gauge is controlled by the instrument cluster circuit
board based upon the cluster programming and elec-
tronic messages received by the cluster from the
Powertrain Control Module (PCM) over the Program-
mable Communications Interface (PCI) data bus. The
fuel gauge is an air core magnetic unit that receives
battery current on the instrument cluster electronic
circuit board through the fused ignition switch out-
put (st-run) circuit whenever the ignition switch is in
the On or Start positions. The cluster is programmed
to move the gauge needle back to the low end of the
scale after the ignition switch is turned to the Off
position. The instrument cluster circuitry controls
the gauge needle position and provides the following
features:
• Less Than Empty Percent Tank Full Mes-
sage - Each time the cluster receives a message from
the PCM indicating the percent tank full is less than
empty, the gauge needle is moved to the far left (low)
end of the gauge scale. This message would indicate
that the fuel tank sender input to the PCM is a short
circuit.
• More Than Full Percent Tank Full Message
- Each time the cluster receives a message from the
PCM indicating the percent tank full is more than
full, the gauge needle is moved to the far left (low)
end of the gauge scale. This message would indicate
that the fuel tank sender input to the PCM is an
open circuit.
• Message Failure - If the cluster fails to receive
a percent tank full message, it will hold the gauge
needle at the last indication for about twelve seconds,
until a new message is received, or until the ignition
switch is turned to the Off position, whichever occurs
first.
• Actuator Test - Each time the cluster is put
through the actuator test, the gauge needle will be
swept across the entire gauge scale and back in order
to confirm the functionality of the gauge and the
cluster circuitry.
The PCM continually monitors the fuel tank send-
ing unit, then sends the proper messages to the
instrument cluster. The PCM uses an algorithm to
control the negative effect that fuel sloshing within
the fuel tank has on accurate inputs from the fuel
tank sending unit. For further diagnosis of the fuel
gauge or the instrument cluster circuitry that con-
trols the gauge, (Refer to 8 - ELECTRICAL/INSTRU-
MENT CLUSTER - DIAGNOSIS AND TESTING).
For proper diagnosis of the fuel tank sending unit,
the PCM, the PCI data bus, or the message inputs to
the instrument cluster that control the fuel gauge, a
DRBIII
t scan tool and the appropriate diagnostic
information are required.
HIGH BEAM INDICATOR
DESCRIPTION
A high beam indicator lamp is standard equipment
on all instrument clusters. The high beam indicator
lamp is located in the upper edge of the instrument
cluster overlay, between the tachometer and the
speedometer. The high beam indicator lamp consists
of an International Control and Display Symbol icon
for “High Beam” imprinted on a blue lens. The lens
is located behind a cutout in the opaque layer of the
instrument cluster overlay. The dark outer layer of
the overlay prevents the indicator from being clearly
visible when the lamp is not illuminated. The icon
appears silhouetted against a blue field through the
translucent outer layer of the overlay when the indi-
cator is illuminated from behind by a replaceable
incandescent bulb and bulb holder unit located on
the
instrument
cluster
electronic
circuit
board.
Because this indicator may remain illuminated for
extended periods, it is designed to automatically illu-
minate at a slightly lower intensity whenever the
exterior lighting is turned On. The high beam indica-
tor lens is serviced as a unit with the instrument
cluster lens, hood and mask unit.
OPERATION
The high beam indicator lamp gives an indication
to the vehicle operator when the headlamp high
beams are illuminated. This lamp is controlled by a
transistor on the instrument cluster circuit board
based upon the cluster programming and electronic
messages received by the cluster from the Body Con-
trol Module (BCM) over the Programmable Commu-
nications Interface (PCI) data bus. The high beam
indicator lamp bulb receives ground on the instru-
ment cluster electronic circuit board through the
fused ignition switch output (st-run) circuit whenever
the ignition switch is in the On or Start positions;
therefore, the lamp remains operational regardless of
the ignition switch position. The lamp bulb only illu-
minates when it is provided with battery current by
the instrument cluster transistor. The instrument
cluster will turn on the high beam indicator lamp for
the following reasons:
• High Beam Headlamps-On Message - Each
time the cluster receives a message from the BCM
indicating the headlamp high beams are turned On,
the lamp will be illuminated. The lamp remains illu-
minated until the cluster receives a message from
the BCM indicating that the headlamp high beams
are turned Off, or until the ignition switch is turned
to the Off position, whichever occurs first.
• Actuator Test - Each time the cluster is put
through the actuator test, the lamp will be turned on
WG
INSTRUMENT CLUSTER
8Js - 15
FUEL GAUGE (Continued)
2001 JEEP GRAND CHEROKEE