Jeep 1996 Grand Cherokee Service Manual - page 408
can be detected within three to four minutes of oper-
ation.
3. Using a 12-volt DC voltmeter, contact the rear
glass heating grid terminal A (passenger’s side) with
the negative lead, and terminal B (driver’s side) with
the positive lead (Fig. 1). The voltmeter should read
battery voltage.
The above checks will confirm system operation.
Illumination of the switch LED means that there is
power available at the output of the defogger relay,
but does not confirm that power 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 and in the junction block. The fuses must be tight
in their receptacles and all electrical connections
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
• 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
• 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 rear defogger switch in the On position. The indi-
cator lamp should light. If OK, go to Step 2. If not
OK, see the diagnosis for Defogger Relay in this
group.
(2) Using a 12-volt DC voltmeter, contact the ver-
tical bus bar on the passenger’s side of the vehicle
with the negative lead. With the positive lead, con-
tact the vertical bus bar on the driver’s side of the
vehicle. The voltmeter 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
not change. If OK, go to Step 4. If not OK, repair the
circuit to ground as required.
(4) Connect the negative lead of the voltmeter to
the passenger’s side bus bar and touch each grid line
at mid-point C with the positive lead. A reading of
approximately six volts indicates a line is good. A
reading of zero volts indicates a break in the grid
line between mid-point C and the driver’s side bus
bar. A reading of 10-14 volts indicates a break
between mid-point C and the passenger’s side bus
bar. Move the positive lead on the grid line towards
the break and the voltage reading will change as
soon as the break is crossed.
DEFOGGER SWITCH
For circuit descriptions and diagrams, refer to
8W-48 - Rear Window Defogger in Group 8W - Wir-
ing Diagrams.
WARNING: ON VEHICLES EQUIPPED WITH AIR-
BAGS,
REFER
TO
GROUP
8M
-
PASSIVE
RESTRAINT
SYSTEMS
BEFORE
ATTEMPTING
STEERING
WHEEL,
STEERING
COLUMN,
OR
INSTRUMENT PANEL COMPONENT DIAGNOSIS OR
SERVICE. FAILURE TO TAKE THE PROPER PRE-
CAUTIONS COULD RESULT IN ACCIDENTAL AIR-
BAG DEPLOYMENT AND POSSIBLE PERSONAL
INJURY.
(1) Disconnect and isolate the battery negative
cable. Remove the right instrument panel switch pod
and unplug the switch pod 10-way wiring connector.
Fig. 1 Rear Glass Heating Grid Test
ZJ
ELECTRICALLY HEATED SYSTEMS
8N - 3
DIAGNOSIS AND TESTING (Continued)