Notes On Intermittents

Check circuits 640, 812A, B and C for intermittent shorts to ground.

NOTE: The following step numbers refer to circled numbers on diagnostic chart. See Fig 1 . For circuit identification, see Fig 2 .

Brake application will determine if the BCM is able to "Wake-Up" the CPS and communicate with the OSM's to turn on the brake lamps.
This step checks for ignition-3 (I-3) input to the BCM, if I-3 is not at system voltage the BCM will not turn on the displays.
This checks the CPS for correct 12 volt output on circuit 812A.
Although the fuse is good, the CPS has no voltage, therefore there is an open in the supply to the fuse, or between the fuse and the CPS.
The fuse was blown, therefore replacing the fuse will determine if there is a current or intermittent circuit failure.

Fig 1: Identifying Chart B: Displays Blank Flow Chart

Courtesy of GENERAL MOTORS CORP.

Fig 2: Identifying Chart B: Displays Blank Flow Chart Circuit

Courtesy of GENERAL MOTORS CORP.