Grid Filament

With ignition switch in ACC position and defogger switch activated, test filaments with voltmeter. Wrap negative probe of voltmeter with foil and press against filament. SeeFig 1 .

Fig 1: Attaching Foil To Voltmeter Negative Probe
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Attach positive probe of voltmeter to heat filament positive terminal. Slowly slide negative probe along filament from positive heat wire terminal to negative heat wire terminal. SeeFig 2 .

Fig 2: Testing Defogger Wire Grid
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If voltage reads 4-6 volts, wire is good. If voltage reads zero volts, wire is damaged between wire center and ground. If voltage reads 10 volts, wire is damaged between wire center and positive end.
If filament is good, voltage will read 12 volts at positive end and decrease as probe is moved towards negative end. If voltmeter needle fluctuates, damaged area of filament has been located.