Workshop Repair and Service Manuals GMC SIERRA

04-08-48-001D Rear Window Defogger - Broken Heating Grid Detection Method - (Apr 17 2012) Defoggers Rear Glass


Subject:Rear Window Defogger – Broken Heating Grid Detection Method

#04-08-48-001D: Rear Window Defogger - Broken Heating Grid Detection Method - (Apr 17, 2012) Defoggers Rear Glass

Models: 2013 and Prior GM Passenger Cars and Light Duty Trucks

This bulletin is being revised to add the 2013 model year and change the recommendedrepair. Please discard Corporate Bulletin Number 04-08-48-001C (Section 08 – Bodyand Accessories).

The addition of vertical grid lines to the heated back window defogger circuits hasmade it difficult to detect broken defogger grid lines. In the past, it was a simplematter to use a voltmeter to check the continuity of each grid line in order to locatea non-functional line. Some new design back windows have two vertical grid lines thatconnect all of the horizontal grid lines together, thereby providing alternate routesfor the electricalcurrent to follow. This makes the old test method ineffective. If the vehicle doesnot have the vertical lines, the old (line-by-line) test methods can be used.

Materials RequiredA small ball of fine steel wool Type 00, orOptional — A strip of liquid crystal heat sensitive paper, 51-mm x 305-mm (2-in x12-in) or similar size (Contact Edmund Scientific at 800-728-6999 for part numberCR30723-70 or go to, orOptional — A portable infrared thermometerCorrection

There are three distinct zones across the back window that must be checked. They are:

the driver's side outboard of the two vertical linesthe passenger side outboard of the two vertical linesthe central zone that falls between the two vertical lines

To detect a broken grid line in any of the above three zones and to isolate the exactlocation of the break, perform the following steps:


Approved safety glasses and gloves should be worn when performing this procedure toreduce the chance of personal injury.Cover the rear shelf area to prevent damage to the interior trim material.Start the engine and turn on the back window defogger.Take the ball of fine steel wool and twist one end to a point. Move the point slowlyacross each grid line. Be sure to start at the far side of the zone and move it tothe opposite side of the zone. When you bridge the grid line break with the steelwool, you will see a small spark. Repeat the test over the same area to be sure youhave accurately located the break. Mark the exact location of the grid line break.Repeat this portionof the test for each grid line. If you do not see a spark at any point, it is possiblethat there are two breaks in the same line and zone. Close visual inspection usinga magnifying glass may be the only way to locate breaks in this case.The following are provided as an alternative way to detect a non-functional grid line.If available, use in addition to the steel wool.Method using liquid crystal heat sensitive paper:

Important:The first part of the test must be completed quickly before the entire surface ofthe back window becomes warm.

From outside the vehicle, place the heat sensitive paper (dull surface in contactwith the glass) against the top driver side grid line.
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