Some modern vehicles use priority lighting to ensure the vehicle stays illuminated in the case of a bulb or circuit failure. When this system is used, all bulbs fitted to the rear light clusters are 21 Watt bulbs. A control module, normally fitted in the rear of the vehicle, controls the operation of all rear lights. During tail light operation, a 21 Watt bulb is controlled by ≈25% Duty Cycle to provide an output similar to a 5 Watt bulb. It must be noted that the bulbs are controlled on the supply side and have a constant ground.
Should a bulb fail, the Control Module can use another bulb to ensure the vehicle is still illuminated correctly.
The diagram below shows a basic overview of the system.
Right hand rear light cluster:
- Brake Light
- Indicator Light
- Fog Light
- Reverse Light
- Tail Light
The waveform below shows the duty cycle for tail light operation.
Full system voltage is applied to a bulb for the following operations:
- Brake Light
- Fog Light
- Reverse Light
Connecting a scan tool to a vehicle with a fault may indicate a similar malfunction to the case study below:
B3878 - Right Brake Light Circuit Open or High Voltage
An oscilloscope was connected to the output from the Control Module to the right hand brake light bulb, which was illuminated, a voltage of 12 volts was present on the wire at all times with and without the brake pedal depressed. The wire was tested for a short to positive, the integrity of the wire was found to be acceptable. This indicated a fault within the Control Module. The module was replaced and programmed/configured. The fault memory was cleared and the operation of the brake lights tested. The vehicle now behaved as expected.
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