Heated Glass Element: Description and Operation
REAR WINDOW DEFOGGERRear Window Defogger System Schematic:
The Rear Window Defogger electrical system involves the operation of the Manual Temperature Control (MTC) or Automatic Temperature Control (ATC) head, the Body Control Module (BCM), and the rear window defogger relay and wiring. The system consists of a rear glass with two vertical bus bars and a series of electrically connected grid lines fired on the inside surface. The Body Control Module (BCM) operates a relay located in the right side trunk area on all models.
Vehicles with Rear Window Antenna refer to Audio Systems.
Vehicles equipped with an electrically heated rear window defogger also have a 90 amp generator.
ATC Control Head:
The passenger can push the Rear Window Defogger button on the MTC or ATC head to defog the rear glass. The system will only operate when the ignition switch is in the run position. Once the Rear Widow Defogger button is pushed, the button indicator will light and the system will operate for a period of time and then automatically turn OFF. Refer to the owner's manual for operation of the MTC or ATC system.
When the rear window defogger relay is ON, current is directed to the rear defogger grid lines. The heated grid lines heat the rear glass to clear the surface of fog or frost.
Circuit protection is provided by a fusible link, located in the charging circuit, for the heated grid circuit and by a fuse for the relay control circuit.
The MTC or the ATC head sends a signal to the BCM, which indicates to the BCM that the operator wants the rear window grid to heat. An output wire from the BCM connects to the coil of the rear window defogger relay. The other side of the coil is connected to a fuse. The BCM turns on the relay by connecting the output line of the BCM to vehicle ground. The relay contacts will then close supplying vehicle power to the rear window defogger grid.
CAUTION:
Grid lines can be damaged or scraped OFF with sharp instruments, care should be taken in cleaning glass or removing foreign materials, decals or stickers. Normal glass cleaning solvents or hot water used with rags or toweling is recommended.
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 right rear trunk area behind the carpeting.
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 bezel.
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 A/C HEATER CONTROL - REMOVAL)
On Manual Temperature Control systems when the rear window defogger switch push button is depressed, it provides a hard wired signal by momentarily closing the rear window defogger switch sense circuit which is monitored by the BCM. On Automatic Temperature Control (ATC) systems, when the defogger switch is depressed, the ATC head sends a switch status message to the BCM over the Programmable Communications Interface (PCI) data bus network. 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. Each time the BCM rear window defogger timer and logic circuitry sees another request, 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.