Rear Window Defroster Functional Check

The rear window defroster heating element consists of a horizontal grid of conductive strips bonded to the inner surface of the glass. Power is supplied to the defroster through a large fuse from the power distribution box located in the engine compartment. The defroster is switched on by the switch located on the instrument panel.

Minor breaks in the conductive strips of the heating element grid can be repaired without removing the rear window glass.
CHECK
PROCEDURE

1. Turn the ignition on and switch on the rear window defroster.
2. Connect the positive voltmeter probe to the positive terminal of the heating element grid and the negative probe to the negative terminal. The meter should read battery voltage. If not, check the fuse, switch and connecting wiring. If voltage is normal but part of the heater does not work, proceed to the next step.

3. To measure voltage during the checks described below, wrap the positive voltmeter probe with metal foil, which can then be pressed by finger directly onto the conductive strip of the heating element grid. Connect the negative meter probe to the negative terminal of the grid.

4. Measure the voltage at the center of each grid strip. A reading of 5 to 6 V indicates the strip is good. A zero reading indicates an open between the center of the strip and the positive side of the element. A reading doubled (10 to 12 V) indicates an open between the center of the strip and the negative side of the element. Check each grid strip in turn.

5. If there are no open circuits, connect the negative probe to chassis ground and repeat the check. The readings should not change; otherwise, the grid ground connection is faulty.

6. To locate an open, connect the negative voltmeter probe to the negative terminal of the heating element grid. Using the foil-wrapped positive probe, probe the conductive strips of the grid, moving from the power supply side toward ground. At the point of the open, the meter reading should drop from several volts to zero.