Several gauges in a tractor's cab flicker when the blower and wipers run, and a poor cab-to-frame ground strap is suspected. With the blower and wipers running, where should the DMM leads go to read the voltage drop of the strap and its bolted connections?

On cab and frame metal beside each strap bolt

On frame metal and the battery negative post

On the strap's own two ring terminals at each end

On cab metal and the battery positive post

To check whether a section of a truck's feed wire is open, a technician bypasses it with a jumper wire from a known-good feed point to the load's feed terminal. Why does the jumper carry its own fuse?

It protects against a short through the jumper

It blows to show the bypassed section is open

It limits the current to the load's normal draw

It drops the voltage to match the bypassed wire

On a wiring schematic, a diode is drawn as a triangle pointing at a short straight bar. In which direction does the diode let conventional current (positive to negative) flow?

From the triangle's side toward the bar

Both ways, once a set voltage is reached

From the bar's side toward the triangle

Only while a third terminal gets a signal

Before a conductance test, the technician enters the battery's cold cranking amp (CCA) rating into the tester. Why is the rating needed?

The tester judges its reading against what that rating should give

The tester needs it to calculate the battery's open-circuit voltage

The tester sets its carbon-pile load to one-half of that rating

The tester uses it to set the charge rate after the test

What does a low voltage disconnect (LVD) module do on a truck with a sleeper cab?

Sheds chosen loads when battery voltage stays too low

Cuts power to the starter when cranking voltage falls too low

Opens the main battery cable whenever the key is turned off

Raises the charging voltage when the batteries are discharged

Why should a truck's batteries never be disconnected with the engine running, as a check of whether the alternator can keep the engine running by itself?

The voltage surge can damage electronic modules

The alternator's field polarity can be reversed

The regulator loses its setpoint and must relearn it

The stator overheats with no battery load on it

A truck has an insulated-ground alternator, whose negative output terminal is cabled to the battery negative posts instead of grounding through the alternator housing. Any of these could raise the charging circuit's ground-side voltage drop EXCEPT:

paint on the mounting bracket's contact faces.

a corroded lug on the negative output terminal.

broken strands inside the negative output cable.

a loose connection at the battery negative post.

A tractor's right rear incandescent combination lamp goes out now and then on rough roads and comes back on by itself. Any of these could be the cause EXCEPT:

a bulb whose filament has burned open.

a weak socket spring that lets the bulb's base bounce.

a loose ground eyelet on the lamp's frame stud.

a wire broken inside its insulation at a harness clamp.

A truck's oil pressure gauge, driven directly by a variable-resistance sender, reads zero with the engine running, while a mechanical test gauge shows normal oil pressure. Which test shows whether the dash gauge and its wire respond correctly, separately from the sender?

Apply known resistances at the sender wire

Measure the sender's resistance to ground

Install a new sender and recheck the gauge

Check voltage at the sender wire, engine running

10 Free ASE T6 Electrical/Electronic Systems Questions

Ten ASE T6 Electrical/Electronic Systems questions in the ASE format: four options, one best answer. They are free to use, print and share, and there is nothing to sign up for on this page.

01How to use these

The questions come from the ToolboxPrep T6 bank and are spread across the test's content areas in the proportion the ASE test specification gives them: A (General Electrical/Electronic System Diagnosis), B (Battery and Starting System Diagnosis and Repair), C (Charging System Diagnosis and Repair), D (Lighting Systems Diagnosis and Repair), E (Related Vehicle Systems Diagnosis and Repair). Answer each one before opening the explanation. Every explanation covers all four options, because knowing why the wrong answers are wrong is most of the skill.

Pace. The T6 test gives you 90 minutes for 60 questions (50 scored, 10 unscored), about 1 min 30 s a question. Try these at that pace.
Print. The printed version shows the questions and the answer key without the explanations, so it works as a handout.

02The questions

Recall · General Electrical/Electronic System Diagnosis

Several gauges in a tractor's cab flicker when the blower and wipers run, and a poor cab-to-frame ground strap is suspected. With the blower and wipers running, where should the DMM leads go to read the voltage drop of the strap and its bolted connections?
1. On frame metal and the battery negative post
2. On the strap's own two ring terminals at each end
3. On cab and frame metal beside each strap bolt
4. On cab metal and the battery positive post
Show the answer

C. Correct. With the leads on the cab metal next to one bolt and the frame metal next to the other, the reading takes in the strap and both bolted joints, where corrosion and paint usually add resistance. The loads are on, so any resistance shows as a voltage drop to compare with the manufacturer's limit.

Why not the others

A. This reads the frame-to-battery part of the ground path, a different section that does not include the cab strap. The leads go on the cab metal and the frame metal on either side of the strap.

B. Leads on the strap's own ring terminals read only the strap and its crimps, leaving out the joints between the terminals and the cab and frame metal. Placing them on the metal beside each bolt includes those joints.

D. Cab metal to the battery positive post reads close to battery voltage and shows the supply, not the small loss across the strap. The leads go across the strap's two mounting points, cab metal to frame metal.

ID T6-A-003 · Task T6-A-1 (ASE task list)
Recall · General Electrical/Electronic System Diagnosis

To check whether a section of a truck's feed wire is open, a technician bypasses it with a jumper wire from a known-good feed point to the load's feed terminal. Why does the jumper carry its own fuse?
1. It blows to show the bypassed section is open
2. It limits the current to the load's normal draw
3. It drops the voltage to match the bypassed wire
4. It protects against a short through the jumper
Show the answer

D. Correct. If a jumper end touches ground, or lands where it bypasses the load, the jumper becomes a dead short across the source; its fuse opens before the jumper or the truck's wiring overheats. The jumper's fuse is rated no higher than the protection of the circuit being tested.

Why not the others

A. Whether the bypassed section is open shows by the load working once the jumper is in place. The fuse is there to open if the jumper accidentally creates a short to ground.

B. A fuse does not limit current while it is intact; the load's resistance sets the current. The fuse only opens if the current climbs high enough, as it would in a short to ground.

C. An intact fuse drops almost no voltage. The fuse is there to open if the jumper accidentally creates a short to ground.

ID T6-A-019 · Task T6-A-4 (ASE task list)
Recall · General Electrical/Electronic System Diagnosis

On a wiring schematic, a diode is drawn as a triangle pointing at a short straight bar. In which direction does the diode let conventional current (positive to negative) flow?
1. Both ways, once a set voltage is reached
2. From the bar's side toward the triangle
3. From the triangle's side toward the bar
4. Only while a third terminal gets a signal
Show the answer

C. Correct. The triangle is the anode and the bar is the cathode; the triangle points the way conventional current can flow. On the part itself, the painted band marks the cathode end, matching the bar in the symbol.

Why not the others

A. Conducting in reverse above a set voltage describes a zener diode, which is drawn with a bent or flagged bar. A standard diode with a straight bar passes current only from the triangle side to the bar.

B. That is the reverse direction, which a diode blocks. Current flows from the anode (the triangle) to the cathode (the bar), the way the triangle points.

D. A device with a third, controlling terminal is a transistor or a controlled rectifier and has its own symbol. A two-terminal diode conducts whenever it is forward biased, from the triangle side to the bar.

ID T6-A-037 · Task T6-A-9 (ASE task list)
Recall · Battery and Starting System Diagnosis and Repair

Before a conductance test, the technician enters the battery's cold cranking amp (CCA) rating into the tester. Why is the rating needed?
1. The tester needs it to calculate the battery's open-circuit voltage
2. The tester sets its carbon-pile load to one-half of that rating
3. The tester uses it to set the charge rate after the test
4. The tester judges its reading against what that rating should give
Show the answer

D. Correct. A conductance tester measures how readily the battery passes a small AC signal and turns that into an estimate of cranking ability. It needs the rating to judge whether that estimate is good enough for this battery; a wrong rating entered gives a wrong verdict.

Why not the others

A. Open-circuit voltage is measured directly at the posts, not calculated from a rating. The rating is the yardstick the tester compares its measured result against.

B. A conductance tester applies no heavy load; that describes a carbon-pile load test. The rating is entered so the tester can compare its measured result with what a good battery of that rating should show.

C. A conductance tester does not charge the battery. It uses the rating to judge its measured result against what a good battery of that rating should give.

ID T6-B-005 · Task T6-B-2 (ASE task list)
Recall · Battery and Starting System Diagnosis and Repair

What does a low voltage disconnect (LVD) module do on a truck with a sleeper cab?
1. Cuts power to the starter when cranking voltage falls too low
2. Sheds chosen loads when battery voltage stays too low
3. Opens the main battery cable whenever the key is turned off
4. Raises the charging voltage when the batteries are discharged
Show the answer

B. Correct. The module watches battery voltage and, when it stays below its set point for its set time, switches off non-essential loads such as sleeper accessories. That keeps enough charge in the batteries to crank the engine; the loads reconnect when voltage rises, as after a start.

Why not the others

A. Cranking voltage always dips during a start, and the module does not control the starter. It sheds chosen loads when battery voltage stays low with the engine off, to save enough charge for cranking.

C. Isolating the batteries by hand is the job of a battery disconnect switch. The module sheds only chosen loads, and only when battery voltage stays below its set point.

D. Charging voltage is set by the alternator's regulator. The module protects cranking by shedding chosen loads when battery voltage stays below its set point.

ID T6-B-019 · Task T6-B-7 (ASE task list)
Recall · Charging System Diagnosis and Repair

Why should a truck's batteries never be disconnected with the engine running, as a check of whether the alternator can keep the engine running by itself?
1. The voltage surge can damage electronic modules
2. The alternator's field polarity can be reversed
3. The regulator loses its setpoint and must relearn it
4. The stator overheats with no battery load on it
Show the answer

A. Correct. The batteries absorb the system's voltage swings; when they drop off while the alternator is charging, the regulator cannot cut the field current fast enough and output can surge far above normal for a fraction of a second (a load dump). That surge can damage the electronic modules left connected, so output is tested with meters and a load tester instead.

Why not the others

B. Reversed polarity was a concern with old DC generators, which had to be polarized after service. An alternator's rectifier fixes its output polarity; the risk here is the voltage surge when the batteries stop absorbing output.

C. An alternator's regulator does not lose a setting when the batteries are removed. The danger is the brief high-voltage surge that follows the lost load, which can damage electronic modules.

D. Without the batteries the alternator has less load, not more, so the stator does not overheat. The danger is the voltage surge as the regulator reacts to the sudden loss of load.

ID T6-C-008 · Task T6-C-2 (ASE task list)
EXCEPT · Charging System Diagnosis and Repair

A truck has an insulated-ground alternator, whose negative output terminal is cabled to the battery negative posts instead of grounding through the alternator housing. Any of these could raise the charging circuit's ground-side voltage drop EXCEPT:
1. a corroded lug on the negative output terminal.
2. paint on the mounting bracket's contact faces.
3. broken strands inside the negative output cable.
4. a loose connection at the battery negative post.
Show the answer

B. Correct. On an insulated-ground alternator the housing is not part of the charging circuit, so paint at the mounting adds no resistance to the ground side. Only the negative output cable and its connections are in the ground-side path.

Why not the others

A. The negative output terminal is where this alternator's ground current leaves, so corrosion on its lug adds resistance right in the ground path. It would show in a loaded ground-side drop test.

C. Broken strands reduce the copper carrying the ground current and raise the cable's resistance. The drop would rise with current even if the cable looks sound outside.

D. The ground-side path ends at the battery negative posts, so a loose connection there adds resistance in series with the charging current. Clean and tighten it to the manufacturer's torque.

ID T6-C-019 · Task T6-C-5 (ASE task list)
EXCEPT · Lighting Systems Diagnosis and Repair

A tractor's right rear incandescent combination lamp goes out now and then on rough roads and comes back on by itself. Any of these could be the cause EXCEPT:
1. a weak socket spring that lets the bulb's base bounce.
2. a loose ground eyelet on the lamp's frame stud.
3. a wire broken inside its insulation at a harness clamp.
4. a bulb whose filament has burned open.
Show the answer

D. Correct. A filament that has burned open is a permanent break, so the lamp would stay out instead of coming back on by itself. A lamp that drops out over bumps and recovers points to a connection or conductor that opens and closes as it moves.

Why not the others

A. A weak socket spring no longer holds the bulb's base contacts firmly against the socket terminals, so they lose touch as the lamp shakes and the lamp goes out over bumps, then returns when the base reseats. Dull or arc-marked contacts point to it, and the socket is replaced.

B. A loose ground eyelet can lose and regain contact with its stud as the frame vibrates, opening the lamp's ground path for moments at a time. Cleaning the stud and eyelet and tightening the nut fixes it.

C. Strands broken inside the insulation can separate and touch again as the harness flexes at a clamp, giving an intermittent open that a wiggle test with the lamp on can locate.

ID T6-D-004 · Task T6-D-1 (ASE task list)
Recall · Related Vehicle Systems Diagnosis and Repair

A truck's oil pressure gauge, driven directly by a variable-resistance sender, reads zero with the engine running, while a mechanical test gauge shows normal oil pressure. Which test shows whether the dash gauge and its wire respond correctly, separately from the sender?
1. Apply known resistances at the sender wire
2. Measure the sender's resistance to ground
3. Install a new sender and recheck the gauge
4. Check voltage at the sender wire, engine running
Show the answer

A. Correct. With the sender unplugged, a resistance substitution box or sender tester on the sender wire stands in for the sender at the values in the manufacturer's chart. If the gauge tracks each value, the gauge and wire are good and the sender is at fault; if not, the fault is in the wire or gauge.

Why not the others

B. Measuring the sender tests the sender only. It says nothing about whether the gauge and its wire respond, which is what is being asked; substituting known resistances at the sender wire tests those.

C. Fitting a part to see what happens is not a test, and if the wire or gauge is at fault the new sender changes nothing. Known resistances applied at the sender wire show whether the gauge and wire respond before any part is bought.

D. A voltage reading at the sender wire shows only that the gauge circuit is powered; it does not show whether the needle follows changes in resistance. Applying known resistances at the sender wire does.

ID T6-E-007 · Task T6-E-3 (ASE task list)
Recall · Related Vehicle Systems Diagnosis and Repair

An inverter supplies a 1,000-watt, 120-volt AC load from a truck's 12-volt batteries, and its maker rates its efficiency at 90 percent at this load. About how much DC current does it draw from the batteries at 12.0 volts?
1. 9.3 A
2. 75.0 A
3. 83.3 A
4. 92.6 A
Show the answer

D. Correct. The inverter's input is its output divided by its efficiency: 1,000 / 0.90 = 1,111 watts, and 1,111 / 12.0 = 92.6 amps. That is why an inverter needs heavy DC cables, a fuse near the battery and a battery bank able to supply that current.

Why not the others

A. This divides by the 120-volt AC output instead of the 12-volt battery side: 1,000 / 0.90 / 120 = 9.3 amps. The batteries must supply the power at 12.0 volts: 1,000 / 0.90 / 12.0 = 92.6 amps.

B. This multiplies by the efficiency instead of dividing: 1,000 x 0.90 / 12.0 = 75.0 amps. The inverter's losses add to what it takes from the batteries, so the input is 1,000 / 0.90 = 1,111 watts, or 92.6 amps at 12.0 volts.

C. This ignores the inverter's losses: 1,000 / 12.0 = 83.3 amps. At 90 percent efficiency the inverter takes 1,000 / 0.90 = 1,111 watts from the batteries, which is 92.6 amps at 12.0 volts.

ID T6-E-028 · Task T6-E-16 (ASE task list)

03Answer key

Question	Answer	Content area
Q1	C	General Electrical/Electronic System Diagnosis
Q2	D	General Electrical/Electronic System Diagnosis
Q3	C	General Electrical/Electronic System Diagnosis
Q4	D	Battery and Starting System Diagnosis and Repair
Q5	B	Battery and Starting System Diagnosis and Repair
Q6	A	Charging System Diagnosis and Repair
Q7	B	Charging System Diagnosis and Repair
Q8	D	Lighting Systems Diagnosis and Repair
Q9	A	Related Vehicle Systems Diagnosis and Repair
Q10	D	Related Vehicle Systems Diagnosis and Repair

04Questions by content area

Working on one part of the T6 test? Each content area has its own page with five more free questions.

05More free questions

The T6 study guide covers each content area and how to study for it, and the ASE T6 Electrical/Electronic Systems practice test page describes the full T6 question bank. The same free format is available for the other 15 tests.

How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.

ToolboxPrep is an independent study site, not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original, written from ASE’s published test specifications and task lists and other public sources. They are not actual ASE test questions and not recalled or “brain dump” questions. ASE is a trademark of its owner, and we never use the ASE logo. Standards and service information are cited, not reproduced.