Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsLighting Systems Diagnosis and RepairMedium
A vehicle uses a Body Control Module (BCM) to control its exterior lighting. A technician finds that the left headlight is inoperative, but the right headlight works. When checking the left headlight connector, there is no voltage present. All fuses related to the headlights are confirmed good. What is the most likely cause?
- AA faulty headlight switch
- BAn open circuit in the wiring between the BCM and the left headlight
- CA faulty left headlight bulb
- DA short to ground in the left headlight circuit
Show answer & explanationAnswer & explanation
Correct answer: B. An open circuit in the wiring between the BCM and the left headlight
Since the right headlight works and all fuses are good, the BCM is likely receiving the command and has power. The absence of voltage at the left headlight connector, despite good fuses, suggests an open circuit (break) in the wiring between the BCM's output for the left headlight and the headlight itself.
Why the other options are wrong
- A. A faulty headlight switch would likely affect both headlights or prevent the BCM from sending power to either, not just one side while the other works.
- C. A faulty bulb would still allow voltage to be present at the connector, even if the bulb itself doesn't light up.
- D. A short to ground would typically cause an excessive current draw, blowing the fuse (which are confirmed good) or potentially damaging the BCM output, but not simply an absence of voltage without a blown fuse.
BCM Controlled Circuit Diagnosis
Troubleshooting circuits managed by a BCM involves verifying BCM input, BCM output, and wiring integrity to the load.
- BCM acts as an intelligent switch/relay.
- Check input signals to BCM (e.g., switch position).
- Check output signals from BCM to component.
- Wiring integrity between BCM and component is crucial.
Memory trick: BCM's path, wiring's wrath!