Dodge RAM TRUCK 1500 1995 Service Manual page 413

B R — INSTRUMENT PANEL AND GAUGES 8E - 3
TRANS
TEMP
J948E-37
Fig. 3 Message Center
One of the coils has a fixed current flowing through
it to maintain a constant magnetic field strength.
Current flow through the second coil changes, which
causes changes in its magnetic field strength. The
current flowing through the second coil can be
changed by:
• a variable resistor-type sending unit (fuel level,
coolant temperature, or oil pressure)
• changes in electrical system voltage (voltmeter)
• electronic control circuitry (speedometer/odometer,
tachometer).
The gauge needle moves as the movable permanent
magnet aligns itself to the changing magnetic fields
created around it by the electromagnets.
COOLANT TEMPERATURE GAUGE
The coolant temperature gauge gives an indication
of engine coolant temperature. The coolant tempera­
ture sending unit is a thermistor that changes elec­
trical resistance with changes in engine coolant
temperature. Some gauge needle cycling, depending
upon vehicle load, is normal on models equipped with
the diesel engine option. High sending unit resis­
tance causes low coolant temperature readings. Low
resistance causes high coolant temperature readings.
Sending unit resistance values are shown in a chart
in Specifications.
FUEL GAUGE
The fuel gauge gives an indication of the level of
fuel in the fuel tank. The fuel gauge sending unit has
a float attached to a swing-arm in the fuel tank. The
float moves up or down within the fuel tank as fuel
level changes. As the float moves, an electrical con­
tact on the swing-arm wipes across a resistor coil,
which changes sending unit resistance. High sending
unit resistance causes low fuel level readings. Low
resistance causes high fuel level readings. Sending
unit resistance values are shown in a chart in Spec­
ifications.
GEAR SELECTOR INDICATOR
The gear selector indicator is used on models with
an automatic transmission to show the position of
the transmission gear selector lever. The indicator is
mechanically actuated by a cable connected to the
PRNDL driver of the gear selector lever mechanism
on the steering column. Refer to Group 21 - Trans­
mission and Transfer Case for diagnosis, service, and
adjustment of this component.
OIL PRESSURE GAUGE
The oil pressure gauge gives an indication of en­
gine oil pressure. The combination oil pressure send­
ing unit contains a flexible diaphragm. The
diaphragm moves in response to changes in engine
oil pressure. As the diaphragm moves, resistance in­
creases or decreases. High resistance on the gauge
side of the sending unit causes low oil pressure read­
ings. Low resistance causes high oil pressure read­
ings. Sending unit resistance values are shown in a
chart in Specifications.
SPEEDOMETER/ODOMETER
The speedometer/odometer gives an indication of
vehicle speed and travel distance. The speedometer
receives a vehicle speed pulse signal from the Vehicle
Speed Sensor (VSS). An electronic integrated circuit
contained within the speedometer reads and analyzes
the pulse signal. It then adjusts the ground path re­
sistance of one electromagnet in the gauge to control
needle movement. It also sends signals to an electric
stepper motor to control movement of the odometer
number rolls. Frequency values for the pulse signal
are shown in a chart in Specifications.
The VSS is mounted to an adapter near the trans­
mission (two-wheel drive) or transfer case (four-wheel
drive) output shaft. The sensor is driven through the
adapter by a speedometer pinion gear. The adapter
and pinion vary with transmission, transfer case,
axle ratio and tire size. Refer to Group 21 - Trans­
mission and Transfer Case for more information.
TACHOMETER
The tachometer gives an indication of engine speed
in Revolutions-Per-Minute (RPM). With the engine