stuck on, or blow a fuse, not prevent it from coming on with the door open.
Ground-Side Control
A circuit design where a constant voltage is supplied to a component, and a switch or control module completes the circuit by providing a ground path.
- Component always has power
- Switch/module provides ground to activate
- Common in lighting and relay circuits
Memory trick: Manual works, door fails; ground path prevails.