Testing Grid Lines

Use the following procedure in order to locate inoperative grid lines:

1. Start the engine.
2. Turn the rear window defogger system ON.
3. Ground the negative lead of the J 39200.
4. Lightly touch the positive lead of the J 39200 to each grid line. The voltage will decrease as you move the lead farther away from the feed bus bar.
   Fig 1: Testing Grid Lines

   Courtesy of GENERAL MOTORS CORP.
5. If the J 39200 reads a high voltage at both ends of the grid lines, inspect for the following conditions:
   • A loose terminal-to-bus bar connection
   • A loose ground wire-to-body connection
6. Test all the grid lines in at least two places in order to eliminate the possibility of bridging a break. For best results, contact each grid line a few millimeters on either side of the window centerline.
7. If an abnormal meter reading is apparent on a specific grid line, perform the following steps:
   1. 7.1. Place the lead on the grid.
   2. 7.2. Move the lead along the grid until the voltage reading drops significantly. This condition indicates a break in the continuity of the grid line.
   3. 7.3. Mark the grid line breaks on the outside of the window using a wax pencil or a crayon. Refer to Grid Line Repair .