chapter 10 bennet
TRANSCRIPT
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Chapter 10
Chassis Electrical Circuits
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Objectives (1 of 2)
Describe how a light bulb functions.
Explain the operating principles of halogen andhigh-intensity discharge (HID) lamps.
Describe the function of the reflector and lens in aheadlamp assembly.
Aim truck headlights.
Troubleshoot lighting circuit malfunctions.
Describe the operation of typical truck auxiliaryequipment.
Explain how a trailer electrical plug and connectorare connected.
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Objectives (2 of 2)
Outline the operating principles of truck instrument clustercomponents.
Diagnose and repair some typical truck instrument clusterfailures.
Explain the function and operation of warning and shutdownsystems.
Identify the types of circuit protection used in truck electricalsystems including fuses and cycling and non-cycling circuitbreakers.
Describe the procedure and material required to solder a pair ofcopper wires.
Outline the procedure required to quickly check out a truckelectrical system.
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Lighting Systems (1 of 2)
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Lighting Systems (2 of 2)
Light bulb operation
Fluorescent lights
Halogen lamps
Halogen infrared (HIR) headlamps
Xenon headlamps
HID optics
Managed headlamp voltage
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Light-emitting Diodes (LEDS)
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Caution
When replacing
halogen bulbs, take
care not to contact the
bulb with fingers
because this can cause
a rapid failure. When
installing a bulb, handle
it by the base only.
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Shop Talk
Some manufacturers recommend coating the
prongs and base of the new sealed beam
with terminal grease for corrosion protection.
Use an electrical terminal protectiveapproved by the manufacturer.
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Night Vision Systems
Although a typical truck operates only 30 percent of itsoperational life at night, more than half of truck accidents occurwhen it is dark.
This has led to the emergence of infrared thermal imagingsystems such as the Bendix Xvision system.
XVision consists of a front-mounted infrared camera thatsenses heat from the environment and processes the signalelectronically to produce a virtual display as an output on thevehicle dash.
The driver display consists of a black-and-white image. Warmer
objects such as people, animals, and headlights appear inshades of white, while cooler objects such as guardrails, trees,and abutments appear in shades of gray and black.
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Shop Talk
Lights should be turned off when cranking the
engine to avoid transient voltage spikes.
As the engine is cranked, 650 to 1,200 amps
are drawn through the system.
At the precise moment when the starter is
disengaged, electricity may surge into any
closed electrical circuits. This random surge can shorten the life of the
lights, causing them to burn out prematurely.
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Headlight Dimmer Switch
The headlight dimmer switch, or courtesy
switch, can either be mounted on the floor or
it can be a part of the turn signal assembly.
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Stalk Switch
Trucks today may use either electromechanicalswitches or smart switches.
The procedures you use for diagnosing andrepairing switches differ vastly so make sure you
know what you are working on before attemptingany repairs.
The stalk switch or turn light switch may be eithersmart or electromechanical.
A smart stalk switch is designed to send nothingmore than signals to a module.
The module then converts the signals into outputs.
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Turn Signal Switch Diagnosis
Before replacing the turn signal switch, ensure thatthe trouble is in the switch and not elsewhere in thecircuit.
Check that the circuit breaker and fuse are
functional, and inspect the signal light bulbs forbroken filaments.
Also, check the flasher relay, and replace it ifnecessary.
If the turn signal switch must be replaced, makesure the key is off and the battery negative cable isdisconnected.
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Auxiliary Electrical Equipment
Trailer circuit connector
Instrument panels and gauges
Dash design
Dash components
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Thermistor Resistance Check
To check thermistor-type
temperature sensors, the
approximate temperature of
the component being
checked must be known todetermine the resistance
that should be read at the
cluster harness connector.
Consult a chart such as this:
See Table 10-3 on
page 263 of the
textbook.
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Caution
Static electricity can cause permanent
damage to the cluster.
Before working on the cluster, be sure to
remove all static electricity from your body bytouching grounded metal.
Do not touch pin connectors during removal
and installation of gauges.
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Fuel Gauge
The fuel level sensor used isa float/rheostat type.
When a sending unit sensoris shorted to ground, thegauge should read empty.
Locate the fuel sensorgauge input in the clusterharness.
If resistance for the fueltank level matches OEM
spec and the gaugereading does notcorrespond, replace thefuel gauge.
See Table 10-4 on
page 264 of the
textbook.
Sh T lk
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Shop Talk
Before checking the fuel gauge, be sure the
cab interior is warmed up (during cold
weather), and that the vehicle has been
sitting still long enough to allow the fuel tosettle.
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Circuit-protection Devices
Fuses
Circuit breakers
Cycling circuit breakers
Non-cycling circuit breakers
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Relays
Terminal Assignments
Terminal Designation Old Number New Number
Coil circuit
coil circuit (Control Pos) 86 1 coil circuit (Control Neg) 85 2
Switched circuit
common (supply current or B+ ) 30 3
normally closed (NC) 87a 4
normally open (NO) 87 5
Standard SAE
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Standard SAE
Relay Terminal Assignments
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R id Ch ki f
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Rapid Checking of
a Truck Electrical Circuit General voltmeter test
Battery voltage
Cranking voltage
Charging voltage
Voltage-drop test
These tests are used to determine the generalcondition of a vehicle electrical system.
The idea is to produce a report card on the battery,the cranking circuit, and the charging circuit.
In fact, the test is so fast and easy to perform, itshould become part of routine service procedure.
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Cranking Voltage
Ensure the engine is not capable of starting by no-fueling the engine.
Connect the voltmeter leads across the crankingmotor terminals.
Crank the engine for 15 continuous seconds. Thevoltmeter reading should read above 9.6V for thefull 15 seconds.
If the reading falls below 9.6V, a problem with oneof the following is indicated:
Defective or corroded battery cables or terminals Defective or discharged batteries
Defective cranking motor, solenoid, or relay
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Charging Voltage
Start the engine and run at 75 percent of
rated speed with no load.
Now turn on all the electrical accessories on
the vehicle. Use the voltmeter to test battery voltage. It
should read between 13.5V and 14.5V.
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Summary (2 of 6)
Truck chassis lighting systems use
incandescent, gaseous-discharge, or LED
operating principles.
Halogen headlamps use a tungsten filamentwithin a gas envelope.
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Summary (3 of 6)
Xenon or HID headlamps are becoming
popular due to longer service life and a
brighter, whiter light characteristic.
LED light units are increasingly being usedon truck chassis marker and taillight
assemblies because they last longer and
illuminate faster.
Faster illumination makes them ideal as brake
lights.
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Summary (4 of 6)
The wiring output junction block for the trailer
is usually located inside the tractor cab, often
directly behind the driver seat.
Most current trucks use computer-managedinstrument clusters that are networked with
the chassis data bus.
For this reason, the OEM service literatureshould be consulted before attempting
troubleshooting.
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Summary (5 of 6)
Relays are a means of using a low-currentcontrol circuit to switch a high-current circuit.Standard SAE relays use standard numericcoding to indicate the terminal assignments.
Fuses and cycling and non-cycling circuitbreakers are all circuit-protection devices.
Fuses fail when overloaded. Circuit breakers
trip when overloaded, opening the circuit; theyreset either automatically or when the circuit isswitched open.
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Summary (6 of 6)
SAE Type 1 circuit breakers are cycling andautomatically reset.
SAE Type 2 circuit breakers are non-cycling and
the circuit must be switched open to allow them to
cool after tripping.
Voltage-drop testing is a key to diagnosing truck
chassis electrical systems.
Note the procedures for performing a fast electricalsystem assessment on a truck in this chapter, and
study voltage-drop testing with a DMM in the next.