Operation; Speedometer Description - Dodge RAM 1500 1998 Owner's Manual

DR
SERVICE 4WD INDICATOR (Continued)
appears in the lower portion of the odometer/trip
odometer Vacuum Fluorescent Display (VFD) unit.
The VFD is soldered onto the cluster electronic cir-
cuit board and is visible through a window with a
smoked clear lens located on the lower edge of the
tachometer gauge dial face of the cluster overlay. The
dark lens over the VFD prevents the indicator from
being clearly visible when it is not illuminated. The
text "SERV 4WD" appears in an amber color and at
the same lighting level as the odometer/trip odometer
information when they are illuminated by the instru-
ment cluster electronic circuit board. The service
4WD indicator is serviced as a unit with the VFD in
the instrument cluster.

OPERATION

The service 4WD indicator gives an indication to
the vehicle operator when the Transfer Case Control
Module (TCCM) has recorded a Diagnostic Trouble
Code (DTC) for an electronic transfer case circuit or
component malfunction. This indicator is controlled
by a transistor on the instrument cluster circuit
board based upon cluster programming and elec-
tronic messages received by the cluster from the
TCCM over the Programmable Communications
Interface (PCI) data bus. The service 4WD indicator
is completely controlled by the instrument cluster
logic circuit, and that logic will only allow this indi-
cator to operate when the instrument cluster receives
a battery current input on the fused ignition switch
output (run-start) circuit. Therefore, the indicator
will always be off when the ignition switch is in any
position except On or Start. The indicator only illu-
minates when it is switched to ground by the instru-
ment cluster circuitry. The instrument cluster will
turn on the service 4WD indicator for the following
reasons:
• Service 4WD Lamp-On Message - Each time
the cluster receives a service 4WD lamp-on message
from the TCCM, the indicator will be illuminated.
The indicator remains illuminated until the cluster
receives a service 4WD lamp-off message from the
TCCM, or until the ignition switch is turned to the
Off position, whichever occurs first.
• Communication Error - If the cluster receives
no messages from the TCCM for five seconds, the
service 4WD indicator is illuminated by the instru-
ment cluster to indicate a loss of TCCM communica-
tion. The indicator
illuminated by the cluster until a valid message is
received from the TCCM.
• Actuator Test - Each time the cluster is put
through the actuator test, the service 4WD indicator
will be turned on, then off again during the VFD por-
tion of the test to confirm the functionality of the
VFD and the cluster control circuitry.
remains controlled
The TCCM continually monitors the electronic
transfer case switch and circuits to determine the
condition of the system. The TCCM then sends the
proper lamp-on or lamp-off messages to the instru-
ment cluster. For further diagnosis of the service
4WD indicator or the instrument cluster circuitry
that controls the VFD, (Refer to 8 - ELECTRICAL/
INSTRUMENT
TESTING). For proper diagnosis of the TCCM, the
PCI data bus, or the electronic message inputs to the
instrument cluster that control the service 4WD indi-
cator, a DRBIII scan tool is required. Refer to the
appropriate diagnostic information.
SPEEDOMETER
DESCRIPTION
Fig. 28 Speedometer Text
A speedometer is standard equipment on all instru-
ment clusters. The speedometer is located next to the
tachometer, just to the right of center in the instru-
ment cluster. The speedometer consists of a movable
gauge needle or pointer controlled by the instrument
cluster circuitry and a fixed 210 degree primary scale
on the gauge dial face that reads left-to-right either
from "0" to "120" mph, or from "0" to "200" km/h,
depending upon the market for which the vehicle is
manufactured. Each version also has a secondary
inner scale on the gauge dial face that provides the
equivalent opposite units from the primary scale.
Text appearing on the cluster overlay just below the
hub of the speedometer needle abbreviates the unit
of measure for the primary scale (i.e.: MPH or km/h),
followed by the unit of measure for the secondary
scale (Fig. 28). The speedometer graphics are black
(primary scale) and blue (secondary scale) against a
white 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 black graphics appear dark blue and
the blue graphics appear light blue. The orange
gauge needle is internally illuminated. Gauge illumi-
and
nation is provided by replaceable incandescent bulb
and bulb holder units located on the instrument clus-
ter electronic circuit board. The speedometer is ser-
viced 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-
INSTRUMENT CLUSTER
CLUSTER - DIAGNOSIS
8J - 37
AND