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Damien Coleman
Snap-on® Diagnostic Software Specialist 

Vauxhall Vectra C:

Brake Light Brilliance 

This 2008 Vauxhall Vectra C had a bulb failure warning light illuminated on the instrument cluster and the brake light on the right hand side of the vehicle appeared to be lighting at full brilliance at all times.

This vehicle has priority lighting. For this purpose, all bulbs fitted to the rear light clusters are 21 watt bulbs. The Rear Electric Centre (REC) 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 REC 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:

  1. Brake Light.
  2. Indicator Light.
  3. Fog Light.
  4. Reverse Light.
  5. 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 

A Scantool was connected to the vehicle and the following fault was stored in the Rear Electric Centre:

B3878 - Right Brake Light Circuit Open or High Voltage

An oscilloscope was connected to the output from the REC 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 and the integrity of the wire was found to be acceptable. This indicated a fault within the REC. 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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