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Defogger Grid Lines Diagnosis

  1. Start the engine.
  2. Turn the rear window defogger ON.
  3. Connect a test lamp to a good ground.
    4. Fig 1: Identifying Defogger Grid Line Zones
    GM71185Courtesy 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.
  4. Move the test lamp probe from zone 5 to zone 1 along each grid line.
    • If the test lamp does not illuminate anywhere on the grid, the problem may be in the circuit. Refer to Defogger Inoperative - Rear Window .
    • 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 for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
      • Fig 2: Identifying Grid Line Test Locations
      GM658924Courtesy of GENERAL MOTORS CORP.
    • If the test lamp goes out, locate the open in that grid line. Test every grid line in at least 2 places (1, 3) to eliminate the possibility of bridging an open (2) in the grid line.
  5. If an open is located, replace the rear window. Refer to Rear Window Replacement .
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