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Defogger Grid Lines Diagnosis

IMPORTANT: This test is for reference only. A grid line fault requires rear window replacement. Refer to Rear Window Replacement .
  1. Start the engine.
  2. Activate the rear window defogger system.
  3. Connect a test lamp to a good ground.
    4. Fig 1: Testing Rear Window Defogger Grid Lines
    GM894100Courtesy 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 the positive to the negative wire.
    • 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.
    • If the test lamp goes out, test the grid line in at least 2 places to eliminate the possibility of bridging the open in the grid line.
  5. Once the open is located, replace the rear window. Refer to Rear Window Replacement .
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