Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSMERCURY1989GRAND MARQUIS LSREPAIR AND DIAGNOSISACCESSORIES & EQUIPMENTHEATED GLASS/DEFOGGERSDEFOGGER - REAR WINDOWTESTINGGRID TEST
1989 Mercury Grand Marquis LS
Grid Test
1989 Mercury Grand Marquis LSSECTION Grid Test
- Shine a strong light through grid from inside vehicle and inspect grid from outside. Broken grid lines will appear as Brown spots. Run engine at idle and turn control switch to "ON" position. The indicator light should come on.
- Using a voltmeter, contact wide Red-Brown strips on rear window (positive lead to battery side). Reading should be 10-13 volts. If voltage is lower, window grid ground wire (pigtail) connection is loose. Contact a known good ground. Reading should not change.
- With voltmeter negative lead grounded, touch each grid line at its midpoint with voltmeter positive lead. A reading of about 6 volts indicates that grid line is good. A zero volt reading indicates that grid line is broken between midpoint and battery side of grid. A 12-volt reading indicates that grid line is broken between midpoint and ground.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.