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The ASE T6 Electrical/Electronic Systems test is the one a lot of heavy-duty techs put off, but it doesn’t have to be scary. Most of the test comes down to a few skills: reading a wiring diagram, using a meter correctly and doing a voltage drop test. Get those down and the rest falls into place.

Here’s what’s on the T6, where the points are and how to study for it.

The T6 test at a glance

The T6 has about 50 scored questions, plus around 10 unscored research questions, with about 90 minutes to finish.

You’ll need two years of hands-on work experience to be certified, and a diesel program can count for up to one year. Registration is $34 per order plus $62 per test.

A. General Electrical/Electronic Diagnosis (about 14 questions)

This is the biggest section and the foundation for everything else.

• Ohm’s law: volts = amps × ohms. Know how added resistance in a circuit lowers current and dims a light or slows a motor.
• Meter use: measure voltage in parallel, current in series and resistance with the circuit unpowered.
• Voltage drop testing: the best way to find a bad connection or ground. Test with the circuit loaded. A good connection drops very little voltage, often a few tenths of a volt or less.
• Wiring diagrams: follow power from the source through fuses, switches and loads to ground.
• Parasitic draw: how to measure key-off battery drain and isolate the circuit by pulling fuses.

B. Battery and Starting Systems (about 11 questions)

• Battery testing: open-circuit voltage (about 12.6 volts at full charge), load testing and conductance testers.
• Battery banks: most heavy trucks run several 12-volt batteries in parallel. Know how parallel and series connections change voltage and capacity, and why one bad battery can drag down the bank.
• Cranking circuit: slow cranking can be batteries, cables, connections or the starter. Voltage drop on the positive and ground sides tells you which.
• Starter controls: magnetic switch, solenoid and neutral safety or clutch switch.

C. Charging System (about 7 questions)

• Output testing: measure charging voltage at the batteries and alternator output under load.
• Charging circuit voltage drop: high resistance in the output or ground cable can make a good alternator look bad.
• Diodes and ripple: a failed diode causes low output and excessive AC ripple voltage.
• Belt drive: slipping belts and worn tensioners cause low output.

D. Lighting Systems (about 6 questions)

• Common problems: dim or flickering lights from bad grounds and corroded connectors. Lights that feed back into other circuits when a ground is missing.
• Trailer wiring: the 7-way connector and trailer light cord, including which pin powers ABS on the trailer.
• LED lighting: why LEDs can cause bulb-out warnings or turn signals that flash too fast.

E. Related Vehicle Systems (about 12 questions)

• Data networks: J1939 CAN bus. The backbone has two 120-ohm terminating resistors, so you should measure about 60 ohms across the two data wires with the key off.
• Gauges and sensors: sending units, sensor reference voltage (often 5 volts) and signal ground.
• Body systems: power windows, door locks, wipers, mirrors and horns.
• Scan tools: reading active and inactive codes and using live data to confirm a repair.

Try a few practice questions

Question 1

An engine cranks slowly. Battery voltage is good, but voltage drop on the positive cable from the battery to the starter is 1.2 volts while cranking. What is the most likely cause?

• A. A weak starter motor
• B. High resistance in the positive cable or its connections
• C. A bad ground at the alternator
• D. A faulty neutral safety switch

Answer: B. A 1.2-volt drop is far too high. Clean or replace the cable and connections, then retest.

Question 2

With the key off, a technician measures resistance across the two J1939 data wires and gets about 120 ohms. What does this mean?

• A. The network is normal
• B. One terminating resistor is missing or open
• C. Both terminating resistors are shorted
• D. The batteries are low

Answer: B. A healthy backbone with two 120-ohm terminators reads about 60 ohms. A 120-ohm reading means one terminator is missing or open.

Question 3

Technician A says current is measured with the meter in series with the circuit. Technician B says resistance should be measured with the circuit powered. Who is right?

• A. Technician A only
• B. Technician B only
• C. Both A and B
• D. Neither A nor B

Answer: A. Technician A only. Resistance must be measured with the circuit unpowered, or the reading will be wrong and the meter can be damaged.

A simple 4-week study plan

• Week 1, fundamentals: Ohm’s law, meter use, reading wiring diagrams and voltage drop testing.
• Week 2, batteries and starting: battery testing, battery banks, cranking circuit diagnosis and parasitic draw.
• Week 3, charging, lighting and networks: alternator testing, lighting and trailer wiring, and J1939 basics.
• Week 4, practice and review: timed practice tests and review every question you missed.

Keep going with the ASE T-Series

Working toward Master Medium/Heavy Truck Technician? Check out our ASE T2 Diesel Engines study guide, then take a free practice test with instant explanations for every answer. T6 practice tests are coming soon.

The ASE T8 Preventive Maintenance Inspection (PMI) test is a great first T-Series certification. It covers the whole truck from bumper to mud flaps, and it tests the inspection skills shops use every day: spotting a problem before it turns into a breakdown, a roadside violation or a crash.

Here’s what’s on the T8, where the points are and how to study for it.

The T8 test at a glance

The T8 has about 50 scored questions, plus around 10 unscored research questions, with about 75 minutes to finish. Frame and chassis is the biggest section by far, so spend your study time there.

Like the other T-Series tests, you need two years of hands-on work experience to be certified, and a diesel program can count for up to one year. Registration is $34 per order plus $62 per test.

A. Engine Systems (about 10 questions)

• Fluids: oil level and condition, coolant level, freeze protection checked with a refractometer, and coolant additive (SCA) levels with test strips.
• Belts and hoses: cracks, glazing, tension and tensioner operation. Soft, swollen or collapsed hoses.
• Fuel system: leaks, water separator draining and fuel filter service intervals.
• Air intake and exhaust: air filter restriction indicator, charge air cooler leaks, exhaust leaks, and DPF and DEF system checks.
• Leaks and noises: oil, coolant, fuel and exhaust leaks, and what each one tells you.

B. Cab and Hood (about 5 questions)

• Safety items: seat belts, seats, mirrors, horn, wipers and washers, defroster and door latches.
• Gauges and warning lights: all instruments working, including the low-air warning.
• Hood and body: hood latches and tilt mechanism, steps and grab handles, and cab mounts.
• HVAC: cabin filter, blower operation and A/C performance.

C. Electrical/Electronics (about 10 questions)

• Batteries: state of charge (about 12.6 volts at rest for a fully charged 12-volt battery), load testing, clean and tight cables, and hold-downs.
• Starting and charging: cranking voltage, charging voltage at the batteries, and voltage drop on cables.
• Lighting: every exterior light and reflector, the trailer light cord, and the 7-way connector.
• Diagnostics: checking for active fault codes with a scan tool as part of the PMI.

D. Frame and Chassis (about 22 questions)

This is almost half the test. Know your measurements and what makes a truck unsafe to drive.

Air brakes

• Pushrod stroke checks at full application, and the adjustment limit for each brake chamber size.
• Air leak-down and build-up tests, governor cut-in and cut-out, and low-air warning operation.
• Drum and lining condition, air lines and chafing, and air dryer and reservoir draining.

Steering and suspension

• Steering wheel free play, tie rod ends, drag link, kingpins and steering gear mounting.
• Springs, U-bolts, shocks, air springs and ride height. Broken leaves and cracked hangers.

Tires, wheels and hubs

• Tread depth: at least 4/32 inch on steer tires and 2/32 inch on all others, under federal rules.
• Inflation, mismatched duals, sidewall damage and cracked wheels.
• Hub oil level and leaks, and wheel bearing end play.

Driveline, frame and fifth wheel

• U-joints, driveshaft slip joints and center bearings.
• Frame cracks, loose crossmembers and fifth-wheel locking jaw adjustment.

E. Road/Operational Test (about 3 questions)

• Brakes stop straight, the parking brake holds, and steering returns to center.
• Shifting, clutch, engine power and any noises or vibrations.
• Gauge readings at operating temperature, and checking for leaks after the drive.

Try a few practice questions

Question 1

During a PMI, a steer tire measures 3/32 inch of tread depth. What should the technician do?

• A. Pass it because it’s above 2/32 inch
• B. Record it and replace the tire before the truck goes back in service
• C. Rotate it to a drive position and pass it
• D. Increase tire pressure

Answer: B. Steer tires need at least 4/32 inch of tread under federal rules. Below that, the tire needs to be replaced.

Question 2

Technician A says coolant freeze protection should be checked with a refractometer. Technician B says SCA levels in conventional coolant can be checked with test strips. Who is right?

• A. Technician A only
• B. Technician B only
• C. Both A and B
• D. Neither A nor B

Answer: C. Both are standard PMI checks.

Question 3

During a PMI, a 12-volt battery reads 12.2 volts after the truck has sat overnight. What does this mean?

• A. The battery is fully charged
• B. The battery is about half charged and needs charging and testing
• C. The alternator is overcharging
• D. The battery is ready for a load test

Answer: B. A fully charged 12-volt battery reads about 12.6 volts at rest. Charge it, then load test it before deciding whether to replace it.

A simple 4-week study plan

• Week 1, frame and chassis brakes: pushrod stroke limits, air system tests and foundation brake inspection.
• Week 2, the rest of frame and chassis: steering, suspension, tires, wheels, hubs and the fifth wheel.
• Week 3, engine and electrical: fluids, belts, cooling, batteries, charging and lighting.
• Week 4, cab, road test and practice: cab safety items, road test checks and timed practice tests.

Keep going with the ASE T-Series

After the T8, many techs move on to the T2 Diesel Engines test. Read our ASE T2 Diesel Engines study guide, then take a free practice test with instant explanations for every answer. T8 practice tests are coming soon.

The ASE T4 Brakes test is where heavy-duty techs prove they can keep an 80,000-pound truck stopping safely. Air brakes make up most of the test, so if you can diagnose an air system from the compressor to the slack adjuster, you’re most of the way there.

This guide covers what’s on the T4, where the points are, the concepts that show up again and again, and a simple plan to get ready.

The T4 test at a glance

The T4 has about 50 scored questions, plus around 10 unscored research questions mixed in. You won’t know which ones don’t count, so treat every question like it does. You get about 75 minutes.

To earn the certification, you also need two years of hands-on work experience. A diesel or heavy-equipment program can count for up to one of those years. Registration is $34 per order plus $62 per test.

A. Air Brakes (about 32 questions)

This is almost two-thirds of the test. Know the whole system: how air is made, stored, delivered and released.

Air supply

• Compressor and governor: the governor tells the compressor when to pump (cut-in) and when to stop (cut-out). Typical cut-out is around 120 to 135 psi. Know how to diagnose a governor that won’t cut out versus a compressor that can’t keep up.
• Air dryer: removes moisture and oil before air reaches the tanks. A dryer that never purges, or tanks full of water, point to dryer or purge-valve problems. Oil in the tanks usually points back to the compressor.
• Reservoirs: supply (wet), primary and secondary tanks. Know the check valves that keep one circuit from draining the other.
• Leak-down and build-up tests: know how to run a static and an applied leak test and what pressure drop per minute is acceptable, plus build-up time and low-air warning operation.

Service, parking and emergency brakes

• Valves: foot (treadle) valve, relay valve, quick-release valve, spring brake control valve and tractor protection valve. Questions often ask which valve causes a slow apply or slow release on one axle.
• Spring brakes: held off by air, applied by spring force. Know how to cage a spring brake safely before working on it.
• Tractor protection: know what happens to the trailer supply line when tractor air pressure drops.

Foundation brakes

• S-cam drum brakes: camshaft bushings, rollers, return springs and shoe wear. Know drum discard dimensions (stamped on the drum) and how to measure them.
• Automatic slack adjusters: check pushrod stroke at a full application. An out-of-adjustment automatic slack means something is wrong with the slack or the foundation brake. Don’t just keep adjusting it by hand.
• Air disc brakes: pad wear, rotor cracks and caliper slide function.

B. Hydraulic Brakes (about 11 questions)

Medium-duty trucks, buses and many vocational units still use hydraulic brakes, often with a hydraulic power assist.

• Master cylinder and lines: spongy pedal versus pedal that sinks slowly. Air in the system versus internal master cylinder leakage.
• Power assist: vacuum boosters and hydraulic boosters (powered by the power steering pump), plus the electric backup pump many systems use.
• Bleeding: correct sequence and when a scan tool is needed to bleed an ABS unit.
• Disc and drum service: rotor thickness, runout and parallelism, and caliper slide problems that cause uneven pad wear.

C. ABS, Traction Control, Stability Control and Collision Warning (about 7 questions)

• Wheel speed sensors: air gap, resistance checks and AC voltage output while spinning the wheel. A loose sensor or excessive bearing end play can set codes.
• Modulator valves: how they hold or release air at a wheel, and how to test them.
• Codes: reading blink codes and using a scan tool. Know which faults put the ABS lamp on.
• Stability control and collision mitigation: steering angle and yaw sensors, and radar or camera alignment after a front-end repair or bumper replacement.

Try a few practice questions

Question 1

An air brake system takes too long to build pressure from 85 to 100 psi. Technician A says a leaking reservoir drain valve could cause this. Technician B says a worn compressor could cause this. Who is right?

• A. Technician A only
• B. Technician B only
• C. Both A and B
• D. Neither A nor B

Answer: C. Both A and B. Any leak or a compressor that can’t pump enough air will slow build-up. Do a leak test first, then check compressor output.

Question 2

A truck with automatic slack adjusters has one brake with excessive pushrod stroke. What should the technician do first?

• A. Manually adjust the slack and release the truck
• B. Replace all slack adjusters on the axle
• C. Inspect the foundation brake and the slack adjuster for the cause
• D. Increase governor cut-out pressure

Answer: C. An automatic slack that’s out of adjustment is a symptom. Find the cause, such as worn cam bushings or a failed slack, before returning the truck to service.

Question 3

An ABS wheel speed sensor reads correct resistance, but the ABS sets a fault for that wheel at road speed. Which is the most likely cause?

• A. Excessive sensor air gap
• B. Low battery voltage
• C. Wrong brake fluid
• D. Weak spring brake

Answer: A. If the sensor has pushed back away from the tone ring, its signal is too weak at speed even though resistance tests fine. Push the sensor back in and check wheel bearing end play.

A simple 4-week study plan

• Week 1, air supply: compressor, governor, dryer, reservoirs and leak tests. This is the biggest piece of the test.
• Week 2, valves and foundation brakes: trace air through each valve on a system diagram, then study slack adjusters, S-cams and air disc brakes.
• Week 3, hydraulic brakes and ABS: master cylinders, boosters, bleeding, wheel speed sensors and modulators.
• Week 4, practice and review: take timed practice tests, then go back over every question you missed.

Keep going with the ASE T-Series

Working on your full Medium/Heavy Truck certification? Start with our ASE T2 Diesel Engines study guide, then take a free practice test with instant explanations for every answer. T4 Brakes practice tests are coming next.

The ASE T2 Diesel Engines test is one of the core certifications in the Medium/Heavy Truck (T-Series). Pass it along with the other T-Series tests and you’re on your way to Master Medium/Heavy Truck Technician status. It also tells every shop you apply to that you can do more than change oil.

This guide breaks down what’s on the test, where the points are, and how to study so your time goes where the questions are.

The T2 test at a glance

The T2 has 40 scored questions across seven content areas. ASE may also include a few extra unscored questions it’s testing for future exams. You won’t know which ones they are, so treat every question like it counts.

Where the points are: Fuel, Air Induction/Exhaust, Cooling, and General Diagnosis add up to 29 of the 40 questions. Head, block, and lube are only 11 combined. If your study time is limited, spend most of it on diagnosis, air, fuel, and cooling.

A. General Engine Diagnosis (7 questions)

This section tests whether you can read the engine before you pick up a wrench: fluid levels and condition, leaks, noises, vibration, no-start and hard-start complaints, power problems, and pulling codes and live data.

• Know your smoke colors. Black smoke means too much fuel for the air available, so think air restriction, boost leaks, a bad turbo, or overfueling. Blue smoke means oil is being burned. White smoke usually means fuel that isn’t burning completely, often a cold engine, low compression, or timing problems. White smoke can also be coolant getting into the cylinders.
• Diagnose in order. Expect “what should the technician do first?” questions. The answer is almost always the simplest, least invasive check: verify the complaint, check fluids, pull codes, then test.
• Codes point you somewhere. They don’t replace testing. A code tells you which circuit or system to look at. It doesn’t tell you which part to replace.

B. Cylinder Head and Valve Train (4 questions)

This section covers head removal and inspection, checking for cracks and warpage, valves, guides, seats and springs, injector sleeves and protrusion, overhead cams, and adjusting valve bridges and valve lash.

• Know that a valve bridge (crosshead) is adjusted before setting valve clearance on engines that have them.
• Know how head warpage is checked: a precision straightedge and feeler gauge across the mating surface, compared against the manufacturer’s spec.
• Injector sleeve problems can show up as coolant in the fuel or fuel in the coolant. Remember that connection.

C. Engine Block (3 questions)

This is the biggest task list and the fewest questions. It covers crankcase pressure testing, liners and liner height, crankshaft and bearings, gear train timing, pistons and rings, piston cooling jets, the vibration damper, and the flywheel housing.

• Crankcase pressure (blow-by): high readings point to worn rings or liners. They can also come from other sources pushing air into the crankcase, like a leaking turbocharger seal or air compressor.
• Liner protrusion (height) is measured and has to be within spec, or you’ll have head gasket problems.
• A damaged or out-of-spec vibration damper can lead to crankshaft damage. Know that it’s an inspection item.

D. Lubrication System (4 questions)

This section covers oil pressure testing and sensors, oil level, condition and consumption, the oil pump and pickup, pressure regulator and relief valves, the oil cooler, turbo lubrication, and choosing the right oil.

• Confirm a low oil pressure reading with a mechanical gauge before condemning the engine. The sensor or gauge could be the problem.
• Know what a stuck regulator or relief valve does to oil pressure, both when it’s stuck open and when it’s stuck closed.
• Coolant in the oil can come from an oil cooler failure. Don’t jump straight to a head gasket.

E. Cooling System (7 questions)

This section covers coolant type, level and condition, temperature sensors, belts and tensioners, thermostats, flushing and bleeding, coolant filters, water pumps, radiator and cap pressure testing, and fan drives and shrouds.

• A refractometer is the accurate way to check coolant freeze protection.
• Don’t mix coolant types. Know the difference between conventional coolant that needs supplemental coolant additives (SCAs) and extended-life coolant.
• Overheating questions often come down to airflow: a missing fan shroud, a fan clutch that isn’t engaging, or plugged radiator fins.
• A thermostat stuck open causes an engine that runs cold, not one that overheats.

F. Air Induction, Exhaust Systems and Engine Brakes (7 questions)

This section covers intake restriction and leak tests, boost tests, exhaust back pressure, turbochargers (including variable geometry), charge air coolers, exhaust aftertreatment, intake heaters and glow plugs, EGR, and engine brakes.

• Test intake restriction at full load and rated speed, because that’s when the engine pulls the most air. A test at idle won’t show a restriction.
• A leaking charge air cooler means low boost, low power, and black smoke. Know how it’s pressure tested.
• High exhaust back pressure, for example from a plugged DPF, causes low power and high exhaust temperatures.
• Know the basics of how compression (engine) brakes work and when they’re adjusted.

G. Fuel System (8 questions)

This is the biggest section. It’s split between the fuel supply system (tanks, lines, transfer pump, filters, water separators, priming and bleeding) and the electronic fuel management system (circuits, sensors, scan tools, unit injectors, HEUI, and common rail).

• Restriction on the suction side of the transfer pump and air getting into the supply system are classic causes of low power and hard starting.
• Know your electrical basics: voltage drop testing, measuring resistance, and checking power and grounds before replacing a module.
• A cylinder cutout/contribution test with diagnostic software is the fastest way to find a weak cylinder or bad injector.
• Common rail systems run extremely high pressures. Follow the safety procedures before opening any high-pressure line.

Try a few practice questions

1. A diesel engine produces black smoke under load. Which of these is the LEAST likely cause?
A. Restricted air filter
B. Leaking charge air cooler
C. Thermostat stuck open
D. Damaged turbocharger

Answer: C. A stuck-open thermostat makes the engine run cold. The other three all reduce airflow, which causes black smoke.

2. Crankcase pressure is above spec. Technician A says worn piston rings could be the cause. Technician B says a leaking turbocharger seal could be the cause. Who is right?
A. A only
B. B only
C. Both A and B
D. Neither A nor B

Answer: C. Anything that pushes combustion gases or pressurized air into the crankcase raises crankcase pressure.

3. Which tool gives the most accurate reading of coolant freeze protection?
A. Hydrometer
B. Refractometer
C. Test strips
D. Infrared thermometer

Answer: B. A refractometer is the most accurate. Test strips are useful for checking SCA levels.

4. An intake air restriction test is most accurate when performed:
A. At low idle
B. At high idle with no load
C. At rated speed under full load
D. With the engine off

Answer: C. Restriction shows up when airflow is highest.

5. An electronically controlled engine has a miss at idle. What is the most efficient first test?
A. Replace all injectors
B. Perform a cylinder cutout/contribution test
C. Pull the cylinder head
D. Replace the fuel filters

Answer: B. It tells you which cylinder is the problem before you spend money on parts.

A simple 4-week study plan

• Week 1: General diagnosis and the fuel system. That’s 15 questions’ worth.
• Week 2: Air induction, exhaust, aftertreatment, and engine brakes.
• Week 3: Cooling and lubrication.
• Week 4: Head, valve train, and block, then full-length practice tests. Review every question you miss until you understand why the right answer is right.

Read the “Technician A / Technician B” questions slowly. Decide whether each statement is true on its own before you look at the answer choices.

Always check the official ASE Medium/Heavy Truck study guide at ase.com for the current task list before your test date. ASE updates its specifications from time to time.

Get the full ASE prep series

This is the first in a series of T-Series study guides on Learn Diesel, with practice tests on the way. Sign up on the homepage to get each new guide when it’s published.

Related: 5 Careers in the Diesel Industry