Online Diesel Mechanic Training Course | Penn Foster Career School

Diesel Mechanics/Heavy Truck Maintenance Training

Why Penn Foster Career School?

  • Regionally & Nationally Accredited
  • Self-Paced
  • Supportive
  • Low monthly payments

Program Overview

We support NATEFPenn Foster Career School’s Diesel Mechanics/Heavy Truck Maintenance Training Program will cover everything from basic diesel engine operations to computerized truck management systems. You can complete this program in as little as seven months, and with Penn Foster Career School you train online - at home or on the go - at a pace that’s right for you. Our program can help you take the first step toward your future career, includes student discounts from Snap-on and helps you prepare for the National Institute for Automotive Service Excellence (ASE) certification Medium/Heavy Truck diesel engine test.

You don’t need previous work experience for this program, and diesel mechanics can work for an established repair shop, repair and maintain their own rig, or even start their own business.

Curriculum Details

Program Goal and Outcomes

Program Goal

The Diesel Mechanics/Heavy Truck Maintenance Career Diploma program provides students with knowledge of skills ranging from basic diesel engine operations to computerized truck management systems. Students will learn how to maintain and do repairs on their own trucks, work for a truck fleet or dealership, or start a full- or part-time business.

Program Outcomes

Upon completion of the program, students will be able to:

  • Recognize the role of a diesel repair technician and the many exciting career opportunities available to skilled technicians
  • Know the common tools and shop safety procedures used in the repair of heavy-duty vehicles
  • Understand the basic operation of a diesel engine and its major components
  • Describe the operation of the cooling and lubrication systems of a typical diesel engine
  • Identify the various procedures used in the inspection and rebuilding of a typical diesel engine
  • Understand the operation of a mechanical fuel injection system, the fuel-delivery system, as well as the properties of the various types of diesel fuels
  • Know the basics of computers and the specifics of the electronic operation of vehicle and engine control systems
  • Be aware of the basic theory and repair of the various electrical systems, including the battery, charging system, starting system, and lighting systems
  • Know the basic operation and repair of heavy-duty truck steering and suspension systems, along with service and repair of vehicle wheels and tires
  • Understand the operation of hydraulic and pneumatic braking devices and control systems, the interpretation of fault-indication messages, and the troubleshooting of brake system problems
  • Describe the operation and repair procedures for common heavy-duty truck manual transmissions and clutch systems
  • Identify the principles of operation, troubleshooting steps, and repair procedures for the most common types of automatic transmissions used in today's heavy-duty trucks
  • Identify the function and operation of driveline components, troubleshooting procedures, and the tasks required for servicing driveline problems
  • Recognize the function and operation of heating, ventilation, and air conditioning systems used on heavy-duty trucks

Unit 1

Starting Your Program

Succeed by learning how to use your Penn Foster program.


  • Understand how to use your Student Portal, including your My Homepage and My Courses pages.
  • Access the Penn Foster Community and use it to find answers.
  • Connect with Penn Foster on various social media sites.
Introduction to Diesel Truck Mechanics

Learn all about the growing diesel-engine-powered vehicle repair field and the increasing need for professional technicians. You’ll discover the many exciting career opportunities that are available to skilled technicians. Finally, you’ll learn about the basic operation of engines and other vehicle systems.


  • Name and describe the eight basic operating systems in a vehicle.
  • Describe the basic operation of a four-cycle diesel internal combustion engine.
  • Define the terms horsepower, displacement, and compression ratio.
  • Discuss the employment possibilities available in the heavy-duty vehicle repair field.
  • List and discuss the steps needed to attain certification in the heavy-duty vehicle servicing field.

Unit 2

Diesel Engine Fundamentals, Part 1

This lesson is an introduction to diesel engines, introducing the fundamental concepts you’ll need to understand as you begin your studies of diesel mechanics. It discusses the modern trucking industry and the important role truck technicians play in it. This lesson also gives you a guide to a wide variety of tools that diesel technicians use. It also covers the important topic of safety—including personal safety equipment, workplace hazards, and oxyacetylene safety—which is particularly important to a diesel mechanic.


  • Discuss today’s over-the-road trucking industry and the contemporary truck technician’s qualifications and responsibilities.
  • Describe how to use common hand, precision measuring, and shop tools associated with diesel engine work.
  • Identify SAE fastener grades and torques.
  • Understand how to make conversions between English and metric units of measurement.
  • Discuss workplace hazards and personal and procedural safety precautions that minimize their danger.
  • Describe the steps of the diesel cycle, the two-stroke cycle, and the Otto cycle.
  • List the components that make up engine systems and circuits.
  • Discuss major innovations in the history of the diesel engine.
Diesel Engine Fundamentals, Part 2

This lesson focuses primarily on power produced by a vehicle’s diesel engine. You’ll learn about power formulas and equations, including brake power and indicated power calculations, and engine powertrain components. Finally, you’ll cover the engine feedback assembly.


  • Understand basic power terminology and how to make common power calculations.
  • Describe combustion chamber designs and piston assembly types.
  • Discuss piston ring types as well as their function, geometry, components, inspection, and installation.
  • Explain connecting-rod inspection practices and describe the loads to which the rods are subjected.
  • Describe crankshaft construction, failure modes, and inspection procedures.
  • Understand the role of rod and main bearings in the powertrain.
  • Explain the function of vibration dampers and flywheels.
  • Describe the construction, installation, inspection, and removal of timing gears and camshafts.
  • Understand the functions served by valve/injector trains, rocker assemblies, and cylinder-head valves.
  • Explain how to adjust valve lash.
Diesel Engine Fundamentals, Part 3

This lesson covers three critical aspects of the diesel engine: engine housing components, lubrication systems, and cooling systems. The first part of this lesson discusses the components that enclose the internal engine components. The most important housing component that you’ll learn about is the cylinder block, which serves as the engine’s central frame. In this section, you’ll also learn about cylinder heads and gain a better understanding of intake and exhaust manifolds. The second part of this lesson is an in-depth discussion of diesel engine lubrication systems. This system may seem complex. However, you’ll find that it’s easy to comprehend once you understand the system’s components and the basic principles associated with lubrication. The third part of this lesson focuses on diesel engine cooling systems. Very simply, these systems transfer heat away from critical engine components. You’ll specifically learn about engine coolant and the components for storing, pumping, conditioning, and managing coolant flow and temperature.


  • Describe the engine cylinder block and cylinder liners.
  • Explain cylinder head installation, disassembly, inspection, reconditioning, and torque-down procedures.
  • List engine lubricating oil types and understand terminology related to lubrication systems.
  • Describe the components of a lubrication system in a diesel engine.
  • Explain how to interpret oil analyses.
  • Describe engine coolant and cooling-system components in a diesel engine.
  • Discuss the management of diesel engine cooling systems.
Diesel Engine Fundamentals, Part 4

This lesson provides both theoretical and practical information about diesel engines. You’ll learn the theories about engine breathing and engine retarders. You’ll also gain a practical understanding of engine disassembly, cleaning and inspection, reassembly, and performance testing.


  • Describe intake system components.
  • Explain the function of roots blowers, turbochargers, the intake manifold, and charge-air heat exchangers as they relate to engine breathing.
  • Describe exhaust system components.
  • Understand engine-brake operation and control circuits.
  • Explain the general tasks required to remove and disassemble an engine.
  • Understand the guidelines for inspecting, cleaning, and reassembling an engine.
  • Explain run-in procedures.
  • Describe how chassis and engine-dynamometer tests are performed.
Additional Course Material

Textbook: Medium/Heavy Duty Truck Engines, Fuel & Computerized Management Systems

Unit 3

Hydromechanical Diesel Fuel Injection Systems, Part 1

This lesson covers a broad range of topics, from chemistry to diesel fuel to hydromechanical injection principles. First, you’ll be given an overview of basic chemistry. You’ll learn about matter, elements, mixtures, and compounds. Having an understanding of these principles will help you grasp the concept of combustion and cylinder head dynamics. Next, you’ll study diesel fuel. You’ll learn many terms that will help you understand chemistry and combustion of diesel fuel. You’ll learn about petroleum: how it’s formed, refined, and chemically modified. The characteristics of diesel fuel are discussed. Fuel storage, deterioration, and performance are also discussed. There’s a helpful section on alternatives to fuel. You’ll learn about the group of components responsible for fuel storage and transportation of fuel to the injection pumping system. Finally, this lesson introduces hydromechanical and electronic management. You’ll learn the principles of fuel injection. In addition, you’ll learn about units of pressure and linear measurement.


  • Describe atomic structure and states of matter.
  • Explain the properties of common elements, mixtures, and compounds.
  • Discuss combustion and cylinder gas dynamics.
  • Calculate air/fuel ratios.
  • Understand fuel terminology and fuel characteristics.
  • Discuss fuel storage, deterioration, performance, and conditioners.
  • Understand the function of fuel tanks, filters, water separators, heaters, and pumps.
  • Explain how to prime a fuel system.
  • Discuss the principles of fuel injection.
  • Understand units of pressure and linear measurement.
Hydromechanical Diesel Fuel Injection Systems, Part 2

This lesson focuses exclusively on different types of injection systems. First, you’ll study port-helix metering injection pumps. You’ll develop a clear understanding of the components of these pumps and their interactions. An important part of this section covers timing principles and procedures. Second, you’ll study Detroit Diesel mechanical unit injection systems. The focus will be on learning the system components and how to tune up these systems. Third, you’ll study Caterpillar’s mechanical unit injection system. You’ll learn how these injectors operate and how they’re tested, adjusted, and synchronized. Also, you’ll learn about governor types. Finally, you’ll study the Cummins pressure-time (PT) fuel system. This section presents a theoretical framework for the operation of this system. You’ll learn about the Cummins PT components and circuit schematics. You’ll also learn how to test and adjust PT pumps and injectors. This section provides step-by-step instructions for calibrating a PT pump.


  • Understand port-helix metering injector pump principles and components.
  • Describe charging pumps and governors.
  • Understand injection and fuel rates and timing principles of injection pumps.
  • Describe phasing, calibration, delivery, injection, and combustion of port helix metering injection pumps.
  • Explain Detroit Diesel mechanical unit injector components, operation, and tune-up.
  • Describe the Caterpillar mechanical unit injector components, operation, and testing.
  • Explain the theory and schematics of a Cummins PT.
  • Describe PT injector testing, installation, adjustment, and calibration.
  • Understand the PACE and PACER computer control of a Cummins PT.
Hydromechanical Diesel Fuel Injection Systems, Part 3

This lesson begins with a chapter on rotary distribution pumps, a simplified pumping design seen mainly on small-bore engines. The second part of this lesson covers an important engine component: the governor. You’ll learn about governor types and classifications. You’ll also learn governor terminology, which will help prepare you for the later lesson material on troubleshooting. But before that, you’ll look at some alternatives to diesel fuel that are available now and may increase in importance in the future. The final part of this lesson covers engine and fuel system failure analysis, troubleshooting, and diagnosis strategies. You’ll learn how to follow step-by-step procedures for troubleshooting, how to identify causes of failures, and how to analyze them.


  • Describe the function of a hydromechanical governor.
  • Explain operating principles of a simple mechanical governor.
  • Classify and describe various governor types.
  • Understand engine and fuel system failure analysis and diagnostic strategies.
  • Describe troubleshooting practices for diesel engines.

Unit 4

Computerized Truck and Bus Systems Management, Part 1

This lesson focuses on electronics and computer systems, two technologies associated with diesel engines. First, you’ll study electricity and electronics, learning the basic principles of electricity, including current flow, magnetism, capacitance, and circuits. You’ll also learn about different types of conductors, transistors, and how these various components are joined to form electrical systems. Second, you’ll study digital computers. You’ll learn the components of a computer and how a computer works. Third, you’ll learn about the Internet and telecommunications systems. You’ll also learn about global positioning satellites. Next, you’ll study vehicle computer systems. You’ll learn about the information processing cycle, including data retention and input and output circuits. You’ll also learn about engine/electronic control modules and engine management systems. Finally, you’ll study electronic service tools. You’ll learn how these instruments are used for troubleshooting.

When you complete this lesson, you’ll be able to

  • Describe atomic structure and electron movement.
  • Explain the function of circuits, conductors, semiconductors, insulators, and transistors.
  • Understand electronic principles of magnetism, current flow, and capacitance.
  • Describe computer components and operation.
  • Explain how the trucking industry makes use of networking and communications systems.
  • Describe the technology of global positioning.
  • Understand the information processing cycle in vehicle computer systems.
  • Describe ECMs, including data retention, inputs, outputs, and programming.
  • Identify the tools needed to troubleshoot vehicles’ electronic systems.
  • Understand how to repair common problems with electrical wiring, connectors, and terminals.
Computerized Truck and Bus Systems Management, Part 2

This lesson focuses on the electronic management systems of various vehicle manufacturers. You’ll study Caterpillar’s programmable electronic engine control system (PEEC), Mack Trucks’ Vehicle Management and Control (V-MAC) systems, Detroit Diesel’s Electronic Controls (DDEC), Caterpillar’s ADEMS (advanced electronic engine management system), and the Cummins CELECT and CELECT Plus systems. You’ll learn the difference between partial and full authority systems. You’ll be given an in-depth look at the layouts of each of these systems, their fuel subsystems, ECMs, inputs, outputs, and other components. You’ll also learn how electronic management systems operate and how to troubleshoot these important diesel engine systems.


  • Describe the system layout and diagnostics of the Bosch common rail (CR) system.
  • Explain the function of injection pumps in Mack Trucks’ Vehicle Management and Control (V-MAC) systems.
  • Describe V-MAC electronics and troubleshooting.
  • Understand the fuel subsystem, ECM, inputs, outputs, and governing of Detroit Diesel Electronic Controls (DDEC) systems.
  • Explain engine diagnostics, calibration, reprogramming, and tune-up procedures used for DDEC systems.
  • Understand the fuel subsystem, ECM, inputs, outputs, and data links for Caterpillar’s advanced electronic engine management system (ADEMS).
  • Describe how to diagnose Caterpillar’s ADEMS.
  • Understand the fuel subsystem, ECM, inputs, and outputs for the Cummins CELECT and CELECT Plus.
  • Describe CELECT/CELECT Plus injectors, engine protection system, programmable features, and troubleshooting procedures.
Computerized Truck and Bus Systems Management, Part 3

This lesson continues your study of electronic management systems, covering electronic unit pump (EUP) technology, hydraulically actuated electronically controlled unit injectors (HEUI), and High Pressure Injection-Time Pressure (HPI-TP) systems. You’ll learn the components and operating principles of these systems. In addition, you’ll learn about emissions. You’ll gain an understanding of photochemical smog, emission control techniques, analysis, and testing.


  • Describe the fuel subsystem, input circuit, and engine controllers of the Bosch electronic unit pump (EUP).
  • Discuss the removal and installation of EUPs.
  • Explain the operating principles of a hydraulically actuated electronically controlled unit injector (HEUI).
  • Identify HEUI subsystems, components, and principles of electronic management, switching, and diagnostics.
  • Describe the features of the Cummins Signature 600 engine series.
  • Discuss electronic circuitry and operating principles of High Pressure Injection-Time Pressure (HPI-TP) systems.
  • Understand the operating principles of the Cummins Accumulator Pump System (CAPS) and the Cummins Interact Systems series.
  • Explain how photochemical smog is formed and how emissions testing, controls, and analysis can reduce it.

Unit 5

Electricity and Diesel Engine Electrical Systems

This lesson introduces you to basic principles of electricity and electronics as they relate to diesel trucks. First, you’ll learn about how to protect yourself and how to properly operate equipment and tools to keep the shop a safe place to work. You’ll then learn fundamental principles of electricity and how to use electrical diagnostic tools. You’ll learn about the effect of wire size on circuit capacity, terminals and connectors, printed circuits, and wiring harnesses. You’ll also learn how protective devices are wired and routed. You’ll learn how to replace fusible links, how to repair connector terminals, and how to work with wiring diagrams. This lesson also introduces you to electrical components and basic electrical troubleshooting and service. Finally, you’ll learn about service and repair procedures.


  • Explain personal safety practices and procedures for working on the electrical systems of heavy-duty vehicles.
  • Identify the proper methods for using electrical diagnostic tools, including test lights, various meters, and oscilloscopes.
  • Define electrical and electronic principles as they apply to a vehicle’s circuits.
  • Understand the effect of wire size on electrical circuits.
  • Describe the functions served by several electrical components, including terminals, conductors, printed circuit boards, wiring harnesses, buzzers, solenoids, semi-conductors, and transistors.
  • Identify common electrical schematic symbols and understand how to interpret wiring diagrams.
  • Explain how to repair wires and connector terminals.
  • Identify and understand how to test various electrical components, including switches, relays, stepped resistors, variable resistors, diodes, and circuit protection devices.
  • Understand how to check for shorted, grounded, and opened circuits and how to measure the voltage drop across a component or circuit.
Electronic Engine Controls

This lesson discusses the diesel engine’s battery, which is the heart of a truck’s electrical system. You’ll first be introduced to the functions of a battery, and you’ll learn how the components of a battery, known as cells, harness the energy produced by chemical reaction to generate electrical current. This lesson discusses various types of batteries; battery installation, problem diagnosis, and service practices are also addressed. You’ll learn about the operating principles of starter motors and starting systems. You’ll also learn the design features of different starter-motor types, including series-wound, shunt-wound, and compound motors. Finally, this lesson includes instruction on diagnosing and servicing starting systems and their components. You’ll learn how to perform the following tests: quick testing, current draw, starter voltage, battery cable, voltage drop, and no-crank tests.


  • Explain how a battery’s elements function to produce electrical power.
  • Interpret battery capacity ratings.
  • Describe how to make series and parallel hookups.
  • List the actions required to properly inspect, clean, and service batteries, battery terminals, and cables.
  • Describe the procedure for applying both fast and slow charges to batteries.
  • Explain testing procedures for batteries, including state of charge, open circuit voltage, capacity, three-minute charge, conductance, and battery drain testing.
  • Understand the functions served by the components of a typical starting system.
  • Describe how to clean, inspect, repair, and reassemble a starter motor.
  • Understand the operating principles of starter drive mechanisms and starter circuits.
  • Explain safe procedures for troubleshooting and otherwise servicing starting systems.
  • Describe how to conduct various starting system tests, including quick-tests, current draw, voltage drop, no-load, and component checks.
Additional Course Material

Textbook: Diesel Technology

Unit 6

Truck Electrical Systems

This lesson covers charging, lighting, and instrumentation systems. You’ll study the components of an alternator, including the rotor, brush, stator, rectifier bridge, housing, and cooling fan. You’ll also learn two methods technicians use to determine the condition of the charging system: measuring amperage and voltage. You’ll learn about testing, troubleshooting, and servicing the charging system. The lesson continues with a discussion of truck and trailer lighting systems, including an overview of various lighting technologies, regulations, and standards. The third section of this lesson discusses instrumentation and warning systems. The final section of this lesson discusses common electrical accessories for diesel trucks.


  • Identify the major charging-system components and their functions.
  • Describe voltage regulation and the circuits involved in the process.
  • Explain the operation of transformer-rectifier units and charge indicators.
  • Understand how to inspect a charging system for undercharge, overcharge, and no-charge conditions.
  • Describe how to perform output, voltage, and in-vehicle tests on the charging system.
  • Explain how to remove, disassemble, and reinstall an AC generator.
  • Describe the operation of headlight lamps, LEDs, exterior lights, and interior lights.
  • Explain the operation of various lighting switches, including the headlight switch and dimmer switch.
  • Describe circuits and wiring for trailer light operation.
  • Understand how to diagnose lighting-system problems.
  • Understand how to repair headlights, dimmer switches, and taillights.
  • Discuss the operation and repair of speedometers, odometers, bimetallic and electromagnetic gauges, gauge sending units, pyrometers, warning indicators, and engine shutdown systems.
  • Explain how to remove instrument panels and printed circuits.
  • Describe the operation and repair of horn circuits, wiper systems, blower motor circuits, heated and lighted mirrors, engine retarders, and thermostatic engine fan systems.
Truck Computer Systems

In the study of ignition systems, you’ll first learn about the gasoline internal combustion engine, particularly the four-stroke cycle. This study will provide you with a foundation from which you’ll progress to the study of diesel engine ignition systems. You’ll learn about ignition components and the electrical circuit that connects them. Next, you’ll study ignition system diagnosis and service. You’ll follow a logical troubleshooting sequence and advance your knowledge of diagnostic theory. You’ll learn how to service spark plugs, set ignition timing, and test individual components. You’ll study gasoline engine computerized control diagnostics and OBD II standards. In the study of diesel engine computer control systems, you’ll learn about the electronic devices that store and process data for managing and controlling the vehicle’s systems. Finally, the lesson focuses on the external devices that interface directly with an ECM.


  • Describe the function of an ignition system.
  • Explain the gasoline engine’s four-stroke cycle.
  • Describe the operation of a gasoline engine’s ignition system components, including ignition coils, spark plugs, and ignition cables.
  • Understand the major electrical circuits in a gasoline engine’s ignition system and the principles of computer spark control and timing.
  • Follow logical ignition system diagnostic procedures.
  • Describe how to perform spark test procedures, ignition timing testing, and individual component testing.
  • Describe the function of a diesel engine’s control computer, including the role of the CPU, inputs, outputs, and electronic unit injectors.
  • Explain how analog and digital voltage signals, computer communications, computer language, and logic gate operations relate to the control of a diesel engine.
  • Understand how to service engine control systems.
  • Describe how to perform basic sensor tests and how to replace PROM chips.
Additional Course Material

Textbook: Truck and Trailer Systems

Unit 7

Tires, Wheels, and Trailer Maintenance

You’ll begin this lesson with a review of safety practices. Then you’ll move on to an overview of basic theories, which will help you develop a fundamental understanding of the principles involved in the complex systems you’ll learn about later. You’ll first learn about Newton’s three laws of motion, including laws that apply to inertia, force, and action/reaction. You’ll learn how a vehicle’s systems convert energy into more useful forms. You’ll learn the operating principles that describe how tires and wheels behave in motion and the results of these assemblies being in or out of balance. This lesson also introduces the tools and shop practices common to most wheel, suspension, and steering work. Once you have an understanding of the basic principles and tools associated with truck steering and suspension systems, you’ll learn specifically about tires, wheels, and wheel bearings. This lesson also introduces various types and designs of tires.You’ll learn about how tires are properly inflated and rotated. Finally, you’ll learn common, safe procedures for servicing and diagnosing in accordance with OSHA regulations.


  • Work with the physical laws that apply to accelerating, controlling, and stopping a vehicle.
  • Explain the use of various suspension and steering service tools.
  • List precautions you should closely follow when using truck lifts, overhead hoists, and trolleys.
  • Explain the characteristics of bias, radial, and low-profile radial tires.
  • Describe ways of retaining tires on rims and disk wheels on wheel hubs.
  • Understand the importance of and steps taken to achieve a properly balanced wheel.
  • Explain the purpose for and capabilities of wheel bearings.
  • Describe how to remove, clean, evaluate, replace, and adjust wheel bearings.
  • Describe the purpose of garter springs, snap rings, shields, seals, flutes, spoke and wheel disks, grit guards, and wiper rings.
  • Understand how to service wheels, tires, and rims.
  • List the steps required to perform rim runout tests, pre-trip inspections, and wheel-balancing checks.
Suspension and Steering System

In this lesson, you’ll be introduced to the basic operating concepts and components of a heavy-duty vehicle’s steering system. This lesson covers steering columns, manual steering gears, steering linkages, power steering pumps, and power steering gears.


  • Understand the characteristics of different steering column types and how they’re diagnosed, removed, inspected, and repaired.
  • Describe the purpose, design, operation, maintenance practices, and adjustment procedures for manual steering gears.
  • Define steering system characteristics, such as directional stability, understeer, oversteer, load distribution, wheel jounce, and rebound.
  • Explain the operation and advantages of all-wheel steering systems.
  • Explain the purpose of steering linkage components and how they’re removed and replaced, including tie rod ends, pitman arms, drag links, and idler arms.
  • Differentiate between types of power steering pumps, describing their operating characteristics.
  • Understand how power steering gears work and how they’re repaired and maintained.
  • Diagnose power steering belt problems.
  • Understand how to remove and replace power steering pump components, including pulleys, reservoirs, valves, seals, and O-rings.
  • Explain the operation of air-assisted and variable-assist power steering systems.

Unit 8

Drivelines, Clutches, and Axles

In this lesson, you’ll learn how frames are reinforced, including how to repair existing frams and how to add new reinforcements to a vehicle’s frame. You’ll learn about the operation, rating, and maintenance of fifth wheels. The final portion of this lesson focuses on suspension systems.


  • Explain material properties as they relate to the strength of a vehicle’s frame.
  • Identify differences in frame designs and reinforcement schemes.
  • Describe types of frame defects and procedures for identifying and correcting them.
  • Explain the characteristics of and uses for various types of fifth wheels.
  • Describe the proper procedures for coupling and uncoupling fifth wheels.
  • Discuss fifth wheel service, including mounting, shimming, repositioning, and adjusting.
  • Identify the differences in various types of front suspensions.
  • Explain the features and operating principles of various suspension system components, including coil springs, leaf springs, spring-mounting hardware, ball joints, control arms, steering knuckles, front axles, shock absorbers, equalizers, torque rods, and equalizing beams.
  • Explain how to remove, inspect, and replace various suspension system components.
  • Describe how to inspect the overall front and rear suspension systems.
Manual and Automatic Transmissions

This lesson introduces various air suspension designs. You’ll learn specifically how the air system works, including how these systems maintain normal ride height by controlling the supply of pressurized air to the air springs. You’ll study the advantages of air suspension systems, including improved ride quality, improved braking capability, better vehicle stability, and increased tire life. The second major topic in this lesson is wheel alignment. You’ll learn about the two critical alignment concepts: camber and caster. The final section in this lesson covers commercial vehicle inspections.


  • Describe the design characteristics and operating principles of various air suspension systems.
  • Explain the operation of various air suspension system components, including the pressure control valve, height control valve, relay valve, and external dock lock mechanism.
  • List the advantages of air spring suspension systems.
  • Describe how to perform preoperational inspections, regular inspections, preventive maintenance, and component replacement on tractor and trailer air suspension systems.
  • Explain the principles related to wheel alignment, the variables that affect alignment, and the hazards resulting from incorrect wheel alignment.
  • Understand how to measure and adjust/repair front wheel camber, caster, kingpin inclination, and front wheel toe.
  • Explain how to use a computer wheel alignment system to measure front and rear suspension angles.
  • Describe how to measure rear axle alignment with a laser aligner and with a trammel bar.
  • Explain how to use bending equipment to correct front and rear axle misalignment.
  • Understand how to perform commercial vehicle inspections related to fifth wheels, frames, steering, suspension, tires, wheels, and rims.

Unit 9

Truck Brake Systems, Part 1

This lesson begins your study of truck brakes. It first covers safety practices that you should follow when working on the brakes of a diesel powered truck and basic theories that will help you to understand the more complex aspects of brake systems. Second, you’ll learn about wheel bearings. Finally, you’ll cover hydraulic brake systems.


  • Explain the general safety precautions to be followed when servicing vehicle brake systems.
  • Describe how to safely handle diesel fuel, brake fluid, and other hazardous waste materials.
  • Discuss the physical and electrical principles associated with vehicle brake systems.
  • Describe the operation of basic air brake systems as well as air brake chambers, wedge-type brakes, and balanced brake systems.
  • Explain the basic operation and purpose of a dial indicator, micrometer, pressure bleeder, scan tool, and other brake service tools.
  • Discuss how to diagnose, remove, replace, and adjust vehicle wheel bearings and seals.
  • Describe the operating principles of master cylinders, adjuster wheel cylinders, parking brake cylinders, sliding brake calipers and pistons, metering valves, proportioning valves, and pressure differential valves.
  • Explain how to test, diagnose, and service brake system components, including the cylinders, drums, rotors, calipers, valves, brake lines, and hoses.
  • Explain the brake shoe mounting and adjustment procedures for various kinds of hydraulic brake assemblies.
Truck Brake Systems, Part 2

In this lesson, you’ll cover hydraulic brake power assist units, hydraulic parking brake systems, air brake air supply systems, and air service-brake systems. You’ll learn about hydraulic brake booster types, operating principles, component functions, and diagnostic techniques. Your study of parking brake systems will focus on three major types: mechanical propeller shaft systems, electric/hydraulic systems, and the hydraulic parking brake systems. Your study of electric/hydraulic type parking brake systems will concentrate on the parking brake control module (PBCM). Your study of air brake air supply systems will begin with an explanation of why air brakes (instead of hydraulic brakes) are used on trucks, tractors, and trailers. Then, you’ll be given detailed explanations of the purpose, design, operation, and serving techniques for the various air supply system components.


  • Describe the operating characteristics and service procedures for hydraulic brake booster systems including the Hydrovac system.
  • Understand the purpose and operation of various hydraulic brake booster system components, including the flow switch, electrohydraulic pump, warning lights, and the stoplight circuit.
  • Explain how to adjust, diagnose, and repair stoplight switches and circuits.
  • Describe the operation of and service techniques for a mechanical propeller parking brake.
  • Explain the function of and service techniques for the electric/hydraulic parking brake system, including the operation of the parking brake control module, parking brake switch, pressure sensor, travel sensor, actuator, cable, brake shoes, and warning lights.
  • Describe the function of and service techniques for the hydraulic parking brake system, including the purpose of the control valve and the unitized valve.
  • Describe the operation of and service techniques for air brake air supply system components, including the air compressor, governor, valves, reservoir, low pressure warning circuit, alcohol evaporator, alcohol injector, and air dryer.
  • Outline procedures for testing compressor air pressure buildup time, unloader piston leakage, air compressor discharge valve leakage, compressor piston ring leakage, governor cut-out pressure, and governor leakage.
  • Explain the operational characteristics of and service techniques for the brake application valve, single and double check valves, quick release valves, service brake relay valves, limiting valves, the limiting quick release valve, pressure protection valves, pressure reducing valves, trailer control valves, and tractor protection valves.
HVAC Systems

In this lesson you’ll learn how a vehicle’s temperature control systems operate and how these systems are serviced. You’ll also study the government-mandated material-handling guidelines that apply when servicing air conditioning systems.


  • Explain the regulatory system in place to control the handling of A/C system refrigerants and the technician’s role in meeting system guidelines.
  • Describe how an air conditioning system operates.
  • List general steps to be taken when troubleshooting A/C system complaints.
  • Understand how a vehicle’s air ventilation system operates.
  • Explain how the heat from engine coolant can be extracted and controlled to heat the vehicle’s operator compartment.

Unit 10

Truck Maintenance Fundamentals

This lesson focuses on the operating principles of manual transmissions and clutches. You’ll learn how manual transmissions, clutches, and related components are removed, installed, and serviced.


  • Describe the operating principles of a clutch.
  • Identify and describe the functions of clutch components.
  • Troubleshoot clutch problems.
  • Outline the procedure for removing, servicing, and replacing clutches.
  • Describe how a manual transmission operates.
  • Explain the function of each major component in a manual transmission.
  • Identify shifting system types and their operating characteristics.
  • Outline the procedure for maintaining and servicing manual transmissions.
  • Troubleshoot a transmission and shift system to identify faults.
  • List the tasks required to overhaul a manual transmission.
Review of Diesel Engine Topics

In this lesson you’ll learn how an automatic transmission operates as well as how its internal components are removed, serviced, and/or replaced. You’ll also be introduced to other components that directly influence the operation of an automatic transmission. Finally, you’ll learn how automatic transmissions and related components are removed, installed, and serviced.


  • Describe the operating principles of a torque converter.
  • Identify and describe the functions of torque converter components.
  • Troubleshoot torque converter problems.
  • Outline the procedure for removing, servicing, and replacing torque converters.
  • Describe how an automatic transmission operates and how its shift points are controlled.
  • Explain the function of each major subsystem of an automatic transmission.
  • Describe how hydraulic retarders operate.
  • Outline the procedure for maintaining and servicing automatic transmissions.
  • Troubleshoot an automatic transmission to identify faults.
  • List the tasks required to overhaul an automatic transmission.
Troubleshooting and Repair

In this lesson you’ll apply much of what you’ve learned about the general operation of automatic transmissions to the CEC and World models of Allison transmissions. You’ll also be introduced to the detailed workings of a transmission’s electronic control system.


  • Describe the operating principles of an Allison CEC transmission, including how electronic signals are converted into hydromechanical ones.
  • Identify and describe the functions of Allison transmission components.
  • Explain how an Allison World transmission operates, including the functions served by its electronic control system.
  • Describe the design differences between Allison CEC and World transmission models.
  • Troubleshoot problems with CEC and Allison World transmissions.
  • Outline the procedure for removing, servicing, and replacing CEC and Allison World transmissions.
Review of Electrical Starting and Cooling Systems

In this lesson, you’ll study critical components from the rest of the driveline: drive shafts and axle assemblies. You’ll learn how drive shafts operate, how they’re serviced, and the types of drive problems they’re likely to encounter. Axles and differentials (found on drive axles) are also discussed in this lesson. Topics include their operating characteristics, service, and removal.


  • Explain how drive shafts and other driveline components are connected.
  • Outline the procedure for removing, servicing, and replacing drive shafts, universal joints, and other driveline components.
  • Identify different types of drive axles, their operating characteristics, and the function served by each of their major component parts.
  • Describe the operating principles of a differential.
  • Explain how to service and repair problems with drive axles, differentials, and nondrive axles.
PM, Inspection, and Troubleshooting

In this lesson, you'll study preventative maintenance inspections (PMIs) on over-the-road vehicles and diesel engines. Proper maintenance inspections allow you to catch problems before they become bigger, more expensive ones. This lesson discusses the type of maintenance programs you'll encounter as a diesel technician, and the types of routine inspections you'll be expected to carry out. You'll also study DOT inspection requirements and the characteristics of an effective preventative maintenance (PM) program.

  • Interpret maintenance programs, including maintenance schedules and types of inspections
  • Explain how to perform DOT inspections and list DOT out-of-service criteria
  • Describe the importance of preventative maintenance programs and accurate maintenance records
  • List the tests and checks for modern oil analysis and the instructions for performing these tests
  • Explain how to perform a range of troubleshooting tests with attention paid to troubleshooting charts and service information
  • Prepare diesel engines for long-term and short-term storage
Additional Course Material

Textbook: Troubleshooting and Repairing Diesel Engines

Computer Specifications
You will need high-speed internet access to begin your program. You will need access to a Microsoft® Windows® based computer running Windows XP® or later or an Apple® Mac® computer running OS X® or later, and an email account to complete your program with Penn Foster.

We reserve the right to change program content and materials when it becomes necessary.
Microsoft, Windows, and Windows XP are trademarks of Microsoft Corporation registered in the United States of America and/or other jurisdictions.
Apple, Mac, and OS X are trademarks of Apple, Inc registered in the United States of America and/or other jurisdictions.

Sample Lesson

To view a Diesel Mechanics sample lesson, click here.

Diesel Mechanics Certification

Course materials are aligned with the National Institute for Automotive Service Excellence (ASE) Certification Exam in order to help you prepare for ASE certification in the Medium/Heavy Duty Trucks discipline.

Snap-on Student Excellence Program

Students enrolled in the Diesel Mechanics/Heavy Truck Maintenance Training Program are eligible to participate in the Snap-on Student Excellence Program. This program allows students in trades and technology education programs to purchase high-quality Snap-on tools at a preferred student discount. Some restrictions may apply for total purchase and quantities of certain items. Click here to see the new Snap-on Catalog.

State Licensing Information

Licensing and/or certification requirements for jobs in this field are not the same in every state and may include educational, testing, and/or experiential requirements beyond those offered by your Penn Foster Program. You should contact the state professional licensing board or similar regulatory body in the state(s) where you plan to work to determine their requirements before enrolling in your Program. Please click here for contact information for state licensing/regulatory boards and certain industry licensing information.

Online Diesel Mechanic Training Course

Diesel mechanics work on engine operations and computerized management systems for trucks and other diesel vehicles. Here are a few qualities that effective diesel mechanics have in common:

  • Detail oriented: It’s important to have a sharp eye and check things twice.
  • Problem solving: Finding the missing piece can be challenging and fun.
  • Persistent: You work until the job is done, and done well.
  • Honest: You are ethical in your work for customers.

The quality of materials and staff at Penn Foster are the highest of the various schools I have attended.

- Bill F., Diesel Mechanics Student

I would like to say that enrolling with Penn Foster Career School was the best choice that I ever made. I would recommend Penn Foster. I just have to say 'Thank You' to Penn Foster for achieving my goal.

- Francisco C., Penn Foster Graduate

My overall experience with Penn Foster was great. You are allowed to go at your own speed, and the school gives encouragement to keep you going. I would recommend Penn Foster.

- James R., Penn Foster Graduate

Your program includes:

  • Customized payment plans with 0% interest
  • Books and learning aids to help you prepare for ASE certification
  • Access to Snap-on Student Excellence Program offering up to a 50% discount on Snap-on products
  • Study Planner App to customize your study plans and keep track of your progress
  • Instructional support from our experienced faculty
  • Access 24/7 to Penn Foster’s online community, plus immediate membership in the Diesel Mechanics/Heavy Truck Maintenance academic group
  • Your personalized online student homepage and learning portal
  • Additional resources such as our online library and career guidance from Career Cruising

Do you recommend any additional resources for students in the Diesel Mechanics Program?

Yes, we recommend several additional online sources that are helpful while you are in the program. A few useful websites include:

What certifications are linked to the automotive and engine repair programs?

Students are eligible to sit for numerous certification examinations upon completion of automotive & engine repair programs. An overview of the certifications for each program can be found on the program specific pages.

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