Trace the difference between a driver request, a low-current control circuit, and a high-current lamp circuit.
A switch may request light operation without carrying lamp current. In this simplified model, a BCM interprets the request and grounds a relay coil; relay contacts then connect fused battery power to the lamp. Real vehicles may use solid-state outputs, PWM, or different relay arrangements.
The coil and contact faults affect only the relay architecture. A BCM driver fault affects BCM-controlled modes.
Select a test point and compare command ON versus OFF.
The BCM relay output is a ground-side command: near 0 V when it successfully energizes the coil, and near battery voltage when not grounding an intact coil. A 0 V reading alone does not prove a good driver.
In relay mode, the switch requests ON. The relay coil has 12.6 V at its B+ terminal and the BCM output is near 0 V, but the relay contact output remains 0 V. Which part of the circuit should you investigate?
Choose an answer.
Educational circuit model: 12.6 V nominal supply, simplified static readings and one selected fault. Actual BCM diagnostics require vehicle-specific service information and may require scan-tool data and bidirectional tests.