ASE Certification Guide

Medium/Heavy Truck · Test T6 · Electrical/Electronic Systems

ASE T6 practice test: truck electrical systems

T6 covers truck electrical and electronic systems: 60 questions (50 scored) in 90 minutes, which works out to the most time per question in the series. General diagnosis and related vehicle systems (gauges, wipers, locks, auxiliary power, telematics) carry about half of the scored questions.

  • 50 scoredquestions that count
  • 90 mintesting time
  • 25questions on the recert test
  • 11practice questions here

What T6 covers

Checked Oct 23, 2026 against each source linked in the table. Content areas and scored questions from the spec table in the 2026 study guide, p. 32.
FactDetailSourceChecked
T6 questions60 questions (50 scored + 10 unscored), 90 minutesASE Medium/Heavy Truck Study Guide 2026 · ASE: T1–T8 tests page
Master statusT2–T8 make Master Medium-Heavy Truck Technician; T1 is not requiredeach certification must stay current to keep Master statusASE: T1–T8 tests page · ASE Medium/Heavy Truck Study Guide 2026
Official ASE practice testASE sells official practice tests for T2 (28 questions), T4, T6 and T8 (25 each), 60 days to finishASE shows no price on the pageASE: official practice tests
A · General Electrical/Electronic Diagnosis14 scored questions (28%)Study guide, p. 32—
B · Battery and Starting11 scored questions (22%)Study guide, p. 32—
C · Charging7 scored questions (14%)Study guide, p. 32—
D · Lighting6 scored questions (12%)Study guide, p. 32—
E · Related Vehicle Systems12 scored questions (24%)Study guide, p. 32—

T6 practice questions

0 of 11 answered · 0 correct
Question 1 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

With the engine running, a DMM reads 14.2 V at the alternator output terminal and 12.3 V at the batteries. A clamp-on ammeter shows almost no current in the output cable. The cause could be:

Explanation

Correct: Normal voltage at the alternator but no current reaching the batteries points to an open in the output circuit, such as a blown fuse or fusible link.

Incorrect: A low regulator set point would show low voltage at the alternator, but it reads 14.2 V there.

Incorrect: A slipping belt would lower voltage at the alternator as well, not just stop current in the output cable.

Incorrect: An open diode reduces output current, but current would still reach the batteries.

Question 2 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

After repeated cranking, one battery cable terminal is noticeably hot while the rest of the cable stays cool. This indicates:

Explanation

Correct: Heat at one terminal while the cable stays cool means resistance is concentrated at that connection, so it needs cleaning and tightening.

Incorrect: Normal cranking current warms the whole cable evenly; a single hot terminal is not normal.

Incorrect: An undersized cable would warm along its whole length, not just at one end.

Incorrect: A shorted cell affects battery voltage and charge, not heat at one terminal.

Question 3 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

A starter is being replaced on a diesel truck. Technician A says the batteries need not be disconnected if the key is off. Technician B says to confirm the replacement starter's pinion tooth count and inspect the flywheel ring gear. Who is right?

Explanation

Incorrect: Batteries are disconnected before any starter work because the battery cable at the starter is always live, key on or off; Technician A is wrong.

Correct: Checking the pinion tooth count and the ring gear avoids poor mesh and gear damage; Technician A is wrong because the battery cable at the starter is live even with the key off.

Incorrect: Only Technician B is right; the starter's battery terminal is live regardless of key position.

Incorrect: Technician B is right: the replacement pinion must match and the ring gear should be inspected.

Question 4 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

Several electronic systems on a truck malfunction at the same time. Technician A says a data-link (CAN bus) fault can cause multiple systems to malfunction at once. Technician B says disconnecting modules with the key on is an accepted way to isolate a bus fault. Who is right?

Explanation

Correct: A data-link fault can take out the messages several modules depend on, so several systems fail together; Technician B is wrong because modules are unplugged with the key off.

Incorrect: Unplugging modules with the key on can cause voltage spikes and set extra codes; modules are disconnected with the key off, so Technician B is wrong.

Incorrect: Only Technician A is right; modules are isolated with the key off, not on.

Incorrect: Technician A is right: a CAN bus fault can make many systems malfunction at the same time.

Question 5 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

This question contains the word EXCEPT. A truck's cab blower operates on high speed only; all lower speeds are dead. Any of these could be the cause EXCEPT:

Explanation

Incorrect: A broken wire to the resistor cuts the lower speeds, while high is fed directly.

Incorrect: An open resistor assembly kills every speed that runs through it, leaving only high, which bypasses it.

Incorrect: An open thermal limiter opens the resistor circuit, so only high speed works.

Correct: A failed high-speed relay would kill the high speed, the one speed that still works; it cannot explain dead lower speeds.

Question 6 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

An A/C compressor clutch will not engage. When the clutch is commanded on, 12.4 V and a good ground are present at the clutch coil, and coil resistance is within specification. The cause could be:

Explanation

Incorrect: An open pressure switch would also remove power from the coil, but power is present.

Incorrect: A failed relay would leave no voltage at the coil, but 12.4 V is there.

Correct: With voltage, ground and coil resistance all good, the magnet cannot pull the plate across an air gap that is too wide.

Incorrect: The coil resistance is within spec, so its winding is not open.

Question 7 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

This question contains the word EXCEPT. A truck's electronic primary air pressure gauge reads about 20 psi lower than a master gauge installed at the primary reservoir. Any of these could be the cause EXCEPT:

Explanation

Incorrect: A low 5 V reference lowers the sensor's output signal, so the gauge reads low.

Incorrect: A wrong calibration in the cluster can display a pressure that is consistently off.

Incorrect: A faulty pressure sensor can send a reading that is lower than real pressure.

Correct: A leaking drain valve really lowers reservoir pressure, so the master gauge would read low as well; it cannot explain the dash gauge reading 20 psi lower.

Question 8 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

A truck's batteries are deeply discharged and the charging system is suspected. Technician A says the batteries should be recharged by running the engine at fast idle before any testing. Technician B says the alternator can be tested accurately with the batteries still discharged. Who is right?

Explanation

Incorrect: An alternator is built to maintain charged batteries, not to recover deeply discharged ones; running it at full output that long overheats it, so Technician A is wrong.

Incorrect: Alternator tests assume charged batteries; discharged ones pull the voltage down and hide the true output, so Technician B is wrong.

Incorrect: Both statements are wrong: discharged batteries are recharged with a charger, and the alternator is tested only after that.

Correct: The batteries are recharged with a battery charger and then tested, and only then is the alternator tested, so neither statement is right.

Question 9 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

A trailer's LED clearance lamps are dim. Technician A says an LED lamp cannot be affected by ground resistance because it draws so little current. Technician B says a reading of 12.6 V at the socket with the lamp removed proves the circuit is good. Who is right?

Explanation

Incorrect: LEDs draw little current but are very sensitive to voltage, so ground resistance can dim them; Technician A is wrong.

Incorrect: With no lamp in the socket there is no current flow, so the reading cannot show voltage drop; Technician B is wrong.

Incorrect: Both claims miss how voltage drop works: LEDs are voltage sensitive, and drop shows up only under load.

Correct: Ground resistance can dim LEDs, and a voltage reading with no lamp installed proves nothing; the circuit must be tested under load.

Question 10 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

A truck has dim lights and low charging voltage. Which single fault could cause both symptoms?

Explanation

Incorrect: An open tail lamp ground puts out the tail lamps; it does not lower charging voltage.

Incorrect: A blown trailer fuse kills the trailer lights; it does not dim the truck's lights or lower charging voltage.

Correct: A poor engine ground strap adds resistance to the alternator and lighting return paths, so charging voltage drops and the lights dim.

Incorrect: A faulty headlamp switch could dim the headlamps, but it would not lower charging voltage.

Question 11 of 11 · T6 Electrical/Electronic Systems · 50 itemsNot answered

A solenoid-controlled valve does not actuate. Supply voltage at the connector is correct, and coil resistance measures infinite (OL). Which action should the technician take NEXT?

Explanation

Incorrect: The ground path does not matter yet: a coil that reads OL is open and will not work with any ground.

Incorrect: The supply voltage is already correct; the coil reading of OL explains the fault.

Incorrect: Testing the ECU driver comes after the coil is proven good; an open coil already explains the fault.

Correct: An open (OL) coil cannot make a magnetic field no matter how good the supply is, so the coil assembly is replaced.

Answer every question to see your score

Area by area

General electrical/electronic diagnosis

14 scored questions: voltage, voltage drop, current and resistance readings, shorts and opens, parasitic draw, circuit protection, relays and diodes, wiring diagrams, waveforms, and faults on the multiplexed data network. Fault codes come as a suspect parameter (SPN) plus a failure mode (FMI): FMI 3 is voltage above normal or shorted high, FMI 4 voltage below normal or shorted low, FMI 5 current below normal or an open circuit.

Battery & starting

11 scored questions: state of charge by terminal voltage, load and capacitance tests, charging methods by battery type, jump-starting, low-voltage disconnects, cranking voltage and voltage drop, starter controls and cranking inhibit systems. Charge a low battery before load testing it.

Charging

7 scored questions: no-charge, low-charge and overcharge complaints, AC ripple and DC output, charging circuit voltage drop, alternators and their regulators.

Lighting

6 scored questions: headlight aim and switches, truck and trailer lighting circuits, the trailer light cord, stoplights, turn signals and hazards, backup lights and alarms.

12 scored questions: gauges and electronic clusters, warning devices, horn, airbag circuit, wipers and washers, mirrors, power windows and locks, auxiliary power units, cruise control, cooling fan and fuel pump controls, telematics and electronic logging devices.

About 60 ohms is the healthy reading

A J1939 backbone has a 120-ohm terminating resistor at each end. With the key off, the two in parallel read about 60 ohms across CAN high and CAN low. About 120 ohms means one terminator is missing or open; a reading near zero means the two wires are shorted together.

Truck or car electrical

T6 and the car test A6 cover similar circuits on different vehicles, and each is a certification of its own. T6 is also one of the four tests with an official practice test; the table above has the row.

Other T-series tests

T6, quick answers

How many questions are on the ASE T6 test?

60 questions in 90 minutes; 50 count toward the result. To recertify: 25 questions, 45 minutes.