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Your Rear Car Defroster: How Does It Work and How Can You Repair It?

a frozen rear window with a broken car defroster

You probably don’t think much about your rear car defroster until it stops working properly. Usually, the decline is gradual, with only part of the back glass getting cleared of fog, snow and ice. Then, the problem eventually spreads to the point where your visibility in the rearview mirror is significantly compromised.

What can you do to fix the situation? Here’s a closer look at the rear defroster and how it can be repaired.

Rear Car Defroster Design

Car defrosterAlso known as a defogger, your rear defroster has to work very differently from the one up front, due largely to the absence of any hot air ducts linking it to the vehicle’s climate control system. Fortunately, the solution to this quandary is very simple, and it involves harnessing the 12-volt electrical system in an automobile to generate heat.

Instead of hot air, the rear car defroster sends electrical current through a small grid made of a mixture of metal and resin that is attached to the glass with an adhesive. When activated, the resistance in the metal generates enough heat to evaporate fog. In most cases, it can also melt snow and ice to prevent it from sticking to the window.

Vulnerable to Damage

As you might imagine, the defrost lines that make up this grid are fairly sensitive to physical damage. The act of scraping or cleaning a rear window can sometimes sweep away enough of the metal to interrupt the circuit, which, in turn, prevents electricity from being transmitted to that segment of the car defroster. This is why this component often seems to fail in sections, as one part of the grid and then another gets cut off from the main circuit over time.

Paint It Back

There are a couple of ways you can repair your defroster yourself. The first is to make sure that the main tabs on the side of the window — the ones that carry the electricity to the grid — are strongly attached to the defroster itself. You can do this with a tab adhesive product. Next, you can correct any breaks in the defroster grid by painting on conductive paint using a stencil, connecting the gaps in the grid and ensuring that power flows through the entire car defroster.

Visibility — in all four seasons — is an important part of driving safe.

Check out all the electrical system products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on your rear car defroster and how to repair it, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Morguefile.


Benjamin Hunting View All

Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time.  I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.

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