Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A vehicle owner reports that their vehicle's interior dome light does not turn on when any door is opened, but it does turn on when activated manually. The fuse for the dome light is good. When testing the door jamb switch for the driver's door, the technician measures 0.0 volts at the switch terminal when the door is open. What is the most likely cause of this issue?
- AA faulty dome light bulb.
- BA short to power in the door jamb switch wiring.
- CAn open circuit in the ground wire from the door jamb switch to the dome light control module.
- DA faulty dome light control module.
Show answer & explanationAnswer & explanation
Correct answer: C. An open circuit in the ground wire from the door jamb switch to the dome light control module.
Many dome lights are ground-switched. When the door is opened, the door jamb switch completes the ground circuit to activate the dome light. If the switch terminal reads 0.0 volts when the door is open, it implies that the switch is not receiving a ground signal, likely due to an open circuit in the ground wire leading from the switch to the control module or the main ground point.
Why the other options are wrong
- A. The dome light works manually, so the bulb is good.
- B. A short to power would likely keep the light on constantly or blow a fuse, not prevent it from turning on.
- D. While possible, the 0.0V reading at the switch points to an issue with the switch's input, not necessarily the module's output, especially if the module receives its ground signal from the switch.
Ground-Side Switching
An electrical circuit design where a switch completes the ground path for a component to activate it, rather than switching the positive power supply.
- Common in interior lighting and other low-current circuits.
- Component always has constant power, waiting for ground.
- Testing requires confirming power to the component and checking for ground activation at the switch.
Memory trick: The switch ADDS the ground to light it up.