Rear Window Defogger: Inspection

CAUTION: Wrap tin foil around the end of the voltmeter test lead to prevent damaging the heater line.

Apply finger pressure on the tin foil, moving the tin foil along the grid line to check for open circuits.

Fig 1: Illustrating Testing Procedure
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Turn on the defogger switch and use a voltmeter to measure the voltage of each heater line at the glass center point. If a voltage of approximately 6V is indicated by the voltmeter, the heater line of the rear window is considered satisfactory.

Fig 2: Testing Defogger Grid Lines
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If a heater line is burned out between the center point and (+) terminal, the voltmeter will indicate 12V.

Fig 3: Isolating Grid Line Break Between Center Point & Voltage Source
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If a heater line is burned out between the center point and (-) terminal, the voltmeter will indicate 0V.

Fig 4: Isolating Grid Line Break Between Center Point & Ground
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To check for open circuits, slowly move the test lead in the direction that the open circuit seems to exist. Try to find a point where a voltage is generated or changes to 0V. The point where the voltage has changed is the open-circuit point.

Fig 5: Isolating Exact Grid Line Break Point
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Use an ohmmeter to measure the resistance of each heater line between a terminal and the center of a grid line, and between the same terminal and the center of one adjacent heater line. The section with a broken heater line will have a resistance twice as that in other sections. In the affected section, move the test lead to a position where the resistance sharply changes.

Fig 6: Measuring Grid Line Resistance
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