Five free practice questions on ABS, ATC, stability control and collision mitigation: content area C of the ASE T4 Brakes test, 7 of its 50 scored questions (14%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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A tractor's ABS module lists one of its stored DTCs as inactive. What does that mean?
- The fault is present, but only while the truck is moving.
- The fault is present now, but the lamp has been switched off.
- The fault was repaired, and its code was cleared from memory.
- The fault was detected earlier but is not present now.
Show the answer
D. Correct. An inactive (stored or historical) DTC records a fault the module saw earlier that is not present now, usually with an occurrence count. It is a lead for an intermittent fault: check the circuit it names, including a wiggle test, before clearing it.
Why not the others
A. A fault the module sees while the truck is moving is listed as active while it is present. Inactive means the fault is not present at the moment, whatever conditions set it before.
B. A fault that is present now is listed as active and keeps the ABS lamp on. Inactive means the module saw the fault before but does not see it now.
C. A code cleared from memory would not be listed at all. An inactive code is still in memory: the record of a fault that is not present now (on some systems a cleared active code moves into this history), kept so the technician can find an intermittent problem.
ID T4-C-005 · Task T4-C-2 (ASE task list)
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During an ABS stop with ice under a tractor's left wheels and dry pavement under its right, the ABS limits how far the right steer brake's pressure can rise above the left's. What is the purpose of this?
- To keep the trailer from swinging out during the stop
- To keep the right steer wheel from locking on dry pavement
- To keep lining wear even between the two steer brakes
- To keep the steering wheel pull manageable for the driver
Show the answer
D. Correct. Unequal steer brake forces on split traction pull the steering wheel toward the high-traction side, so the steer axle's pressure difference is limited to keep that pull within what the driver can hold. It gives up a little stopping force on the dry side in exchange for control.
Why not the others
A. Trailer swing is controlled by the trailer's own ABS and by keeping the tractor's wheels rolling. The limit on the steer axle's pressure difference is there to reduce the steering wheel pull from uneven steer brake forces.
B. Each steer wheel's lock-up is already handled by its own modulator. Limiting the dry-side pressure gives up stopping force that wheel could use; it is done to tame the steering pull, not to prevent lock-up.
C. Lining wear is not an ABS function, and a brief split-traction stop does little to it. The limit exists to keep uneven steer brake forces from pulling the steering wheel out of the driver's hands.
ID T4-C-009 · Task T4-C-3 (ASE task list)
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Both drive wheels of a tractor with ATC spin together as it accelerates on an icy grade. How does the ATC reduce the spin?
- It switches on the engine compression brake to slow the wheels.
- It applies the trailer brakes through the trailer control line.
- It asks the engine ECM over the data link to cut engine torque.
- It applies the steer axle brakes to hold back the tractor.
Show the answer
C. Correct. Braking one wheel only helps when the other grips; with both spinning, the ATC module requests reduced engine torque from the engine ECM over the J1939 data link until the wheels regain traction. If that data link is lost, this part of ATC cannot work.
Why not the others
A. An engine compression brake works only with the driver off the throttle; it does nothing to counter wheel spin under power. The ATC asks the engine ECM to reduce torque instead.
B. Applying trailer brakes is a stability control action used to keep a combination from rolling over or jackknifing. ATC deals with drive wheel spin by braking a wheel and by asking the engine for less torque.
D. Braking the steer axle would not stop the drive wheels from spinning, since it does nothing to the torque reaching them. The ATC module asks the engine ECM to cut torque.
ID T4-C-013 · Task T4-C-5 (ASE task list)
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A tractor's forward radar, mounted low in the center of the front bumper, reports a blocked condition and turns off collision mitigation. Any of these could cause this EXCEPT:
- packed snow and ice over the radar cover.
- an aftermarket metal grille guard in front of the radar.
- metallic-flake paint applied over the radar cover.
- a plastic bug deflector mounted on the hood.
Show the answer
D. Correct. A hood deflector sits above and well behind the face of a bumper-mounted radar, outside the beam that leaves the front of the sensor, so it cannot block it. Blockage comes from something in the beam's path ahead of the radar, such as snow and ice, a metal guard or metallic paint on the cover.
Why not the others
A. This could cause it. Snow and ice absorb and scatter the radar signal, and the radar reports itself blocked until the cover is cleared.
B. This could cause it. Metal in the beam's path reflects the signal back at the sensor, which can read as a blocked radar; the radar and truck makers advise against guards that intrude on the radar's field of view.
C. This could cause it. Metal flakes in paint reflect and weaken radar signals, which is why the maker restricts what can be applied over the radar cover.
ID T4-C-020 · Task T4-C-8 (ASE task list) · NHTSA
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Why should a DMM probe not be pushed into the front of a female terminal in an ABS harness connector?
- It adds enough resistance to spoil the reading.
- It can spread the terminal so it grips its pin loosely.
- It can feed meter current back into the module.
- It can bend the connector's lock tab so it won't latch.
Show the answer
B. Correct. A probe is larger than the mating pin, so it opens up the female terminal's contact, and the reconnected pin then makes a loose, intermittent contact. Use the maker's matching test adapter or a breakout lead, and check a suspect terminal's grip with a matching male terminal.
Why not the others
A. A probe's contact resistance is tiny compared with an ABS circuit's values. The harm is mechanical: the oversize probe spreads the female terminal.
C. A DMM draws almost no current on voltage and sends very little on resistance, so this is not why probing a terminal's face is avoided. The concern is the oversize probe stretching the terminal.
D. Probing the terminal's face does not reach the connector's lock tab. The risk is to the terminal itself, which the oversize probe spreads.
ID T4-C-023 · Task T4-C-10 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | D |
| Q2 | D |
| Q3 | C |
| Q4 | D |
| Q5 | B |
03What area C covers
Here the subject is the electronic controls that work through a truck's air or hydraulic brakes: antilock brakes on the tractor and trailer, automatic traction control, roll and electronic stability control, and radar and camera collision warning and mitigation. The area takes in wheel speed sensors and modulator valves, steering angle, yaw rate and pressure sensors, blink codes and scan tools, harness repair, calibration, and faults from vehicle modifications.
The ASE test specification calls this content area “Antilock Brake System (ABS), Automatic Traction Control (ATC), Electronic Stability Control System, and Collision Warning/Avoidance System Diagnosis and Repair”. It carries 7 of the 50 scored questions, 14% of the test.
- Watch the ABS self-test and the tractor and trailer lamps
- Read blink codes and scan tool DTCs
- Test wheel speed sensors, tone rings and modulator valves
- Diagnose traction control and stability control faults
- Calibrate steering angle, yaw rate and pressure sensors
- Aim and calibrate collision warning radar and cameras
- Repair harnesses; trace faults to modifications or other systems
- Clear codes and confirm the repair
04What the questions turn on
What an unpowered modulator does
Each drive-axle modulator sits between the relay valve and its own chamber. Unpowered, it passes relay valve air straight through, so an open solenoid circuit leaves that brake working from the treadle, but with no antilock at that wheel.
One wheel spinning or both
When one drive wheel spins, traction control brakes it so torque goes through the differential to the wheel with grip. When both spin, the module asks the engine ECM over the J1939 data link to cut torque, which cannot happen if that link is lost.
Calibrate after the work is done
A steering angle sensor is calibrated after the alignment, since it records the straight-ahead point. A camera refitted to new glass is calibrated, and a replaced radar has its aim checked with the maker's tools, because misaim can cause false alerts without a DTC.
Fix the ABS fault first
Collision mitigation brakes the truck through the ABS and stability valves. If the radar reports brake control unavailable while the ABS module holds an active wheel speed sensor code, that sensor fault is repaired first, then codes are cleared and mitigation braking confirmed.
05The other content areas
The T4 test has three content areas. Each has its own page of free questions:
The ASE T4 Brakes practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
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