General Information; Description And Operation; Body Control Module; Defogger Relay - Jeep 1997 Grand Cherokee Service Manual

8N - 2 ELECTRICALLY HEATED SYSTEMS
GENERAL INFORMATION (Continued)
NOTE: This group covers both Left-Hand Drive
(LHD) and Right-Hand Drive (RHD) versions of this
model. Whenever required and feasible, the RHD
versions of affected vehicle components have been
constructed as mirror-image of the LHD versions.
While most of the illustrations used in this group
represent only the LHD version, the diagnostic and
service procedures outlined can generally be
applied to either version. Exceptions to this rule
have been clearly identified as LHD or RHD, if a
special illustration or procedure is required.

DESCRIPTION AND OPERATION

REAR GLASS HEATING GRID

The heated rear window glass has two electrically
conductive vertical bus bars and a series of horizon-
tal grid lines made of a silver-ceramic material,
which is baked on and bonded to the inside surface of
the glass. The grid lines and bus bars comprise a
parallel electrical circuit.
When the rear window defogger switch is placed in
the On position, electrical current is directed to the
rear window grid lines through the bus bars. The
grid lines heat the rear window to clear the surface
of fog or snow. Protection for the heated grid circuit
is provided by a fuse in the junction block.
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 clean-
ing the glass or removing foreign materials, decals,
or stickers from the glass. Normal glass cleaning sol-
vents or hot water used with rags or toweling is rec-
ommended.
A repair kit is available to repair the grid lines and
bus bars, or to reinstall the heated glass pigtail
wires.

HEATED MIRROR

The heated mirrors are controlled by the rear win-
dow defogger switch. The heater elements in the mir-
ror are activated only when the rear window defogger
switch is in the On position. The heater elements in
the mirrors cannot be repaired and, if faulty, the
entire mirror unit must be replaced. Refer to Group
8T - Power Mirrors for the diagnosis and service of
this component.

DEFOGGER SWITCH

The rear window defogger switch is mounted in the
inboard instrument panel switch pod, inboard of the
steering column. The momentary-type switch pro-
vides a hard-wired ground signal to the Body Control
Module (BCM) each time it is depressed. The BCM
rear window defogger timer and logic circuitry
responds by energizing or de-energizing the rear win-
dow defogger relay.
Energizing the rear window defogger relay pro-
vides electrical current to the rear window defogger
grid and the Light-Emitting Diode (LED) indicator in
the switch, which lights to indicate that the defogger
system is turned On. The defogger switch and LED
cannot be repaired and, if faulty, the inboard switch
pod unit must be replaced.

DEFOGGER RELAY

The rear window defogger relay is a International
Standards Organization (ISO)-type relay. The rear
window defogger relay is a electromechanical device
that switches fused battery current to the rear glass
heating grid and the Light-Emitting Diode (LED)
indicator of the defogger switch, when the Body Con-
trol Module (BCM) rear window defogger timer and
logic circuitry grounds the relay coil. See the Diagno-
sis and Testing section of this group for more infor-
mation on the operation of the rear window defogger
relay.
The rear window defogger relay is located in the
junction block, on the right cowl side panel below the
instrument panel in the passenger compartment.
The rear window defogger relay cannot be repaired
and, if faulty or damaged, it must be replaced.

BODY CONTROL MODULE

A Body Control Module (BCM) is used on this
model to control and integrate many of the electronic
functions and features included on the vehicle. The
BCM contains a central processing unit and inter-
faces with other modules in the vehicle on the
Chrysler Collision Detection (CCD) data bus network.
The CCD data bus network allows the sharing of
sensor information. This helps to reduce wire har-
ness complexity, reduce internal controller hardware,
and reduce component sensor current loads. At the
same time, this system provides increased reliability,
enhanced diagnostics, and allows the addition of
many new feature capabilities.
One of the systems that the BCM supports and
controls, is the rear window defogger system. In its
role as the defogger system timer and controller, the
BCM receives hard-wired inputs from the defogger
switch and the ignition switch. The programming in
the BCM allows it to process the information from
these inputs and send a control output to energize or
de-energize the defogger relay. The BCM also sends a
defogger switch status message to the Driver Door
Module (DDM) and Passenger Door Module (PDM)
on the CCD data bus. The DDM and PDM respond
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