How to test a rear window heater circuit

Rear window doesn't defrost like it should, I will show you how to easily diagnose it and repair it.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• DC Voltmeter
• 12 Volt Test Lamp
• Grid Repair Kit

1. To test your rear window heater you must first turn it on, the switch is usually located in the heater control panel. Turn it on and see if the button lights up. If it doesn't illuminate, check the fuse for your rear window defroster circuit (look in your owners manual for location).
   If the fuse is bad replace and retest switch. Most rear window defrost circuits are tied to side mirror defrosters, which can go bad as well.

2. The light comes on in the heater panel, you may have to have your car running for this test. Park outside so the carbon monoxide doesn't kill you. A normal rear window heater can pull 20-30 amps and this will run down a battery quite fast without it running.

3. 
   Typical rear window grid circuit, A=12 volts, B=6 volts, and C=0 volts

   Crawl into your back seat with your voltmeter and connect the ground (BLACK) lead to a good known ground. Door striker pins are usually good grounds, connect to metal parts. Set your voltmeter to DC volts and touch the positive (RED) lead to the right side of the grid and read your meter. It will read 12 or 0 depending on which side the power is attached to. You should get 12 volts on at least one side or you have an open circuit to the rear heater, corroded connectors can cause this problem. Repair the path for power and this one is fixed.

4. 
   Broken grid circuit

   If you have 12 volts on both sides then you have an open circuit to ground, this can be a broken wire, corroded connector on the ground side or a broken terminal. Repair the path to ground and this one is fixed.

5. If you have parts of the rear window that don't defrost, then you have an open circuit on an individual grid, this can also be repaired with a kit in resources below. Using the voltmeter follow the grid line, it will show 12 volts until it gets to the open circuit then it will drop to zero. Using the kit you will tape on both sides of the grid line then paint it back on using a metallic paint. You have this one fixed!

Tips & Warnings

• If you short a power circuit to ground you will probably blow a fuse, then you will have to go back to the owners manual and find the fuse to replace.

Resources

• Repairing a rear defroster grid
• JC Whitney repair kit for grid