Jeep Wrangler TJ. Manual - part 308
TACHOMETER
DESCRIPTION
A tachometer is standard equipment on all instru-
ment clusters. The tachometer is located to the left of
the speedometer, just to the left of center in the
instrument cluster. The tachometer consists of a
movable gauge needle or pointer controlled by the
instrument cluster circuitry and a fixed 210 degree
scale on the cluster overlay that reads left-to-right
from 0 to 6. The text “X1000 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
(Fig. 26). Red graduations at the right (high) end of
the gauge scale designate the engine overspeed area
of the gauge. The tachometer graphics are white
against a black field, except for the red graduations,
making them clearly visible within the instrument
cluster in daylight. When illuminated from behind by
the panel lamps dimmer controlled cluster illumina-
tion 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 circuit board based upon
cluster
programming
and
electronic
messages
received by the cluster from the Powertrain Control
Module (PCM) over the Programmable Communica-
tions Interface (PCI) data bus. The tachometer 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 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 electronic circuitry controls
the gauge needle position and provides the following
features:
• Engine Speed Message - Each time the cluster
receives an engine speed message from the PCM it
will calculate the correct engine speed reading and
position the gauge needle at that relative speed posi-
tion on the gauge scale. The cluster will receive a
new engine speed message and reposition the gauge
pointer accordingly about every 86 milliseconds. The
gauge needle will continually be repositioned on the
gauge scale until the engine stops running, or until
the ignition switch is turned to the Off position,
whichever occurs first.
• Communication Error - If the cluster fails to
receive an engine speed message, it will hold the
gauge needle at the last indication for about six sec-
onds, or until the ignition switch is turned to the Off
position, whichever occurs first. After six seconds, the
gauge needle will return to the left (low) end of the
gauge scale.
• 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 to confirm the functionality
of the gauge and the cluster control circuitry.
The PCM continually monitors the crankshaft posi-
tion sensor to determine the engine speed. The PCM
then sends the proper engine speed messages to the
instrument cluster. For further diagnosis of the
tachometer 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 crankshaft position
sensor, the PCM, the PCI data bus, or the electronic
message inputs to the instrument cluster that control
the tachometer, a DRBIII
t scan tool is required.
Refer to the appropriate diagnostic information.
TURN SIGNAL INDICATOR
DESCRIPTION
Two turn signal indicators, one right and one left,
are standard equipment on all instrument clusters
(Fig. 27). The turn signal indicators are located near
the upper edge of the instrument cluster, between
the speedometer and the tachometer. Each turn sig-
nal indicator consists of a stencil-like cutout of the
International Control and Display Symbol icon for
“Turn Warning” in the opaque layer of the instru-
ment cluster overlay. The dark outer layer of the
overlay prevents these icons from being clearly visi-
ble when they are not illuminated. A green Light
Emitting Diode (LED) behind each turn signal indi-
cator cutout in the opaque layer of the cluster over-
Fig. 26 Tachometer Text
Fig. 27 Turn Signal Indicators
TJ
INSTRUMENT CLUSTER
8J - 41