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Repairing Your Rear Window Defroster

It snowed last night. I mean it REALLY snowed last night...

» Warning!
» Doesn't Work At All
» Uh oh...
» Easy Repairs

Uh oh...

The fuse is good and the terminals are attached to the glass correctly and it still doesn't work. If this is the case then we have to look for a bad rear window defroster switch, rear window defroster timer or relay. You'll need a wiring diagram and a little experience in tracking this down. There is an article entitled Electrical Troubleshooting elsewhere in the Troubleshooting section that will be a big help here.

In general here's what you can do. As always these are general procedures and you should always use the specific manual for your car for exact procedures. Remove the rear window defroster switch and using your wiring diagram, determine which wire feeds into the switch and which wire feeds out to the defroster grid. If the switch only has two wires, this will be easy. Jump the two wires and if the grid starts working, you know you have a bad switch. If you have no power going to the switch, and the fuse is good, then there is a break in the wire between the fuse box and the switch.

If the switch is good, then we need to look at the relay and timer. Some cars combine the relay and timer into one unit so keep this in mind when checking your car. Now the rear window defroster relay has two sides, the coil side that activates the switch and the switch side that provides the electrical connection. Generally the coil side has two light wires and the switch side has two heavier wires. Most manufacturers bury this relay in the dash or some have the good sense to put it in a relay box for easy access. I hope yours is easy to get to.

If the coil side of the relay has continuity and there is power going to it, the relay and power supply is good. If there is no power then you have a break in the wire from the switch to the relay. If you jumper the two heavier wires, and check the wiring diagram to be sure they are for the rear window defroster, and the defroster works, you have a bad relay. If not, and you have power coming in, the relay is bad. If there is no power going to this side of the relay, there is a break in the wire from the power supply to the relay.

If there is another, exactly the same, relay in the relay box, you can swap the relays and see if the rear window defroster now works. If it does, go over to the dealer and get another relay.

If the rear window defroster switch, timer and relay are good, then we have a break in the wire going to the rear window. Now keep in mind that the rear window defroster is on a ten or 15-minute timer so you have that much time to find where the break is. If the timer shuts off the power while you are checking the wiring, you'll be chasing wild geese.

Rear window defrosters draw a lot of current, up to 20 amps depending on the car. Most cars put the rear window defroster on a 10 or 15-minute timer so it shuts itself off. This is for a couple of reasons. First off the rear window will overheat if left on too long. If you accidentally turn it on in the summer and go on a long trip the heat of the sun and the heat from the grid could cause the window to shatter and, at the very least, destroy the rubber window seal.

The other reason is to lighten the electrical load. In the winter when you have the headlights on, the heater on high, windshield wipers going and the rear window defroster on, you will be placing a high demand on the electrical system. Possibly more than the alternator can supply. So by shutting off the rear window defroster after a period of time, you will lighten the electrical load.

» Part 1       » Part 2       » Part 3       » Part 4
Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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