Airbag Indicator Lamp - Jeep 1997 Grand Cherokee Service Manual

8E - 6 INSTRUMENT PANEL SYSTEMS
DESCRIPTION AND OPERATION (Continued)
received from the Powertrain Control Module (PCM)
on the Chrysler Collision Detection (CCD) data bus.
The PCM uses an input from the engine oil pres-
sure sensor and internal programming to decide
what engine oil pressure message is required. The
PCM then sends the proper message to the instru-
ment cluster and the Body Control Module (BCM) on
the CCD data bus.
The BCM monitors the PCM engine oil pressure
messages. If the PCM message indicates that engine
oil pressure is too low, the BCM sends a message to
the instrument cluster to turn on the Check Gauges
lamp and to drive the oil pressure gauge needle to
the zero end of the gauge scale.
The engine oil pressure sensor is installed in a
threaded hole that penetrates an oil passage of the
engine. The engine oil pressure sensor contains a
flexible diaphragm and a variable resistor coil. The
diaphragm moves in response to changes in the
engine oil pressure, which changes the internal elec-
trical resistance of the sensor. Refer to Group 14 -
Fuel Systems for more information on the PCM and
the engine oil pressure sensor.
SPEEDOMETER
The speedometer gives an indication of the current
vehicle speed. The instrument cluster circuitry con-
trols the gauge pointer position. The instrument clus-
ter circuitry calculates the proper gauge pointer
position based upon a vehicle speed message received
from the Powertrain Control Module (PCM) on the
Chrysler Collision Detection (CCD) data bus.
The PCM uses an input from the Vehicle Speed
Sensor (VSS) and internal programming to decide
what vehicle speed message is required. The PCM
then sends the proper message to the instrument
cluster on the CCD data bus.
The VSS is a hall-effect sensor that is installed in
the transmission (two-wheel drive) or transfer case
(four-wheel drive), and is driven by the output shaft
through a speedometer pinion gear. Incorrect tire
size, incorrect axle ratio, a faulty or incorrect speed-
ometer pinion gear, or a faulty VSS can each result
in inaccurate speedometer readings. Refer to Group
14 - Fuel Systems for more information on the PCM
and the VSS. Refer to Group 21 - Transmission for
more information on the speedometer pinion gear.
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 decide what
engine speed message is required. The PCM then
sends the proper message to the instrument cluster
on the CCD data bus.
The crankshaft position sensor is installed near the
rear of the engine, where it is aimed at the trigger
wheel attached to the rear flange of the crankshaft.
For models with a gasoline engine, refer to Group 8D
- Ignition Systems for more information on the
crankshaft position sensor. For models with a diesel
engine, refer to Group 14 - Fuel Systems for more
information on the crankshaft position sensor.
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 14 - Fuel System for more informa-
tion on the PCM. Refer to Group 8C - Charging Sys-
tem for more information on charging system
components and diagnosis.

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.
ZG