Airbag Indicator Lamp; Anti-Lock Brake System Lamp; Brake Warning Lamp - Jeep 1996 Grand Cherokee Service Manual

8E - 6 INSTRUMENT PANEL SYSTEMS
DESCRIPTION AND OPERATION (Continued)
The PCM uses an input from the Vehicle Speed
Sensor (VSS) and internal programming to calculate
what vehicle speed message is required. The PCM
then sends the proper message to the instrument
cluster on the CCD bus.
The VSS is a hall-effect sensor that is installed in
the transmission or transfer case, and is driven by
the output shaft through a speedometer gear. Incor-
rect tire size, incorrect axle ratio, a faulty or incor-
rect speedometer gear, or a faulty VSS can each
result in inaccurate speedometer readings. Refer to
Group 14 - Fuel System for more information.
TACHOMETER
The tachometer gives an indication of the engine
speed in revolutions-per-minute (rpm). The instru-
ment cluster circuitry controls the gauge pointer
position. The instrument cluster circuitry calculates
the proper gauge pointer position based upon an
engine speed message received from the Powertrain
Control Module (PCM) on the Chrysler Collision
Detection (CCD) data bus.
The PCM uses an input from the crankshaft posi-
tion sensor and internal programming to calculate
what engine speed message is required. The PCM
then sends the proper message to the instrument
cluster on the CCD data bus.
The is installed near the rear of the engine, where
it is aimed at the trigger wheel attached to the rear
flange of the crankshaft. Refer to Group 8D - Ignition
Systems for more information.
VOLTMETER
The voltmeter gives an indication of the electrical
system voltage. The instrument cluster circuitry con-
trols the gauge pointer position. The instrument clus-
ter circuitry calculates the proper gauge pointer
position based upon a system voltage message
received from the Powertrain Control Module (PCM)
on the Chrysler Collision Detection (CCD) data bus.
The PCM uses an input from the electrical system
and internal programming to decide what system
voltage message is required. The PCM then sends
the proper message to the instrument cluster on the
CCD data bus. Refer to Group 8C - Charging System
for more information.

AIRBAG INDICATOR LAMP

The airbag indicator lamp gives an indication when
the airbag system is faulty or inoperative. The lamp
is turned on by the instrument cluster circuitry for
about seven 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 Airbag Control Module (ACM) on the Chrysler
Collision Detection (CCD) data bus.
The ACM continually monitors the airbag system
circuits and sensors to decide whether the system is
in good operating condition. The ACM then sends the
proper message to the instrument cluster on the
CCD data bus to turn the lamp on or off. If the ACM
turns the lamp on after the bulb test, it indicates the
ACM has detected a system malfunction and/or that
the airbag system has become inoperative. Each time
the instrument cluster circuitry receives a lamp-on
message from the ACM, it will light the lamp for
twelve seconds or the duration of the airbag system
malfunction, whichever is longer.
The airbag indicator lamp also has a lamp backup
feature. Following the seat belt reminder lamp dis-
play function, if the instrument cluster circuitry has
detected an inoperative airbag warning lamp circuit
it will flash the seat belt reminder lamp on and off
for twenty seconds. Once the instrument cluster cir-
cuitry has detected an inoperative airbag warning
lamp circuit, if it receives a lamp-on message from
the ACM, it will flash the seatbelt reminder lamp for
twelve seconds or the duration of the airbag system
malfunction, whichever is longer.
Refer to Group 8M - Passive Restraint Systems for
more information.

ANTI-LOCK BRAKE SYSTEM LAMP

The Anti-Lock Brake System (ABS) lamp gives an
indication when the ABS system is faulty or inoper-
ative. The lamp is hard-wired in the instrument clus-
ter, and is completely controlled by the Controller
Anti-lock Brake (CAB). It receives battery voltage
through the instrument cluster fused ignition switch
output feed circuit, and is grounded by the CAB. The
lamp is turned on by the CAB for about two seconds
when the ignition switch is turned to the On position
as a bulb test. After the bulb test, the CAB turns the
lamp on or off based upon the results of the ABS sys-
tem self-tests.
The CAB continually monitors the ABS circuits
and sensors to decide whether the system is in good
operating condition. If the CAB turns the lamp on
after the bulb test, it indicates that the CAB has
detected a system malfunction and/or that the ABS
system has become inoperative. Refer to Group 5 -
Brakes for more information.

BRAKE WARNING LAMP

The brake warning lamp gives an indication when
the parking brake is applied, or when the pressures
in the two halves of the split brake hydraulic system
are unequal. The lamp is turned on by the instru-
ment cluster circuitry for about four seconds when
the ignition switch is turned to the On position as a
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