A customer reports an inoperative headlamp. Find the fault by checking requests, commands and circuit measurements—not by guessing.
“My headlamp does not come on when I turn the switch on.” The circuit is a simplified BCM-controlled relay: the switch sends a request to the BCM, the BCM grounds the relay coil, and the relay contacts deliver fused battery voltage to the lamp.
One hidden fault per case. Switch OFF and ON while comparing readings. The meter black lead is connected to battery negative; readings are idealized for teaching.
Turn the switch ON to inspect the complaint.
Simplified data items: driver request and BCM lamp command. A command PID reports what the module intends—not proof of current flow.
Check both data items before deciding whether the fault is on the input, control or load side.
Compare ON and OFF readings; avoid drawing conclusions from a single voltage reading.
A 0 V ground-side command does not by itself prove a working relay coil. An open coil can also leave its controlled terminal near 0 V in this idealized model.
Collect evidence before submitting.
Training-only, one-fault circuit model. Real vehicles may use PWM, high-side or low-side solid-state outputs, fault-protection logic, and networked switches. Verify OEM wiring and service procedures before vehicle diagnosis.