Jeep 200 Cherokee, Service Manual

Page: 540 / 2119

control output signal at the defogger relay. For diag-
nosis of the CCD data bus and the data bus message
inputs, a DRB scan tool and the proper Diagnostic
Procedures manual are recommended.
Refer to Instrument Cluster in Group 8E - Instru-
ment Panel Systems for the service procedures for
the instrument cluster. The rear window defogger
timer and logic circuitry cannot be adjusted or
repaired and, if faulty or damaged, the instrument
cluster assembly 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
and outside mirror heating grids, and the indicator
lamp of the defogger switch, when the instrument
cluster rear window defogger timer and logic cir-
cuitry grounds the relay coil. See Defogger Relay in
the Diagnosis and Testing section of this group for
more information.
The rear window defogger relay is located in the
junction block, on the right cowl side inner panel
below the instrument panel in the passenger com-
partment. The rear window defogger relay cannot be
repaired and, if faulty or damaged, it must be
replaced.
DIAGNOSIS AND TESTING
DEFOGGER SYSTEM
For circuit descriptions and diagrams, refer to
8W-48 - Rear Window Defogger or 8W-62 Power Mir-
rors in Group 8W - Wiring Diagrams. The operation
of the electrically heated rear window defogger sys-
tem can be confirmed in one of the following man-
ners:
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.
The above checks will confirm system operation.
Illumination of the defogger switch indicator lamp
means that there is electrical current available at the
output of the defogger relay, but does not confirm
that the electrical 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 or outside rear view mirror heating grid is
still inoperative, one or more of the following is
faulty:
•
Defogger switch
•
Defogger relay
•
Instrument cluster circuitry
•
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)
•
Outside rear view mirror heating grid.
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 or outside rear view mirror heating
grids.
Fig. 1 Rear Window Glass Grid Test
1 – TERMINAL “A”
2 – TERMINAL “B”
3 – FEED WIRE
4 – MID-POINT “C” (TYPICAL)
5 – HEATED REAR WINDOW DEFOGGER GRID
6 – GROUND WIRE
XJ
ELECTRICALLY HEATED SYSTEMS 8N - 3
DESCRIPTION AND OPERATION (Continued)