Defogger System Testing

Electrically Heated Rear window defogger operation can be checked in vehicle in the following manner:
1. Turn ignition ON.
2. Make sure defogger OFF.
3. Using a ammeter on the battery. Turn the Defogger control switch ON, a distinct increase in amperage draw should be noted.

Grid Line Test:

4. The rear window defogger operation can be checked by feeling the glass. A distinct difference in temperature between the grid lines and adjacent clear glass can be detected in 3 to 4 minutes of operation.
5. Using a DC voltmeter contact terminal B with the negative lead, and terminal A with the positive lead. The voltmeter should read 10-14 Volts.
6. Step 3, Step 4 or Step 5 above will confirm system operation. Indicator light illumination means that there is power available at the output of the relay only, and does not necessarily verify system operation.
7. If turning the switch ON produced no distinct current draw on the ammeter the problem should be isolated in the following manner:
a. Confirm the ignition switch is ON.
b. Ensure that the heated rear glass feed wire is connected to the terminal or pigtail and that the ground wire is in fact grounded.
c. Ensure that the fusible link and control circuit fuse is operational and all electrical connections are secure.
8. When the above steps have been completed and the system is still inoperative, one or more of the following is defective:
a. Control switch or timer relay.
b. Rear window grid lines, all grid lines would have to be broken or one of the feed wires are not connected for the system to be inoperative.
9. If turning the switch ON produces severe voltmeter deflection, the circuit should be closely checked for a shorting condition.
10. If the system operation has been verified but indicator bulb does not light, service the A/C heater or ATC controls.

Grid Test
The horizontal grid lines and vertical bus bar lines printed and fired on inside surface of rear window glass comprise an electrical parallel circuit. The electrically conductive lines are composed of a silver-ceramic material which when fired on glass becomes bonded to the glass and is highly resistant to abrasion. It is possible, however, that a break may occur in an individual grid line resulting in no current flow through the line. To detect breaks in grid lines the following procedure is required:
1. Turn ignition ON and turn control switch to ON.The indicator light should come on.
2. Using a DC voltmeter with 0-15 Volt range, contact the negative lead of voltmeter to the ground side. With positive lead of voltmeter, contact the battery feed side. The voltmeter should read 10-14 Volts. A lower voltage reading indicates a poor ground connection.
3. With negative lead of voltmeter, contact a good body ground point. The voltage reading should not change.

Systems Electrical Circuit:

4. Connect negative lead of voltmeter to the ground side and touch each grid line at Mid-Point with Positive lead. A reading of approximately 6 Volts indicates a line is good. A reading of 0 Volts indicates a break in line between Mid-Point C and battery feed side. Reading of 10-14 Volts indicates a break between Mid-Point C and ground side. Move toward break and voltage will change as soon as break is crossed.