Jeep 1996 Grand Cherokee Service Manual - page 304
bulb test. After the bulb test, the lamp is controlled
by the instrument cluster circuitry based upon a
message received from the Body Control Module
(BCM) on the Chrysler Collision Detection (CCD)
data bus.
The BCM uses an input from the parking brake
switch and/or the brake warning switch to decide
whether the brake warning lamp should be on or off.
The BCM then sends the proper message to the
instrument cluster on the CCD data bus to turn the
lamp on or off.
The brake warning switch closes to ground when it
senses unequal hydraulic pressures in the two halves
of the split brake hydraulic system, possibly due to
low brake fluid level or brake fluid leakage. The
parking brake switch closes to ground when the
parking brake is applied. Refer to Group 5 - Brakes
for more information.
CHECK GAUGES LAMP
The check gauges lamp gives an indication when
certain gauges reflect a condition requiring immedi-
ate attention. The lamp is turned on by the instru-
ment cluster circuitry for about three seconds after
the ignition switch is turned to the On position as a
bulb test. After the bulb test, the lamp is controlled
by the instrument cluster circuitry upon a message
received from either the Body Control Module (BCM)
or the Powertrain Control Module (PCM) on the
Chrysler Collision Detection (CCD) data bus.
The BCM and PCM use several inputs to decide
whether a condition exists requiring the check
gauges lamp to be turned on. The responsible module
then sends the proper message to the instrument
cluster on the CCD data bus to turn the lamp on or
off. When the instrument cluster circuitry receives a
check gauges lamp-on message, it sends a message to
the BCM on the CCD data bus for a single chime
tone to sound.
The conditions monitored and the responsible mod-
ules are:
• Engine coolant temperature is high or critical
(BCM)
• Engine oil pressure is low (BCM)
• Charging system failure (PCM)
• System voltage is high (PCM)
• Battery temperature sensor failure (PCM).
CLUSTER ILLUMINATION LAMPS
When the park or head lamps are on, the cluster
illumination lamps light. Illumination brightness is
adjusted by sliding the panel dimmer switch knob
(downwards to dim, upwards to brighten). Each of
the instrument cluster illumination lamps receives
pulse-width modulated battery feed from the Body
Control Module (BCM) on the hard-wired panel
lamps driver circuit. The BCM monitors the panel
dimmer resistor switch to determine the desired dim-
ming level, then adjusts the pulse-width signal
accordingly.
The BCM also sends the proper panel lamps dim-
ming level message on the Chrysler Collision Detec-
tion (CCD) data bus. Other modules on the CCD data
bus (radio, mini trip computer, vehicle information
center) receive this message and adjust their dim-
ming levels to match that of the incandescent cluster
illumination bulbs driven directly by the BCM.
Vehicles equipped with the automatic headlamps
option have an automatic funeral mode. In this
mode, the BCM uses an input from the automatic
headlamps photocell sensor to determine the ambient
light levels. If the BCM decides that the exterior
lighting is turned on in the daylight, it overrides the
selected panel dimmer switch signal by sending a
message on the CCD bus to illuminate all vacuum
fluorescent displays at full brightness for easier visi-
bility in daytime light levels. The automatic funeral
mode has no effect on the incandescent bulb dimming
levels.
Refer to Group 8L - Lamps for more information.
CRUISE-ON INDICATOR LAMP
The cruise-on indicator lamp gives an indication
when the vehicle speed control system is turned on,
even when the system is not currently engaged. The
lamp is turned on by the instrument cluster circuitry
for about four seconds when the ignition switch is
turned to the On position as a bulb test. After the
bulb test, the lamp is controlled by the instrument
cluster circuitry based upon a message received from
the Powertrain Control Module on the Chrysler Col-
lision Detection (CCD) data bus.
The PCM uses an input from the analog resistor-
multiplexed vehicle speed control switches in the
steering wheel to decide whether to turn the lamp on
or off. The PCM then sends the proper message to
the instrument cluster on the CCD data bus. Refer to
Group 8H - Vehicle Speed Control System for more
information.
HEADLAMP HIGH BEAM INDICATOR LAMP
The headlamp high beam indicator lamp gives an
indication when the headlamp high beams are on.
The lamp is turned on by the instrument cluster cir-
cuitry for about four seconds when the ignition
switch is turned to the On position as a bulb test.
After the bulb test, the lamp is controlled by the
instrument cluster circuitry based upon a message
received from the Body Control Module (BCM) on the
Chrysler Collision Detection (CCD) data bus.
The BCM uses an input from the headlamp dim-
mer (multi-function) switch to decide whether the
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INSTRUMENT PANEL SYSTEMS
8E - 7
DESCRIPTION AND OPERATION (Continued)