CHAPTER
12
Instrumentation
Instructor Name: (Your Name)
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Learning Objectives
• List the various types of electromechanical
gauges and explain the operation of each
type
• Describe the four types of magnetic gauges
• Explain the purpose of a bucking coil in a
three coil gauge
• Modify the vehicle speed pulses-per-mile
setting
• Troubleshoot a problem with conventional
instrumentation
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Learning Objectives (continued)
• Explain the concept of multiplexed
instrumentation using J1587/J1708 and
J1939
• Describe the operation of a stepper motor
used in an instrument panel cluster
• Diagnose multiplexed instrumentation
using OEM information
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Conventional Instrumentation
• IPC – Instrument panel cluster
• Conventional Instrumentation – Nonmultiplexed IPC hardwired to input device
• Mechanical Gauges – Gauges that are
physically influenced by the input device
• Electromechanical Gauges – Gauges that
use a sensor to convert physical quantity
into a variable resistance
• Bimetallic Gauges – Uses a heater coil and
bimetal strip to move the gauge needle
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Bimetallic Strip Made of
Brass and Steel
Figure 12-2 Bimetallic strip made of brass and steel.
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Bimetallic Fuel Gauge
Figure 12-3 Bimetallic gauge.
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Fuel Level Sensor Circuit
Figure 12-4 Fuel level sensor circuit.
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Fuel Level Sensor or Sending Unit
Figure 12-5 Fuel level sensor or sending unit.
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Magnetic Gauges
• Coils directly connected to sensors or
voltage being measured
• Modern magnetic gauges may be
controlled by the IPC
• There are four main styles of magnetic
gauges in use; D’Arsonval, Three Coil,
Two Coil, and Air Coil.
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D’Arsonval Gauge Movement
Figure 12-7 D’Arsonval gauge movement.
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Three Coil Gauge
Figure 12-9 Three-coil gauge.
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Three Coil Gauge with Low
Resistance Indicating Low Fuel
Figure 12-10 Sending unit resistance is low, indicating an empty fuel tank.
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Three Coil Gauge with High
Resistance Indicating Full Tank
Figure 12-11 Sending unit resistance is high, indicating a full fuel tank.
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Two Coil Temperature Gauge
Figure 12-12 Two-coil temperature gauge.
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Two Coil Fuel Level Gauge
Figure 12-14 Two-coil fuel level gauge.
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Air Core Fuel Gauge
Figure 12-15 Air core fuel gauge.
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Electronically Controlled Gauges
• Usually controlled by an electronic module
or electronics in the gauge assembly
• Sensor or sending unit acts as input to the
electronic module, not the gauge
• The electronic module supplies voltage to
the gauge’s magnetic coil
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Tachometer Sensor and Ring Gear
Figure 12-16 Tachometer sensor and ring gear installation.
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Electronically Controlled Magnetic Gauge
Figure 12-18 Electronically controlled magnetic gauge.
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DIP Switches Used to Program
Speedometers and Tachometers
Figure 12-21 DIP
switches used to
program speedometer
and tachometer on
older trucks.
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DIP Switch Setting Example for
Tachometer
Figure 12-22 DIP switch settings example for tachometer.
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Light Emitting Diodes (LEDs)
• Light emitting diodes (LEDs)
• Numbers 0 to 9 can be displayed through
a seven line segment
• Electronic modules cause certain
segments to illuminate and display digits
• LEDs can be shaped like bars in a circular
arc to mimic a mechanical gauge
• Dot matrix LED displays light specific
LEDs in the matrix to display numbers
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Seven Segment LED
Figure 12-23 Seven-segment LED.
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Truck Instrument Panel Using LEDs
Figure 12-24 Truck instrument panel using LEDs to display information.
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Dot Matrix LED
Figure 12-25 Dot matrix LED.
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Oil Pressure Sending Unit with
Warning Light
Figure 12-26 Oil pressure sending unit used with a low oil
pressure warning lamp.
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Temperature Sending Unit with
Warning Light
Figure 12-27 Temperature-sending unit for a coolant temperature warning lamp.
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Brake Warning Systems
• Air brake systems are required to have low
air warning light
• Air brake low air visual indicator should come
on if system air pressure drops to 60 psi
• Hydraulic brakes often use a normally closed
power steering fluid flow switch to indicate
power steering pressure is lost
• Hydraulic brakes also use a pressure
differential switch to indicate a pressure
difference in the two hydraulic brake circuits
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Low Air Pressure Warning Lamp
and Alarm Circuit
Figure 12-28
Low air pressure
warning lamp
and alarm
circuit.
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Hydraulic Brake Differential
Pressure Switch
Figure 12-29 Hydraulic brake differential pressure switch.
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Tech Tip
The OEM’s troubleshooting information
should be consulted when troubleshooting
instrumentation. Indiscriminate shorts to
ground of instrument circuits or other
random test procedures can cause
damage to the gauges and electronics.
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J1587/J1708 Multiplexed
Instrumentation
• Conventional gauges require a connection
to each sensor or sender
• Modern diesel engines need all engine
data, temperature, oil pressure, engine
speed, etc.
• IPC electronic modules only require a
connection to the J1587/J1708 data link
• IPC will talk to engine to obtain information
needed for dash gauges
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Instrument Panel Using Data Link
Figure 12-30
Instrument panel
cluster using data
link information
to drive enginerelated gauges
and warning
lamps.
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J1939 Multiplexed Information
• J1939 has increased bandwidth for faster
transmission
• Electronic modules throughout the truck
such as engine ECM, automatic
transmission ECU, ABS ECU, and body
control module broadcast on the data link
• Modern IPCs are connected to the J1939
data link
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Stepper Motor Driven Gauges
• Can be driven to precise position and held
there
• An electric motor with no brushes or
commutator
• All windings are typically located on the stator
• The rotor is typically a permanent magnet
type rotor
• The rotor may have six pie shaped magnets
alternating north and south polarity
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Stepped Motor With Coil 1 Energized
Figure 12-32 Stepper motor.
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Stepped Motor With Coil 2 Energized
Figure 12-33 Stepper motor rotating clockwise.
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Tech Tip
By knowing the source of information for
each multiplexed gauge and warning
lamp, you may be able to narrow the
instrumentation problem to a specific
module or a specific electrical connector
based on which gauges or warning lamps
are functional and which are not
functional.
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Sensor Breakout T Used to
Measure Sensor Voltage
Figure 12-34 Sensor breakout T used to measure sensor voltage with a DMM voltmeter.
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Breakout Box with DMM Voltmeter
Figure 12-35 Breakout box used with a DMM voltmeter.
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Summary
• Instrumentation refers to the gauges,
indicator lamps, and audible alarms used to
inform the truck operator of the status of the
truck’s various systems.
• A sensing device called a gauge sender or
sending unit is used to transform pressure,
temperature, level or other physical value into
a corresponding valve of resistance. The
varying resistance causes the current flow
through a coil or a heating element in a
conventional gauge to increase or decrease,
resulting in the gauge needle moving to the
corresponding location.
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Summary (continued)
• The two main categories of electromechanical
gauges are bimetallic and magnetic. Bimetallic
gauges operate on the principle that current
flow through a heating element causes a
bimetallic strip to deflect. Magnetic gauges
contain coils (inductors) and operate on the
principles of electromagnetism.
• An instrument voltage regulator is often used
with bimetallic gauges to maintain a constant
voltage supply for the gauge heating element.
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Summary (continued)
• The four main types of metallic gauges are
D’Arsonval, three coil, two coil, and air core.
• Electronically controlled gauges use an
electronic module to control current flow
through the magnetic type gauge coils. The
sending unit or sensor is an input to the
electronic control module. The electronic
module controls the current supplied to the
gauge accordingly.
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Summary (continued)
• Electronically controlled speedometers
and tachometers typically must be
programmed to correspond to the vehicle
if changes such as rear axle ratios are
made. This programming is performed on
some trucks through a series of DIP
switches in the IPC or through the
modification of a programmable parameter
in the engine ECM or other control
module.
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Summary (continued)
• Trucks may have a variety of warning
indicator lamps. Each lamp is controlled by a
switching device that typically provides a path
to ground to illuminate the indicator lamp.
• Multiplexed instrumentation describes
instrumentation that receives information
from a serial communication network,
commonly know as a data link. The various
electronic module on the vehicle, broadcast
information on the data link. The IPC “listens”
for this information on the data link and drives
the appropriate gauge or warning lamp.
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Summary (continued)
• The term stepper motor describes a type
of motor that can driven to a specific
position and held in that position. Many
modern trucks IPCs make use of stepper
driven motor gauges.
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