Auto Repair Help

by Kyle McFadden

The rear window defogger (or defroster) system operates by supplying electrical current to a grid mounted in the rear glass. The current flowing through the grid will heat the surrounding glass, allowing the window to be cleared of moisture. The defogger system can demand high amperage loads, so consequently most systems will use a relay to provide current flow to the defogger grid. Relay function is controlled by the defogger switch. The duration of defogger operation is controlled by a timer that is normally contained in the defogger switch; refer to your car repair manual for the exact duration spec.

Let’s suppose the defroster grid does not work at all (no areas of the grid are working). Routine diagnostic protocol says to first check the fuse. Defroster grids can draw lots of current (say around 10 – 20 amps), and if the fuse is too small/thin, it surely won’t last. If the fuse doesn’t look blown, test it with a voltmeter. With the ignition key turned on and the defroster on, you should see 12 volts at both fuse terminals. But if the voltage is OK, the problem is either in the wiring or the grid itself. Check the connections from the wiring harness connected to the grid. It is easy for the terminals connected to the grid to become damaged. Generally, the tab that is attached to the glass can break, leaving you with a dangling wire. You have two repair choices: soldering and gluing.

Do you know how to solder? If yes, then this is an easy fix. Use a high-capacity soldering iron (or gun), & solder the tab back on. It may take an extra hand to hold the tab against the grid as you solder it. There is usually a metal strip on the glass under the silk-screening. Clean the surface with alcohol and use a 60-40 rosin-core solder. You need to work fast because excess heat can crack the glass. If you aren’t confident about your soldering skills, or are not ready to risk cracking an expensive piece of glass, there’s another way. Most auto parts stores sell a special electrically conductive epoxy to bond the tab back on. If it’s winter, you will need to work in a heated garage and have the vehicle inside long enough for it to warm up to at least 60F. As with the soldering technique, clean the glass with alcohol first. Mask the glass with tape so as to keep from getting epoxy smeared on it. Mix up a small amount of epoxy and hardener. Put some epoxy on the tab, and use something to hold it in place for 15 minutes or so until the epoxy hardens. Use a wooden stick or the end of your finger to smear the epoxy. Although the epoxy will dry quickly, don’t reattach the wiring until it’s had 24 hours at 70F or more to cure and reach its full strength. Unfortunately, the repair won’t be as strong as the original, so you will need to be careful not to damage it in the future.

If the fuse is OK and the tab is attached to the glass but still the defroster doesn’t work, check for a bad switch, relay, or timer. For this you’re going to need a schematic diagram and experience in troubleshooting wiring problems. Start at the fuse and trace the wiring. If the switch is bad, you can always jump the switch and get 12 volts beyond it for diagnosis. But you may need to replace the timer which often is integrated into a large box of electrical controls buried deep underneath the dashboard. Consult the factory auto repair manual.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)

2 responses to “Rear Defrost”

  1. Pukar Risal says:

    I was hoping you could help me with an issue I have after installing an aftermarket android stereo. So the unit comes with android screen and an integrated climate control unit. All the functions in the climate control unit works except for the rear window defroster. I was told that it was the canbus controlling the rear window defroster. I tried different cabus settings and it still did not work.

    Long story short, I tried to trace the wire and the switch pin that triggers the rear window defroster. To my surprise there wasn’t a wire connecting the rear window defroster relay to the defroster switch in the new unit. I traced the pin that worked when the switch is on.

    The voltage on the relay side (car harness) shows 12V meaning I would need a ground side switching to turn the rear window defroster But the voltage on the switch also shows 12v when I turn it on. meaning the rear window defroster would not turn on because the switch is also supplying positive volt. My plan is to use a relay to invert the voltage from the switch to negative/ground

    My question is, if I am wiring for a ground side switch is it necessary to add a fuse in the circuit? Do you think adding a relay would work in this case.

  2. Evergreen says:

    Do you know if all manufactures use solder or glow or do some manufactures use solder and some use glue to attach the defroster tabs?

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