Jeep 1996 Grand Cherokee Service Manual - page 302
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
System for more information.
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 inside the fuel tank. The send-
ing 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 the resis-
tance of the sending unit. Refer to Group 14 - Fuel
System for more information.
ODOMETER/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, or the trip odometer can
be reset to zero. The odometer and trip odometer dis-
play the distance values that are received from the
Body Control Module (BCM) on the Chrysler Colli-
sion 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 be blank for about four seconds. If there is
still no distance message on the CCD data bus after
four seconds, the instrument panel circuitry will
insert the last normally displayed mileage in the
odometer display.
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 odometer readings. Refer to
Group 14 - Fuel System for more information.
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
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. As the diaphragm moves, resis-
tance in the resistor coil increases or decreases.
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.
ZJ
INSTRUMENT PANEL SYSTEMS
8E - 5
DESCRIPTION AND OPERATION (Continued)