Description And Operation - Jeep 1997 Grand Cherokee Service Manual

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DESCRIPTION AND OPERATION

COOLANT TEMPERATURE GAUGE
The coolant temperature gauge gives an indication
of the engine coolant temperature. The instrument
cluster circuitry controls the gauge pointer position.
The instrument cluster circuitry calculates the
proper gauge pointer position based upon an engine
coolant temperature message received from the Pow-
ertrain Control Module (PCM) on the Chrysler Colli-
sion Detection (CCD) data bus.
The PCM uses an input from the engine coolant
temperature sensor and internal programming to
decide what engine coolant temperature message is
required. The PCM then sends the proper message to
the instrument cluster and the Body Control Module
(BCM) on the CCD data bus.
The BCM monitors the PCM coolant temperature
messages. If the PCM message indicates that coolant
temperature is high or critical, the BCM sends a
message to the instrument cluster to turn on the
Check Gauges lamp and to drive the coolant temper-
ature gauge needle to the corresponding high or crit-
ical position of the gauge scale.
The engine coolant temperature sensor is installed
in a threaded hole that penetrates a coolant passage
of the engine. It is a thermistor-type sensor that
changes its internal resistance with changes in
engine coolant temperature. Refer to Group 14 - Fuel
Systems for more information on the PCM and the
coolant temperature sensor.
FUEL GAUGE
The fuel gauge gives an indication of the level of
fuel in the fuel tank. The instrument cluster circuitry
controls the gauge pointer position. The instrument
cluster circuitry calculates the proper gauge pointer
position based upon a fuel level message received
from the Powertrain Control Module (PCM) on the
Chrysler Collision Detection (CCD) data bus.
The PCM uses an input from the fuel gauge send-
ing unit and internal programming to decide what
fuel level message is required. The PCM then sends
the proper message to the instrument cluster on the
CCD data bus. If the PCM detects a short or open in
the fuel level sending unit circuit, it sends a message
on the CCD data bus that will cause the instrument
cluster circuitry to position the fuel gauge needle at
the Empty stop.
The fuel gauge sending unit is mounted to the elec-
tric fuel pump module located inside the fuel tank.
The sending unit has a float attached to the end of a
swing-arm. The float moves up or down within the
fuel tank as the fuel level changes. As the float
moves, an electrical contact on the pivot end of the
swing-arm wipes across a resistor coil, which changes
INSTRUMENT PANEL SYSTEMS
the internal electrical resistance of the sending unit.
Refer to Group 14 - Fuel Systems for more informa-
tion on the PCM and for the fuel gauge sending unit
service procedures.
ODOMETER AND TRIP ODOMETER
The odometer and the trip odometer share the
same vacuum fluorescent digital display tube in the
instrument cluster circuit board. Each gives an indi-
cation of the distance the vehicle has travelled. How-
ever; by depressing the reset knob on the face of the
instrument cluster, the display can be switched from
odometer to trip odometer. Depressing the reset knob
for longer than two seconds while in the trip odome-
ter mode will reset the trip odometer to zero. The
odometer and trip odometer display the distance val-
ues that are received from the Body Control Module
(BCM) on the Chrysler Collision Detection (CCD)
data bus.
The BCM uses an input from the Powertrain Con-
trol Module (PCM) and internal programming to cal-
culate the distance value. The PCM uses an input
from the Vehicle Speed Sensor (VSS) to send a dis-
tance pulse signal to the BCM on the CCD data bus.
The BCM stores both the odometer and trip odometer
distance information and sends the proper value to
the instrument cluster based upon ignition key-on
and trip odometer reset knob messages received on
the CCD data bus.
If the instrument cluster is not receiving distance
information on the CCD data bus when the ignition
switch is turned to the On position, the odometer dis-
play will remain blank. If the instrument cluster
does not receive a distance message on the CCD data
bus after the ignition switch has been turned to the
On position, the instrument panel circuitry will
insert the last normally displayed distance in the
odometer display.
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 odometer 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.
OIL PRESSURE GAUGE
The oil pressure gauge gives an indication of the
engine oil pressure. The instrument cluster circuitry
controls the gauge pointer position. The instrument
cluster circuitry calculates the proper gauge pointer
position based upon an engine oil pressure message
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