Grid Filament Test

WARNING: This page is about a different car, the 2002 Ford Mustang. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: Perform all voltage and resistance tests using a Digital Volt-Ohm Meter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure. Use ohmmeter to isolate wiring harness shorts or opens.

Shine a strong light through grid from inside vehicle. Inspect grid from outside. Broken grid lines will appear as Brown spots. Run engine at idle. Depress rear window defogger button. Rear window defogger indicator light should illuminate. Go to next step.
Working from inside vehicle, connect negative lead of a DVOM to ground. Touch positive lead to wide Red/Brown strips on rear window. DVOM should indicate 10-13 volts. If less than 10 volts exist, check for faulty ground at connector C402b (Black wire). Connector C402b is located at right side of rear window defogger grid. See Figure. If problem is found, repair as necessary. If no problem is found, go to next step.
With DVOM negative lead grounded, touch DVOM positive lead to each grid line at its midpoint. A reading of about 6 volts indicates grid line is good. If DVOM reads about 12 volts, grid line has a break between midpoint and right (ground) side. If DVOM reads 0 volts, grid line has a break between midpoint and left (feed) side. If problem is found, repair as necessary. See GRID FILAMENT REPAIR under ON-VEHICLE SERVICE.