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Rear Window Defroster Repair

Contributed By: Bill Kelsey

Many FSJ owners have reported problems with the tailgate window defrosters. This is not surprising -- the combination of a movable window with a defroster is almost certain to eventually cause problems. Since I've had my Cherokee, I've had to repair at least 50 breaks in the defroster grid, most due to a bad inside tailgate weatherstrip rubbing against the glass too hard, some to scratches from items carried in the cargo area, and a few from the conductive paint just wearing down.

The FSJ rear defroster is a simple system. A dash switch controls power to a circuit that runs through the defroster grid on the tailgate glass. The grid is entirely on the surface of the glass, and is made up of conductive paint over a thin metal grid bonded to the glass. The most common problems are due to something scratching the inside of the tailgate glass and creating breaks in the conductive paint and sometimes the underlying metal grid, or to the inside tailgate weatherstripping rubbing against the grid as the window is raised or lowered and wearing away the conductive paint. A grid element may fail even if the paint is not entirely worn away, but merely reduced to the point where it will not pass enough voltage to heat that grid element.

If part of the defroster grid works, skip down to "Locating Breaks in the Defroster Grid ".

Checking the system:

  1. The Switch

    The defroster switch (which will automatically switch off after about 10-minutes) is located on the lower left dash. This has a light (green on mine, but apparently the lens color varied). If the light does not come on when the switch is turned on, either there is no power to the switch or the circuit is bad. You can check this by removing the switch from the dash and, using a short wire, shorting the connectors. If the light then comes on, the problem is in the defroster wiring between the switch and tailgate or in the defroster grid. If the light does not come on, check the fuse and the wiring between the switch and the fuse panel.

  2. The Wiring Between the Switch and Tailgate

    Using a voltmeter or test light, turn on the defroster switch and see if a reading is obtained between the lower right (US passenger's side) vertical grid element and a ground. If it is, the grid is receiving power, but there is a major break in the grid, probably in the lower right or left (ground) vertical elements. (If the glass has been badly scratched, this may also indicate that every horizontal grid element has been broken -- a problem I once encountered.)

    If there is no reading, open the tailgate access panel and disconnect the two wires to the defroster and check between those with the voltmeter or test light. If there is no reading, there is a break somewhere between the dash switch and the tailgate. If there is a reading, there is a break in the vertical grid elements near the power or ground connectors.

    If the break is in the wiring between the dash and the tailgate, I'd just install new wiring rather than spending much time trying to track down and fix the break(s). Otherwise, use the usual methods to locate and repair a break in a wire.

Locating Breaks in the Defroster Grid

Items Required:

  • a voltmeter (If you don't have one of these, small battery- powered analog AC/DC multitesters can be bought at auto parts suppliers for under US$10. Digital meters can be bought for about US$20, although better multitesters can cost considerably more.)
  • a felt-tip marker or colored crayon (This will be used to mark the areas needing repair; I have found crayon wax much more difficult to remove than the felt-tip marker ink.)
  • a large piece of white or light colored cardboard or cloth (This is used to enable you to see the grid elements more clearly.)

If some of the grid elements work but others don't, note which elements are not functioning. Obviously, this is easiest on those days when you can simply see which defroster elements aren't melting the frost. If you are uncertain which grid elements are working and which are not, test all of them. This is also useful for identifying grid elements which are functioning but damaged, allowing preventative repairs.

Begin by visually inspecting the grid for any obvious breaks or damaged areas. Be sure your work area is well-lit. It helps to place the white cardboard or cloth below (but near or touching) the glass to provide a high contrast background. Mark the glass next to any suspicious areas with the marker or crayon. Breaks in the positive or negative (grounded) vertical grid elements are unusual, because these typically are overlaid with a heavy wire braid. However, sometime breaks or worn areas will occur at the junction of the horizontal and vertical grid elements. These can be easily missed, as can be areas where the original grid paint has been worn away but the underlying metal strip is intact (and often the same color as the paint).

Turn on the defroster system (the ignition switch must be on, although the engine need not be running).

Set the multitester to read DC voltage and to the 12 volt scale (if there is no 12 volt setting, use the first setting greater than 12 volts). Wrap aluminum foil around the negative probe of the multitester. Touch the positive probe of the multitester against the positive (probably the left) vertical grid element. Press the foil wrapped negative probe to the center of the first horizontal grid element to the tested. If the reading is 6 volts, the grid element is functioning correctly. If the reading is 12 volts, there is a break between the center of the element and the positive vertical grid element. If the reading is 0 volts, there is a break between the center and the negative (grounded) vertical grid element. If the reading is between 6 and 12 volts there is a damaged or worn (but not yet broken) area between the center of the grid element and the positive vertical element, and if the reading is between 0 and 6 volts, there is a damaged or worn area between the center of the grid element and the negative (grounded) vertical element.

To locate the actual break (or damaged/worn area), slowly slide the foil covered negative lead from the positive vertical element towards the negative vertical element. The reading will jump from 0 volts to several volts when the negative lead passes beyond the break; the reading will suddenly increase when an unbroken but damaged/worn area is passed. If there is more than one break or damaged/worn area in a single horizontal element, the readings will change when the break or damaged area nearest the negative vertical element is encountered. The other breaks or damaged areas will show up after these are repaired.

It is possible to find a second break by holding the negative lead to a point on the negative end of the horizontal element that you know is intact and slowly sliding the positive lead along the horizontal element from the positive vertical element. If the multitester reading suddenly drops or changes a second break or damaged area, nearest the positive vertical element, has been located.

Mark the glass next to the broken or damaged areas with the marker or crayon.

If the reading steadily increases from 0 volts to about 12 volts as the negative probe moves from the positive vertical element to the negative vertical element, the horizontal grid element is intact.

Repeat this process for each horizontal element to be tested.

Repairing the Grid

Items Required:

  • defroster repair paint (This is typically a small bottle of conductive epoxy paint, often colored to match the color of your defroster grid, in a kit with a small brush and piece of cardboard with a slit in it. Most auto parts suppliers (including WalMart, KMart, etc.) have these, and kits can also be obtained from JC Whitney. The usual cost is about US$10. I've repaired well-over 50 breaks using a single small bottle of paint (although I had to add some acetone to thin it when it began to dry out.)
  • masking tape (This is preferable to the cardboard paint guide included in the repair kits, both because you can repair a number of areas at the same time and because the slits in the cardboard are typically much wider than the grid elements.)
  • a razor blade (This is used to trim the edges of the repaired grid elements, where the new paint will have a tendency to wick out from the grid. It can also be used to remove the felt-tip marker or crayon elements.)
  • acetone (This is used to thin the paint, which will often begin to dry out in the bottle very quickly. It is only required if you have a number of repairs to make, which allows the acetone in the paint to evaporate.)
  • a large piece of white or light colored cardboard or cloth (This is used to enable you to see the grid elements more clearly.)

Try timing your repair to occur at a time when you can leave the FSJ for about 23 hours. I find doing these repairs in my garage on a Friday afternoon or early evening works well -- the FSJ can be left at least overnight with the tailgate open to allow the paint to dry.

While it is possible to repair the grid with the tailgate closed and window raised, this requires crouching in the cargo area. I find it better to open the tailgate and extend the window so that all of the horizontal grid elements are exposed. This also allows you to sit while working on the defroster. Be sure to support the glass (large cardboard boxes work well, and provide a place to lay the white cardboard or cloth) -- unsupported, the weight of the extended glass will break the window (in which case, ignore the rest of this and go buy another window with a functioning defroster grid!).

Make sure the defroster grid is turned off and is cool.

Following the directions on the defroster repair kit, carefully clean the broken or damaged areas of the defroster grid with alcohol to remove any dirt, oils, or other substances which may prevent the repair paint from adhering tightly.

Carefully place a strip of masking tape along both sides of the broken or damaged grid, as close to the grid element as possible and extending left and right beyond the break about 1/2" to 1". Press the tape firmly to the glass to minimize the repair paint wicking under the tape.

Repeat this until all the broken or damaged areas on the grid are masked with tape.

Carefully brush the repair paint over the breaks or damaged areas, extending about 1/2" to the left and right of the breaks or damaged areas to ensure a good contact with the rest of the grid element.

Following the directions on the repair kit, wait a few minutes until the surface of the paint has dried and repeat for the number of coats of paint desired (usually three or four). If the paint begins to dry out or thicken, add a little acetone to the paint jar and stir or shake it.

Following the directions on the repair kit, carefully remove the tape and use the razor blade to trim the sides of the repaired areas, either by carefully cutting away excess paint that has wicked out, or by carefully pushing the excess paint to the grid element. Press the side of the razor blade against the repaired area to flatten any raised areas to minimize the repair catching on the tailgate weatherstripping.

Leaving the tailgate open and window raised, wait about 24 hours to allow the epoxy paint to fully cure. (If the vehicle is outside and you don't want to leave it open, wait as long as possible before carefully lowering the window, closing the tailgate, and raising the window. You will want to minimize the possibility of the inside tailgate weatherstripping damaging the repaired areas.)

After about 24 hours try the defroster, retesting the horizontal grid elements as necessary for further breaks or damaged areas. If everything is working, use the razor blade to carefully scrape away the marker or crayon marks. (Crayon marks may smear and have to be further wiped away with a towel.)

It is not uncommon to have to repeat this process several times on a grid with a number of breaks. It is sometimes possible to hasten the process by temporarily bridging breaks with a piece of foil, allowing other breaks on the same horizontal grid element to be located. This works best with an assistant who can either hold the foil in place or hold the positive probe in place (allowing you to hold the foil); it is more difficult when working alone, although it is sometimes possible to get a good contact between the positive probe and the positive vertical element by wedging the probe into the tailgate where the positive vertical element emerges or even taping it to the glass (although it is difficult to maintain a good contact with tape).

If the color of the repaired grid is noticably lighter than the original grid (which is often a dark bronze color), the Jeep TSM suggests it can be colored with tincture of iodine.

Sources:

  • 1978 Jeep Technical Service Manual, vol. 3: Body
  • Haynes Automotive Repair Manual: Jeep Wagoneer & Pick-Up, 1972 thru 1991
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