Rear window defroster problems can cause poor visibility through the rear glass and on some models the side view mirrors are connected to the same system. This article covers a few of the common problems with electrically operated defroster systems as well as a few facts about how it operates.
Whether you call it a window defroster, deicer or defogger what were all talking about is heating the rear glass to remove moisture particles that can collect on the glass and reduce visibility. The major components of the rear window defroster on most models will include a manually operated on off switch, one or two relays and the heating elements on the glass surface.
The front windshield uses forced air that has had the moisture removed from it by the air conditioning evaporator to dry the front and side windows with warm low humidity air. This is a more efficient method than the heated grid that is commonly mounted on the rear window glass. Therefore slower defogging should be expected.
Common rear window defroster operation
The electrical rear window defogger is one of those systems that operate basically the same on most makes and models. You may find a few models that operate differently but for the most part standard operation is the same.
By pressing the rear window defroster switch you will energize a relay that sends power to the rear heater grid assembly. On many models full battery voltage is sent to the rear grid. On models with heated side mirrors voltage may also flow to these at the same time.
On most of the systems that I have worked on I find that they are equipped with some type of timing device. Often this automatic rear window defroster timer is set for about 10 minutes. This is to prevent overheating of the glass element. It’s not very common but I have seen rear window defroster problems caused by a timer malfunction. It can heat the glass until it cracks or not turn it on at all.
Even though some rear window defrosters have this automatic shutoff feature it is still wise to turn it off manually after it has completed its task of removing fog and moisture from the inside of the glass. There is no reason to continually send voltage to the heating grid and when this is switched off more efficient battery charging is accomplished.
Common rear window defroster problems
One of the most common problems with rear window defrosters is physical damage to the heating grid that is glued to the rear glass. Damage can be caused by objects rubbing across the inside surface of the glass or in some cases by overzealous cleaning of the inside glass surface using harsh chemicals.
When working at new car dealerships many times we found that the temporary tag that was stuck in the back windshield was glued over the defroster grid. When the customer received their permanent tag and remove the temporary they wound up damaging the electrically heated grid.
When a segment of the grid is damage it opens the circuit. This can cause the grid to become inoperative from the break point forward. An open spot in the grid can be found by using a test light or meter. With the defroster turned on voltage should be present at all points of the grid.
With one side of the test light grounded you can lightly touch the point of the test light to the grid to test for voltage. It is best to start at the voltage connection and work your way towards the ground side. If there is no voltage at all then it will be time to test the switch, fuse and circuit breaker along with any system relays.
If you do find open grids often they can be repaired with a paint like compound that is available at most part stores. If the rear window electrical contact falls off they have a different kit to install it. The repair kit is the Permatex 21351 rear window defogger tab adhesive. The included liquid is conductive. But the glue is not. So use the glue to stick the detached electrical contact that connects the power or ground to the grid back on the window. After the glue dries, paint the joint with the conductive liquid. Apply multiple coats, allowing each to dry. The warmer the glass the better the results. In the end you might have to get the contact soldered back on by a glass company.