Heated Glass Element: Description and Operation

HEATED GLASS
Rear Window Defogger
An electrically heated rear window defogger is standard factory-installed equipment on this model. Electrically heated outside rear view mirrors are available factory-installed optional equipment. When the rear window defogger system is turned ON, electric heater grids on the liftgate flip-up glass and behind both outside rear view mirror glasses are energized. These electric heater grids produce heat to help clear the rear window glass and the outside rear view mirrors of ice, snow, or fog. The rear window defogger system control circuit uses ignition switched battery current, so the system will only operate when the ignition switch is in the ON position.

This group covers the following components of the rear window defogger system:
- Rear glass heating grid
- Rear window defogger relay
- Rear window defogger switch.

Certain functions and features of the rear window defogger system rely upon resources shared with other electronic modules in the vehicle over the Programmable Communications Interface (PCI) data bus network. The PCI data bus network allows the sharing of sensor information. This helps to reduce wire harness complexity, internal controller hardware, and component sensor current loads. At the same time, this system provides increased reliability enhanced diagnostics, and allows the addition of many new feature capabilities. For diagnosis of these electronic modules or of the PCI data bus network, use a DRB III scan tool and (Refer to Appropriate Diagnostic Information).

The other electronic modules that may affect proper system operation are:
- Body Control Module (BCM) - Refer to Electronic Control Modules for more information.
- Driver Door Module (DDM) - Refer to Electronic Control Modules for more information.
- Passenger Door Module (PDM) - Refer to Electronic Control Modules for more information.

The rear window defogger system is controlled by a momentary switch that is integral to the a/c heater control located in the center stack area of the instrument panel. A Light-Emitting Diode (LED) in the switch button will light to indicate when the rear window defogger system is turned on. The BCM, which contains the rear window defogger system timer and control logic, monitors the status of the defogger switch through a hard-wired input. The BCM then sends control outputs through a hard wired circuit to energize or de-energize the defogger relay.

The electrically heated outside rear view mirror heating grids are also controlled by the rear window defogger switch. When the BCM receives an input from the switch, it sends a defogger switch status message to the DDM and the PDM over the PCI data bus. The DDM and PDM respond to the defogger switch status messages by energizing or de-energizing the battery current feed to their respective outside rear view mirror heating grids.

The rear window defogger system will be automatically turned OFF after a programmed time interval of about ten minutes. After the initial time interval has expired, if the defogger switch is turned ON again during the same ignition cycle, the defogger system will automatically turn OFF after about five minutes. The defogger system will automatically shut OFF if the ignition switch is turned to the OFF position, or it can be turned OFF manually by depressing the rear window defogger switch again.

See the owner's manual in the vehicle glove box for more information on the features, use and operation of the rear window defogger system.

REAR WINDOW DEFOGGER GRID
The electrically heated rear window glass is standard equipment on this model. The liftgate flip-up glass has two electrically conductive vertical bus bars and a series of horizontal grid lines made of a silver-ceramic material, which is baked on and bonded to the inside surface of the glass. These grid lines and the bus bars comprise a parallel electrical circuit. A spade type terminal near the top of each bus bar accept the connectors from the two coiled liftgate wire harness take outs.

The grid lines and bus bars are highly resistant to abrasion. However, it is possible for an open circuit to occur in an individual grid line, resulting in no current flow through the line. The grid lines can be damaged or scraped OFF with sharp instruments. Care should be taken when cleaning the glass or removing foreign materials, decals, or stickers from the glass.

Normal glass cleaning solvents or hot water used with rags or toweling is recommended.

A repair kit is available to repair the grid lines and bus bars, or to reinstall the heated glass terminals. (Refer to ELECTRICAL/HEATED GLASS/REAR WINDOW DEFOGGER GRID - STANDARD PROCEDURE).

The rear glass heating grid is energized and de-energized by the rear window defogger relay. The Body Control Module (BCM) monitors the rear window defogger switch. When the BCM receives an input from the switch, it energizes or de-energizes the rear window defogger relay through a hard wired control output. The rear defogger relay switches fused battery current to the rear window grid lines through the bus bars. The grid lines heat the rear window glass to clear the surface of ice, snow or fog. Protection for the rear glass heating grid circuit is provided by a fuse in the Power Distribution Center (PDC).

REAR WINDOW DEFOGGER RELAY
The rear window defogger relay is an electromechanical device that switches fused battery current to the rear glass heating grid and the Light-Emitting Diode (LED) indicator of the rear window defogger switch, when the Body Control Module (BCM) rear window defogger timer and logic circuitry grounds the relay coil. The rear window defogger relay is located in the junction block, under the left end of the instrument panel in the passenger compartment.

The rear window defogger relay is a International Standards Organization (ISO) relay. Relays conforming to the ISO specifications have common physical dimensions, current capacities, terminal patterns, and terminal functions.

The rear window defogger relay cannot be repaired or adjusted and, if faulty or damaged, it must be replaced.

The ISO relay consists of an electromagnetic coil, a resistor or diode, and three (two fixed and one movable) electrical contacts. The movable (common feed) relay contact is held against one of the fixed contacts (normally closed) by spring pressure. When the electromagnetic coil is energized, it draws the movable contact away from the normally closed fixed contact, and holds it against the other (normally open) fixed contact.

When the electromagnetic coil is de-energized, spring pressure returns the movable contact to the normally closed position. The resistor or diode is connected in parallel with the electromagnetic coil in the relay, and helps to dissipate voltage spikes that are produced when the coil is de-energized.

REAR WINDOW DEFOGGER SWITCH
The rear window defogger switch is integral to the a/c heater control, which is located in the instrument panel center stack below the radio receiver. This momentary switch provides a hard wired ground signal to the Body Control Module (BCM) each time it is depressed. A Light Emitting Diode (LED) in the push button for the rear window defogger switch illuminates to indicate when the rear window defogger system is turned on.

The rear window defogger switch and the rear window defogger switch LED indicator cannot be repaired and, if faulty or damaged, the entire a/c heater control must be replaced. (Refer to HEATING & AIR CONDITIONING/CONTROLS/A/C HEATER CONTROL - REMOVAL).

When the rear window defogger switch push button is depressed, it momentarily closes the rear window defogger switch sense circuit for the BCM to ground. The BCM monitors the rear window defogger switch sense circuit. Each time the BCM rear window defogger timer and logic circuitry sees another input from the switch, it toggles a control output to the rear window defogger relay. Energizing the rear window defogger relay provides electrical current to the rear window defogger grid and to the LED indicator in the switch, which lights to indicate when the defogger system is turned on. A dedicated fuse in the junction block protects the rear window defogger relay output circuit to the LED indicator.