copyright © 2014 delmar, cengage learning electrical accessories instructor name: (your name) 10...

Copyright © 2014 Delmar, Cengage Learning

Electrical Accessories

Instructor Name: (Your Name)

10CHAPTER


Learning Objectives

• Troubleshoot an electric horn circuit• Explain how a windshield wiper motor

system operates, including the wiper park feature

• Diagnose the cause of a blower motor not operating

• Troubleshoot the electrical portion of an air conditioning system

• Explain the concept of a latched relay


Learning Objectives (continued)

• Describe the difference between a normally open and normally closed air solenoid

• Troubleshoot an on-off type fan drive system

• Diagnose an inoperative power window• Explain how a compression and exhaust

brake provide braking power• Troubleshoot a Hydro-Max™ booster pump

system


Horn Switches

• Steering wheel mounted switches are a normally open switch

• Slip rings are used to allow the steering wheel to turn and maintain electrical contact

• Trucks with multiple controls on steering wheel have multiple slip rings

• Clocksprings are used to provide continuous electrical contact to air bags


Figure 10-1 Electric horn components.

Figure 10-2 Armature movement due to magnetic field has physically opened the contacts to interrupt the flow of current through the field coil.

Horn Components and Function


Steering Wheel Electrical Components

Figure 10-3 Slip ring provides electrical connection throughout steering-wheel rotation.


CAUTION

It is important to lock the wheels on trucks equipped with clocksprings when the steering linkage is disconnected to prevent clockspring damage. This is especially true on trucks equipped with air bags. Breakage of the clockspring may result in air bag deployment.


Horn Circuit

Figure 10-5 Horn circuit. Capacitors shown in parallel with horns are used for suppression.


Windshield Wiper Motors

• Air motors were used in the past• Currently permanent magnet DC motors

are most commonly used• Permanent magnet motors can be made

to run two-speed with the addition of a high speed motor brush

• Many wiper motors have a built in auto resetting circuit breaker or overload device


Windshield Wiper Motor Components

Figure 10-6 Windshield-wiper motor internal components.


Wiper Motor and Linkage

Figure 10-7 Windshield-wiper linkage and components.


Figure 10-9 Current flow through wiper motor high-speed circuit.

Figure 10-10 Current flow through wiper motor low-speed circuit.

Windshield Wiper – High and Low Speed Operation


Typical Truck HVAC System

Figure 10-13Typical HVAC System.


Blower Motor

• Multi-speed motors are used to circulate air in cab and sleeper

• Utilize squirrel cage type fan• Trucks with sleepers have a second blower

motor in sleeper compartment• Motor speed is typically controlled by stepped

resistor network referred to as the resistor block• Some trucks use a thermal limiter to protect the

blower motor circuit


Instrument Panel Mounted HVAC System

Figure 10-15 Instrument panel mount HVAC assembly.


Typical Resistor Block

Figure 10-16 Resistor block used to control current through blower motor to reduce speed; many trucks use an electronic module to provide this function.


A/C Compressor Clutch• A/C compressor clutch is an electromagnetic

clutch• A large electromagnet magnetically connects

the clutch hub to the belt driven pulley• Typically two pressure switches are wired in

series with the compressor clutch for system protection

• The low pressure switch will open at approximately 20 psi

• The high pressure switch will open at approximately 375 psi


Compressor Clutch Assembly

Figure 10-19 Compressor clutch assembly. Clutch field coil is one of the largest electromagnets found in a truck electrical system.


Typical HVAC Circuit

Figure 10-17 Typical HVAC electrical circuit. Note the switch ganged with the blower control switch, which powers the mode control switch.


Engine Fan Drive• Most truck engine fan drive clutches are

operated by compressed air • They are also know as on-off fan clutches• Fan drive couples the engine fan to the fan

pulley, which is powered by the accessory drive belt

• There are two main types of air powered fan drives: spring engaged and air engaged

• Air solenoid valves are commonly used to control air actuated fan drives


Fan Control Using Pressure, Temperature, and Manual Control

Figure 10-22 Direct control of the fan drive solenoid valve using pressure, temperature, and manual switches.


Power Windows

• Similar to those used on passenger cars• Brushed DC motors• Regulators convert rotary motion of motor

to vertical movement of window• Directional change is accomplished by

reversing polarity of voltage to motor


Current Flow to the Right Rear Window Switch in Down Position

Figure 10-25 Current flow with the right rear window switch in the down position.


Motorized Mirrors• Motorized mirrors with dual axis control

use two motors; one for vertical and one for horizontal control

• Polarity to mirrors is reversed to change direction of motor

• A function table is normally provided with circuit diagrams

• Some truck motorized mirrors only provide horizontal control


Engine Brake Systems

• Compression Brakes – Commonly known as Jake Brakes™, open the exhaust valve as piston approaches TDC of compression stroke

• Bleeder Brakes – Holds exhaust valve slightly open while restricting exhaust flow

• Exhaust Brake – Throttle valve to restrict exhaust flow near exhaust manifold


Compression Brake

Figure 10-29 Compression brake.


Exhaust Brake

Figure 10-30 Exhaust brake.


Exhaust Brake Control Circuit with Electronically Controlled Engine

Figure 10-31 Exhaust brake control circuit with electronically controlled engine.


Hydraulic Brake Booster System

• Medium-duty trucks generally use the power steering system to provide hydraulic assist

• An electric motor and pump act as back up in the event of a stalled engine

• The back up electric motor also operates when brakes are depressed with the key off

• The standard truck hydraulic brake booster system used on modern trucks is the Bosch Hydro-Max™


Hydro-Max™ Brake Boost Pump and Motor

Figure 10-32 Hydro-Max ™ hydraulic brake booster pump and motor.


Relay Used as a Latch

Figure 10-34 Relay used as a latch.


Relay Remains Latched Until ON-Off Switch is Cycled

Figure 10-35 Relay is latched on until the on-off switch is cycled to break the latch.


Summary

• Electrical accessories refers to items that improve safety and diver comfort.

• Electric horns are electromagnetic devices. Current flow through the windings is interrupted repeatedly to cause a diaphragm to move up and down to emit a tone.

• Windshield wiper systems typically use a two speed permanent magnet brushed DC motor. The two speeds are obtained by positioning one of the brushes to change the magnetic field strength to the armature windings.


Summary (continued)

• The wiper motor mechanical linkage system causes the wipers to travel back and forth across the windshield.

• Intermittent wiper systems often use the wiper park system to provide one sweep of the wipers. A brief pulse is supplied to the park motor, causing the wipers to move off the park position and return to the park position.


Summary (continued)

• The electrical portion of the HVAC system on a truck typically includes the blower motor and the A/C clutch. Blower motor speed is often controlled by switching in a series of resistors known as a resistor block.

• The engine fan drive on many trucks is an electrically controlled pneumatic system. The engine fan can be engaged when needed for the A/C system or when the engine coolant temperature increases.


Summary (continued)• Power window motors typically use

permanent magnet DC motors. Reversing the current flow through the motors causes the windows to be raised or lowered. The switches on the power window system control the direction of current flow through the power window motors.

• The brake booster system on a truck with hydraulic brakes may use an electric motor to drive a pump, should the engine stall.

copyright © 2014 delmar, cengage learning electrical accessories instructor name: (your name) 10...

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