Rear Window Defroster Problems

image of rear window defroster repair kit

Defroster grid repair kit

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

image of rear window defroster tester kit

Tester and repair kit

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.

For more information about car accessories and their problems you can visit the auto repair information blog homepage from this page about rear window defroster problems.

16 thoughts on “Rear Window Defroster Problems

  1. Mark Gittelman Post author

    Sally : Hard to say since Mercedes can be so complex when it comes to even basic systems like a rear defroster. On regular cars such a problem would send me towards the switch and the Timer. Rear defrosters have a timer to turn off the grid after 15 minutes to prevent the window from cracking.

  2. Sally wilson

    Hi there, I have a 4 year old Mercedes C class. The rear window defrosting element comes on randomly when I haven’t switched on the button. The first I know of it Is when I smell a scorching smell. The window feels very hot to the touch. Mercedes seem stumped. And maybe? Don ‘t really believe me! A local garage replaced a small square black plastic relay box but that hasn’t stopped it happening. I think it gets hot enough to crack. I would be grateful for any ideas as I can’t find anything online about this problem so it can’t be very common. Sally

  3. Mark Post author

    Steven: Usually the hard part is getting the tab to stay on. Since you have overcome this hurdle you might want to try painting over the top of the tab bridging, the grid and tab with just the conductive paint. It might take several coats this way?

  4. Steven

    Hi. I repaired the tab on my rear window defroster: it worked. However, it became detached, again. Here is the problem. I used the old kit. Apparently, the liquid, which is conductive, was too dry. The glue worked fine, but it’s not conductive. Now, I have the tab on, but the defroster still won’t work. How can I loosen the glue so I can detach the tab and start over with a fresh repair kit? Help! Thanks.

  5. Mark Post author

    John: Technically that kind of damage is not covered under warranty. But the thing is the dealer can cover it as a defect. They do not have to tell the factory about the dog damage. So it’s worth a try. The dealer might cover this for you if you state your a new owner interested in doing all future business with the establishment. I wrote an article about the ins and outs of warranty coverage and how flexible the dealer can be if they want to. http://www.certifiedmastertech.com/wordpress/2011/09/01/is-it-covered-under-warranty/

  6. john

    Hi Mark, Ive just recently bought a Mercedes A 180 ,and at the time of sale I noticed the rear windscreen had quite a few scratches on the inside, I was told that the previous owner`s a dog had done the damage and that all I had to do was to peel off the film covering the window if it bothered me . The damage does look as if the scratches have gone through to the glass itself and the grid on the window seems to have been broken as car is still under warranty, do you think I should speak to the dealer about it or would the damage be covered on my insurance under windscreen repair. or would the problem be sorted if I just peeled off the film and replace it. thanks for any advice you could give me.

  7. Mark Post author

    Joel: You did not talk about the ground side tab but if you have good power to the grid and a strong ground then the dealer might have a point in wanting to replace the glass. You can test the ground side tab by connecting the test light alligator clip to 12Volts and touch the ground tab ( should light up strong ). If you need a rear window shop around with your local glass companies. Some will install junkyard glass.

  8. Joel Tompkins

    Hi Mark! Do you still respond to this page? I accidentally popped a tab off of my 2004 Mustang when applying Armorall to the trim. It is glued back on, it has continuity (zero resistance to the grid), and I even get voltage to the grid when turned on. But it just doesn’t work. Even though I can measure just under 12 volts between the grid and the tab, it doesn’t defrost or defog at all. The Ford dealer could only offer to replace the rear window. I’m looking for a better idea! Do you have any ideas! Thanks!

  9. Billy

    What kind of tester do I need to test the grids on my rear window defogger? Where can I get one?

  10. Mark Post author

    Hi Ruby: Before I tried the tab adhesive again or called the glass company to re-solder it I would want to verify good power and ground connections. You can take a test light and connect the alligator clip to one connector at the defroster tab and touch the point of the test light to the other side. You should have 12 volts. This quick test not only checks your fuse condition but also confirms good power and ground wiring all the way back to the rear defroster.

  11. Ruby

    Hi Mark,

    I have a 95 convertible Mustang and the tab for the rear defrost came off. I picked up some tab Adhesive from Lordco Auto supply and glued it back on. The Adhesive said it was conductive. I’m not sure if I did it correctly as it is not working. I never added any coating after. I called a place to solder and they want to charge $115 which seems a little steep. Would you recommend taking it off and purchasing more tab adhesive to try again? Should I file the connecting piece? I also wanted to check the fuse to be sure but the manufacture info book tells me that the fuse is located under the hood and the battery needs to be unhooked in order to check fuse. Now it’s snowing out! Help! Thanks….

  12. ramkumar

    Hi Mark: Toyota service have checked my defrost and said that there is no problem in the voltage when they checked from one end to another. Can you please advise what else can be wrong?

  13. Mark Post author

    Mark H: They make a special kit for attaching the electrical contact. ” Permatex 21351 Rear Window Defogger Tab Adhesive” The liquid in the kit is conductive. But the glue is not. Use the glue to stick the detached contact clip back on the window. After the glue dries, paint the joint with the included conductive liquid. Apply multiple coats, allowing each to dry, same as any other paint job.

    In my opinion the warmer it is the better the results. You can use the cars interior heater and also a heat gun for better results. If this fails you can call a mobile glass company and they can come out and solder the electrical contact back on to the defroster grid. Although this might cost 50 or 60 bucks it is a lot cheaper then replacing the whole glass.

  14. mark h

    i have a 2000 frd taurus wgn that the clip and the whole wire came off the rear window defogger i tryed to put it back on with a kit i got at a auto parts store and it would not stick do u know of any thing that could help as i do not want to put money into my car as it is only used around town and has 150000 on it any help pls

  15. Mike

    question on the button side that does the activating my dodge neon has three wires going into the circuit of the button (which also has the timer ic). Would that be two negatives and a 5 volt dc line? If I short the relay side the defroster works but I can’t seem to find any power on the low side. What are your thoughts? Maybe a fuse inside the car I missed? The fuse under the hood for the defrost is good.

  16. Steve

    Hi,
    Thanks for writing this article.
    Most people think that they have to buy a rear window in order to fix a bad grid.
    You have shown that they do not and it can be repaired easily.

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