Rear Defroster Diagnosis

Recently we’ve seen an increase in questions on the Auto Glass Safety Council (AGSC) Master Certification and requests for more study materials for the exam. Master certification is different from the Technician or General exam because it tests more advanced services usually offered by the full-service auto glass shops. These services can include:

  • An in depth understanding of regulations and standards pertaining to our industry,
  • Safety related issues and OSHA regulations for the work place,
  • Laminated glass fabrication (glass cutting and finishing),
  • Pattern-making and template construction,
  • Older and more unique vehicle glass replacement, and
  • Understanding electrical circuits and troubleshooting those circuits.

Though information on some of these subjects is harder to find than others, I thought I would begin this series of “Master” posts with the most common electrical problem a glass shops see – rear window defroster malfunction.

There are three defroster malfunctions that present themselves to the customer; either the defroster does not work at all, one or more gridlines do not work, or the defroster connectors come off. Let’s address each of these.

  1. If the defroster does not work at all, it means that power is not getting to the unit. First, you should check the fuse. If the fuse is blown, replace it and make a suggestion to the owner that they get the system checked by the dealer. A blown fuse means there’s something wrong with the system. The system (circuit) consists of:
  • The power source (battery),
  • The wiring from the battery to the fuse block,
  • The wiring to the switch,
  • The wiring from the switch to the connectors on the defroster,
  • The connectors on the defroster both the incoming power and the ground, and
  • The defroster grid itself (Unit).

Replacing the fuse is a short-term fix.  If you’re able to trace the problem to its source, do so. However, if you do not have the expertise to diagnose and fix the problem’s cause, I suggest you refer the customer back to the dealer.

Photo Courtesy Main Image – Ebay


  1. The customer comes in and complains that one or more of the lines are not working. This usually indicates that the grid line(s) is severed and must be repaired.

How do you find the break in the lines and how do you fix it?

You will need a defroster repair kit from your local hardware or auto parts store. It will consist of a small jar of metallic paint, a brush and a template. You’ll need an ohmmeter, which uses an outside source of power for its reading, not a test light. A test light uses the power supplied in the circuit to light the bulb in the tool.  This adds another unit to the circuit and could confuse the internal computer of the added unit and cause further problems.

Turn the vehicle on and connect the ground lead of the ohmmeter to the defroster’s ground connector, this is usually on the passenger side of the back glass. Now take the power lead and gently drag the lead along the suspected grid line. A properly operating grid line would have a slowly decreasing volt reading from 12 volts to 0 as you move the lead from power side to ground side. A broken line would have a consistent 12 volt reading until it suddenly drops to 0. When the voltage drops to zero suddenly, that is where the break in the line is located. Mark the break point and attach the template to its location. Then paint a “bridge” across the two ends of the line with the metallic paint and allow to dry thoroughly. Retest and confirm repair.

  1. The connector tabs frequently break off of the bus bars due to cleaning the inside surface of the back glass or from items stored on the back shelf or storage area. There are several products that do an excellent job of reattaching the tabs and reestablishing the electrical circuit. However, there’s another way that has proven results.

You can simply use a cyanoacrylate glue, such as Super Glue, to reattach the connectors to the glass surface at the point of separation. Then use the defroster metallic paint to make the electrical connection between all circuit components. The circuit components are the connector, the bus bar above the separation point, and the bus bar below the separation point. This will join all of the components and reestablishes the circuit. Then allow the metallic paint to dry thoroughly and test the repair.

Caution:  Do not add concentrated heat to a particular point, such as a solder gun. This could cause a thermal break.

I’ll give you more information concerning the tested knowledge in The AGSC Master’s Certification exam in future posts. My “Lost Arts” series of posts cover the laminated glass fabrication and finishing and can also help when studying for the exam. Please contact me if you have any other questions about Master Certification.