Defogger Grid Lines Diagnosis

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Start the engine.
Activate the rear window defogger system.
Connect a test lamp to a good ground.

Fig 1: Identifying Grid Lines Zones
Courtesy of GENERAL MOTORS CORP.

Important: The test lamp brilliance will decrease proportionately to the increased resistance in the grid line as the probe is moved from the battery positive bus wire to the ground bus wire. The test lamp brilliance may vary from one window to another.
Move the test lamp probe from zone 5 to zone 1 along each grid line.
- If the test lamp shows full brilliance at both ends of the grid lines. Inspect for an open or poor connection in the ground circuit of the rear window defogger grid. Refer to TESTING WIRING SYSTEMS FOR INTERMITTENT & POOR CONNECTIONS and CONNECTOR REPAIRS in Wiring Systems.
  
  Fig 2: Identifying Opening In Grid Line
  Courtesy of GENERAL MOTORS CORP.
- If the test lamp goes out, test the grid line in at least 2 places (1,3) to eliminate the possibility of bridging the open (2) in the grid line.
Once the open (2) is located, repair the grid line. Refer to GRID LINE REPAIR .