Auto Glass Tech Center Knowledge Base


Installation Tips

Rear Defroster Repair

From John Seaborn
of Frost Fighter

    The modern automotive rear window defroster is a sophisticated surface heater. The grid pattern of thin brown horizontal lines are made from a conductive silver ceramic compound which is first silk screened to the inside of the rear window and then baked at very high temperatures. These grids are hooked up to a high amperage circuit to provide rapid heating.

    The average defroster grid heats up to approximately 85 Fahrenheit and is very effective in clearing fog and ice from the rear window. Although effective, rear window defroster grids are vulnerable to damage and with back window glass costing hundreds of dollars to replace, repairing a damaged defroster grid makes a lot of sense.

Trouble Shooting

Typically there are two ways rear window defrosters fail. First, the thin horizontal lines are broken or scratched away from the rear glass causing the defroster to only partially defrost the rear window. Defrosters can also fail when the main power tabs are knocked off of the defroster’s vertical side bars, causing a total defroster failure.

Defroster Grid Repairs

If the defroster clears only part of the rear window, it’s usually a sign of a broken horizontal grid line. The first step in repairing this type of damage is to locate the break by visual inspection. If the break is not obvious after close visual inspection, use a volt meter to help locate the damage. Turn on the defroster and touch one meter lead to each of the wide vertical bars along the sides of the defroster.

The reading should be between 12 and 14 volts. If the volt meter shows a negative reading, reverse the leads and take readings at various places on the grid to isolate the break. When you pass over the break, the reading will change sharply. The smallest break or cracked trace can cause the defroster to fail.

Once a break in the defroster grid has been located, it’s time to consider how to repair the problem. The Frost Fighter Grid Repair Kit is designed to repair this type of damage. Use this kit to precisely mask off the damaged area of the defroster grid. There may be more than one break in the grid so be sure to locate and mask all the damage before proceeding.

After the damaged areas have been located and masked, apply the repair material using the small brushes supplied. Frost Fighter grid repair kits use a highly conductive silver filled polymer repair compound. This polymer bonds aggressively to glass and provides maximum electrical conductivity. The material dries to the touch in 10 minutes and is usable in thirty minutes. The polymer is color matched to the reddish brown colors used by in automotive defrosters for nearly invisible repairs.

Removing the masking tape after the polymer dries leaves a uniform highly conductive defroster repair. It is important that repairs be done at normal room temperatures and that the conductor be allowed to fully cure before activating the defroster. Repairs can be completed even more quickly by applying heat to the repair. A lamp or hair dryer can be used to speed curing. Care must be used not to damage the glass or any tinting that may be on the window.

Defroster Tab Repairs

When the electric defroster grid does not work at all, small breaks in the horizontal grid can be eliminated as the problem. In this case, all of the horizontal defroster grid lines would need to be broken and it is very unlikely that this would happen without it being immediately obvious. Begin trouble shooting a total grid failure by carefully inspecting the thick vertical side bars of the defroster. A break or scratch in one of these bars could cause the defroster to fail completely. Use the Frost Fighter Grid Kit to repair any small breaks or scratches in these critical side bars.

The more likely cause of a total defroster grid failure is a separated main power tab. These critical tabs connect the defroster grid with the vehicle’s high amperage defroster circuit and are located on each of the defroster grid’s vertical side bars These all important main power tabs can be knocked off of the defroster causing total defroster failure. Look for dangling wires near the sides of the defroster as a sure indication of this problem. Once this problem has been identified the challenge becomes reliably reattaching the separated tabs without damaging the back glass or the existing defroster.

To reattach these separated tabs, pull the metal clip out of its connector and use the Frost Fighter Tab Repair Kit to repair the problem. The kit uses a fast curing, two part conductive silver epoxy to reattach the main power tabs to the vertical defroster bars. Frost Fighter silver epoxy comes in two syringes for precise dispensing and is mixed at a simple 1:1 ratio.

Dip the end of the tab into the adhesive making sure to thoroughly coat the area to be rebonded to the defroster grid. The simple Frost Fighter tab repair system is safe for all defroster tab configurations and will not cause additional damage to the defroster as soldering or other methods of tab repair can do.

Replace the separated defroster tab over the area were it was originally separated and rock gently to seat the tab. The highly conductive silver filled epoxy cures quickly and bonds aggressively to glass forming both the electrical and structural connection in one simple operation. Frost Fighter tab repair kits allow repairs to be made in ten minutes when heated.

Other Problems

If the problem isn’t a break in the defroster grid, or a separated main power tab, begin checking by trying to turn the defroster system on. If you hear the system’s relay click and the monitoring lamp is on, you know you have current. Carefully check the switch and the wiring, paying careful attention to the ground side of the wiring. You can check this by placing the volt meter probes on each side of the wide vertical bars of the defroster grid and moving the negative lead to somewhere on the vehicle’s body. If the reading starts at zero and then returns to normal when the lead is moved, the problem is a connection on the ground side of the grid circuit.

By using these tips and through careful trouble shooting, damaged rear window defrosters can be easily and economically repaired, saving hundreds of dollars in the process.

The product featured here is available from Frost Fighter. For more information you can visit their web site @

Click image to close window.