How do secondary heaters work?

Your vehicle is equipped with two heaters/heaters. The main one is in front and is connected to your air conditioner. Turn the controls to defrost, set the temperature and then turn on the fan and you can watch as…

Your vehicle is equipped with two heaters/heaters. The main one is in front and is connected to your air conditioner. Turn the controls to defrost, set the temperature and then turn on the fan and you can watch the moisture evaporate.

At the rear of the car, there is a second defroster on the rear window (note: not all cars have additional defrosters). However, it doesn't work the same way. Instead of blowing air onto the glass, you flip a switch and then watch lines form in the condensation before it eventually disappears completely.

Actually, they work on the same principle as a light bulb and many other electronic components in your car - resistance. The secondary heater is actually an electronic circuit. The lines you see on the glass are actually wires and they connect to the vehicle's wiring harness.

When you flip a switch or press a front panel button that activates the defogger, power is transferred through the system. The wires in the glass resist a small current that heats them up. They don't get hot enough to glow like the filament of a light bulb, but the principle is the same. See a mechanic if the heater switch won't turn on.

The heat from this resistance helps even out the temperature differences that cause fogging, eliminating it and providing a clear view of the rear window. Of course, like any other electronic system in your vehicle, your auxiliary heater is subject to wear and tear. One damaged wire leading to the heater can disable it.