Diagnosis And Testing; Defogger Relay; Defogger Switch; Defogger System - Jeep 1997 Grand Cherokee Service Manual

TERMINAL "A" GROUND WIRE VOLTMETER HEATED REAR WINDOW
ZG
DESCRIPTION AND OPERATION (Continued)
by controlling the current feeds to their respective
outside rear view mirror heating elements.
The BCM is mounted under the driver side out-
board end of the instrument panel, behind the instru-
ment panel support
outboard switch pod. Refer to Group 8E - Instrument
Panel Systems for the removal and installation pro-
cedures. For diagnosis of the BCM or the CCD data
bus, refer to the proper Body Diagnostic Procedures
manual. The BCM can only be serviced by an autho-
rized electronic repair station. Refer to the latest
Warranty Policies and Procedures manual for a cur-
rent listing of authorized electronic repair stations.

DIAGNOSIS AND TESTING

DEFOGGER SYSTEM

For circuit descriptions and diagrams, refer to
8W-48 - Rear Window Defogger in Group 8W - Wir-
ing Diagrams. The operation of the electrically
heated rear window defogger system can be con-
firmed in one of the following manners:
1. Turn the ignition switch to the On position.
While monitoring the instrument panel voltmeter, set
the defogger switch in the On position. When the
defogger switch is turned On, a distinct voltmeter
needle deflection should be noted.
2. Turn the ignition switch to the On position. Set
the defogger switch in the On position. The rear win-
dow defogger operation can be checked by feeling the
rear window or outside rear view mirror glass. A dis-
tinct difference in temperature between the grid lines
and the adjacent clear glass or the mirror glass can
be detected within three to four minutes of operation.
3. Using a 12-volt DC voltmeter, contact the rear
glass heating grid terminal A (right side) with the
negative lead, and terminal B (left side) with the pos-
itive lead (Fig. 1). The voltmeter should read battery
voltage.
Fig. 1 Rear Window Glass Grid Test
DEFOGGER GRID MID-POINT (TYPICAL) "C" TERMINAL "B" FEED WIRE
armature and below
ELECTRICALLY HEATED SYSTEMS
The above checks will confirm system operation.
Illumination of the defogger switch LED means that
there is electrical current available at the output of
the defogger relay, but does not confirm that the elec-
the trical current is reaching the rear glass heating grid
lines.
If the defogger system does not operate, the prob-
lem should be isolated in the following manner:
(1) Confirm that the ignition switch is in the On
position.
(2) Ensure that the rear glass heating grid feed
and ground wires are connected to the glass. Confirm
that the ground wire has continuity to ground.
(3) Check the fuses in the Power Distribution Cen-
ter (PDC) and in the junction block. The fuses must
be tight in their receptacles and all electrical connec-
tions must be secure.
When the above steps have been completed and the
rear glass heating grid is still inoperative, one or
more of the following is faulty:

• Defogger switch

• Defogger relay

• Body Control Module (BCM)
• Rear window grid lines (all grid lines would
have to be broken or one of the feed wires discon-
nected for the entire system to be inoperative).
When the above steps have been completed and the
heated mirror glass heating element is still inopera-
tive, one or more of the following is faulty:
• Body Control Module (BCM)
• Chrysler Collision Detection (CCD) data bus
• Driver or passenger door module
• Outside rear view mirror heating elements.
If setting the defogger switch to the On position
produces a severe voltmeter deflection, check for a
short circuit between the defogger relay output and
the rear glass heating grid.

REAR GLASS HEATING GRID

For circuit descriptions and diagrams, refer to
8W-48 - Rear Window Defogger in Group 8W - Wir-
ing Diagrams. To detect breaks in the grid lines, the
following procedure is required:
(1) Turn the ignition switch to the On position. Set
the defogger switch in the On position. The indicator
lamp should light. If OK, go to Step 2. If not OK, see
the Defogger Relay diagnosis in this group.
(2) Using a 12-volt DC voltmeter, contact the ver-
tical bus bar on the right side of the vehicle with the
negative lead. With the positive lead, contact the ver-
tical bus bar on the left side of the vehicle. The volt-
meter should read battery voltage. If OK, go to Step
3. If not OK, repair the open circuit to the defogger
relay as required.
(3) With the negative lead of the voltmeter, contact
a good body ground point. The voltage reading should
8N - 3