Operation; Tachometer Description - Jeep Wrangler 2001 Service Manual

TJ
SPEEDOMETER (Continued)
equivalent opposite units from the primary scale. A
label on the cluster overlay beneath the hub of the
speedometer pointer abbreviates the unit of measure
for the primary scale in all upper case letters (i.e.:
MPH or KM/H), followed by the unit of measure for
the secondary scale in all lower case letters (i.e.: mph
or km/h). The speedometer graphics are white (pri-
mary scale) and blue (secondary scale) against a
black field, making them clearly visible within the
instrument cluster in daylight. When illuminated
from behind by the panel lamps dimmer controlled
cluster illumination lighting with the exterior lamps
turned On, the white graphics appear white and the
blue graphics appear blue. The orange gauge needle
is internally illuminated. Gauge illumination is pro-
vided by replaceable incandescent bulb and bulb
holder units located on the instrument cluster elec-
tronic circuit board. The speedometer is serviced as a
unit with the instrument cluster.

OPERATION

The speedometer gives an indication to the vehicle
operator of the vehicle road speed. This gauge is con-
trolled by the instrument cluster electronic circuit
board based upon 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
speedometer is an air core magnetic unit that
receives battery current on the instrument cluster
electronic circuit board through the fused ignition
switch output (run-start) circuit whenever the igni-
tion switch is in the On or Start positions. The clus-
ter 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 pro-
vides the following features:
• Vehicle Speed Message - Each time the clus-
ter receives a vehicle speed message from the PCM it
will calculate the correct vehicle speed reading and
position the gauge needle at that speed position on
the gauge scale. The cluster will receive a new vehi-
cle speed message and reposition the gauge pointer
accordingly about every 86 milliseconds. The gauge
needle will continue to be positioned at the actual
vehicle speed position on the gauge scale until the
ignition switch is turned to the Off position.
• Message Failure - If the cluster fails to receive
a speedometer message, it will hold the gauge needle
at the last indication for about six seconds, or until
the ignition switch is turned to the Off position,
whichever occurs first. If a new speedometer message
is not received after about six seconds, the gauge
needle will return to the far left (low) end of the
scale.
2001 JEEP WRANGLER
• Actuator Test - Each time the cluster is put
through the actuator test, the gauge needle will be
swept to several calibration points on the gauge scale
in a prescribed sequence in order to confirm the func-
tionality of the gauge and the cluster control cir-
cuitry.
The PCM continually monitors the vehicle speed
sensor to determine the vehicle road speed, then
sends the proper vehicle speed messages to the
instrument cluster. For further diagnosis of the
speedometer or the instrument cluster circuitry that
controls the gauge, (Refer to 8 - ELECTRICAL/IN-
STRUMENT CLUSTER - DIAGNOSIS AND TEST-
ING). For proper diagnosis of the vehicle speed
sensor, the PCM, the PCI data bus, or the message
inputs to the instrument cluster that control the
speedometer, a DRBIII scan tool is required. Refer
to the appropriate diagnostic information.
TACHOMETER
DESCRIPTION
A tachometer is standard equipment on all instru-
ment clusters. The tachometer is located next to the
speedometer, just to the left of center in the instru-
ment cluster. The tachometer consists of a movable
gauge needle or pointer controlled by the instrument
cluster electronic circuitry and a fixed 210 degree
scale on the cluster overlay that reads left-to-right
from 0 to 6. The text "X 1000 RPM" imprinted on the
cluster overlay directly below the hub of the tachom-
eter needle identifies that each number on the
tachometer scale is to be multiplied by 1000 rpm. A
red line on the high end of the gauge scale desig-
nates the engine overspeed area of the gauge. The
tachometer graphics are white against a black field,
except for the single red line, making them clearly
visible within the instrument cluster in daylight.
When illuminated from behind by the panel lamps
dimmer controlled cluster illumination lighting with
the exterior lamps turned On, the white graphics
appear white and the red graphics appear red. The
orange gauge needle is internally illuminated. Gauge
illumination is provided by replaceable incandescent
bulb and bulb holder units located on the instrument
cluster electronic circuit board. The tachometer is
serviced as a unit with the instrument cluster.
OPERATION
The tachometer gives an indication to the vehicle
operator of the engine speed. This gauge is controlled
by the instrument cluster electronic circuit board
based upon cluster programming and electronic mes-
sages received by the cluster from the Powertrain
Control Module (PCM) over the Programmable Com-
INSTRUMENT CLUSTER 8Js - 33