In order to locate inoperative grid lines, start the engine and turn ON the rear window defogger system. Ground the test lamp lead and lightly touch the other probe to each grid line. Refer to the pattern of test lamp brilliance to be expected with a properly functioning grid:

1. No bulb brilliance
2. One quarter bulb brilliance
3. One half bulb brilliance
4. Three quarters bulb brilliance
5. Full bulb brilliance

If the test lamp bulb shows full brilliance at both ends of the grid lines, test for loose ground wire contact to the body metal.

The range of zones may vary slightly from one window to another; however, the bulb brilliance will decrease proportionately to the increased resistance in the grid line as the probe is moved from the left bus bar to the right. Test all 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. If an abnormal light reading is apparent on a specific grid line, place the test lamp probe on that grid at the left bus bar and move the probe toward the right bus bar until the light goes out. This will indicate a break in the continuity of the grid line.

The following chart identifies the locations shown in the graphic:

1. No light
2. Broken Grid
3. Full brilliance
4. Rear window
5. Feed bus bar
6. Ground bus bar
7. No light
8. Ground bus bar