ASE A5 ABS, Traction and Stability Control — Questions and Answers
Question 1: A technician spins a wheel by hand with a digital multimeter on AC millivolts connected across a passive (variable-reluctance) wheel speed sensor. Which reading is expected from a good sensor?
- A steady 5-volt DC signal that does not change with wheel speed
- An AC voltage that increases as the wheel is spun faster (Correct answer)
- A fixed resistance of 0 ohms that does not vary with wheel movement
- A square-wave signal with constant amplitude at every speed
Correct answer: An AC voltage that increases as the wheel is spun faster
A variable-reluctance sensor generates its own AC signal from the changing magnetic field as the tone ring teeth pass. Both amplitude and frequency rise with wheel speed.
A passive wheel speed sensor is a coil wrapped around a magnet. As the tone ring (reluctor) teeth pass the pole piece, the magnetic field changes and induces an AC voltage in the coil. Both the voltage amplitude and the frequency increase with wheel speed, so a slow hand-spin produces only a small signal. The sensor needs no supply voltage. Technicians also check coil resistance against specification and inspect the air gap and tone ring condition.
Question 2: When a Hall-effect (active) wheel speed sensor is viewed on a lab scope while the wheel turns faster, what change should be seen in the waveform?
- The amplitude rises while the frequency stays the same
- A sine wave whose amplitude drops as speed rises
- A flat line at battery voltage that never changes
- A square wave of constant amplitude whose frequency increases (Correct answer)
Correct answer: A square wave of constant amplitude whose frequency increases
Active sensors are powered by the ABS module and output a digital on/off signal. Speed is carried by frequency, not amplitude.
A Hall-effect or magnetoresistive wheel speed sensor is supplied with reference voltage by the ABS control module. It switches its output between two levels as the encoder ring or magnetic tone ring passes. The result is a square wave of fixed amplitude, and the frequency increases with wheel speed. Unlike passive sensors, active sensors can report very low speeds and sometimes direction of rotation.
Question 3: An ABS warning light comes on only above about 25 mph, and a wheel speed sensor circuit code sets intermittently. The sensor resistance and wiring test good. Which is the most likely cause?
- A cracked or damaged tone ring or debris packed on its teeth (Correct answer)
- A low brake fluid level in the master cylinder reservoir
- A worn brake pad on the same wheel
- A weak rear shock absorber on the opposite side
Correct answer: A cracked or damaged tone ring or debris packed on its teeth
A damaged, corroded or contaminated tone ring causes dropouts in the signal that appear only at certain speeds, even when the sensor itself tests good.
Static electrical tests cannot reveal problems that only appear while the wheel rotates. Missing or cracked tone ring teeth, rust buildup, metal debris on the sensor tip, an excessive air gap, or worn wheel bearing play can all cause erratic signals at speed. The module sees the signal disappear or disagree with other wheels and sets a code. Inspect the tone ring and sensor tip, check bearing play, and view live wheel speed data on a scan tool during a road test.
Question 4: After replacing an ABS hydraulic control unit (modulator), what is the recommended way to remove air trapped inside the unit?
- Run the engine at high idle while opening all bleeders at once
- Gravity bleed the unit by removing the cap and waiting 30 minutes
- Use a scan tool automated bleed procedure to cycle the valves and pump (Correct answer)
- Pump the pedal with the ignition on until the ABS lamp turns off
Correct answer: Use a scan tool automated bleed procedure to cycle the valves and pump
Air in the modulator valve body can only be expelled by commanding the solenoid valves and pump to operate, which a scan tool's bidirectional routine does.
Conventional manual or pressure bleeding clears the lines but cannot always reach air trapped in the modulator's solenoid valve passages and pump circuits. Many manufacturers require a scan tool automated bleed routine that cycles the inlet and outlet valves and runs the pump while fresh fluid is pushed through. Follow it with a normal bleed sequence in the order the manufacturer specifies and verify a firm pedal before road testing.
Question 5: A customer complains of a pulsating brake pedal and a buzzing noise during a panic stop on loose gravel. The ABS warning light stays off and no codes are stored. What should the technician conclude?
- The front rotors are warped and need to be replaced
- The ABS is operating normally and the pedal pulsation is expected (Correct answer)
- The ABS pump motor is failing and the modulator must be replaced
- The wheel speed sensors are out of range and must be adjusted
Correct answer: The ABS is operating normally and the pedal pulsation is expected
During ABS activation the modulator rapidly cycles its valves and pump, which produces pedal pulsation and noise. Low-traction surfaces trigger ABS easily.
When a wheel is about to lock, the ABS module rapidly opens and closes the hold and release valves and runs the return pump. This sends pressure pulses back to the pedal and creates buzzing or grinding sounds. Loose gravel, ice or wet pavement cause earlier ABS activation. With no warning light and no codes, the system is working as designed, and the technician should explain this to the customer.
Question 6: On a vehicle with traction control that shares the ABS hydraulic unit, what can the system do when a drive wheel starts to spin on a slippery surface?
- Release the brakes on the non-spinning drive wheel to send it more torque
- Increase throttle opening to raise engine speed
- Lock the differential by applying all four brakes equally
- Apply the brake at the spinning wheel and/or reduce engine torque (Correct answer)
Correct answer: Apply the brake at the spinning wheel and/or reduce engine torque
Traction control limits wheel spin by braking the spinning wheel and/or reducing engine output through throttle, spark or fuel control.
The module compares wheel speeds, and if a drive wheel is turning faster than the others it intervenes. Brake intervention uses the ABS pump and valves to apply pressure to the spinning wheel, which acts like a limited-slip differential. Engine torque reduction through throttle closing, spark retard or fuel cut works with it. Both actions are coordinated over the network between the ABS and engine control modules.
Question 7: A vehicle has stability control codes and the warning light on after a wheel alignment that changed the steering wheel position. Which service is most likely required?
- Replace the yaw rate sensor and the lateral accelerometer
- Flush the brake fluid and replace the master cylinder
- Calibrate or reset the steering angle sensor with a scan tool (Correct answer)
- Replace both front wheel speed sensors
Correct answer: Calibrate or reset the steering angle sensor with a scan tool
The steering angle sensor zero point must match the straight-ahead position after an alignment or steering component service. A scan tool calibration relearns it.
Stability control compares the driver's intended direction, taken mainly from the steering angle sensor, with the vehicle's actual motion from yaw rate and lateral acceleration. If the stored zero position no longer matches the actual straight-ahead position after an alignment, the data disagree and a code and warning light can result. The standard fix is the manufacturer's steering angle sensor calibration with a scan tool, normally with the wheels straight and the vehicle on level ground.
Question 8: A vehicle is brought in with an illuminated ABS light and traction control light after one tire was replaced with a significantly smaller-diameter tire. What is the most likely explanation?
- The smaller tire rotates faster, so the module sees a wheel speed mismatch (Correct answer)
- The brake fluid in the system has absorbed moisture
- The pump motor relay has failed open
- The brake pedal switch is out of adjustment
Correct answer: The smaller tire rotates faster, so the module sees a wheel speed mismatch
A smaller-diameter tire turns more revolutions per mile, so its wheel speed differs from the other wheels and the module may set a fault or disable the system.
The ABS and stability modules rely on all four wheel speeds being similar when the vehicle is traveling straight. Mismatched tire diameters, such as a temporary spare or a different size, create a constant speed difference. The module may read this as a sensor fault or a wheel slipping, and it can set a code or inhibit ABS and traction functions. Installing matching tires, or following the manufacturer's size rules, resolves the problem.
Question 9: Before disconnecting a brake line on an integral ABS with a high-pressure accumulator, what must be done first to relieve stored pressure?
- Open all four bleeder screws while the pump motor is running
- With the ignition off, pump the brake pedal many times until it feels hard
- Turn the ignition on and hold the pedal down for 10 seconds
- Disconnect the wheel speed sensors and drive the vehicle (Correct answer)
Correct answer: Disconnect the wheel speed sensors and drive the vehicle
Accumulator pressure can reach roughly 2,000 psi or more. It is relieved by pumping the pedal with the ignition off until the pedal becomes firm.
Integral systems (such as Teves Mark II) use a pump and nitrogen-charged accumulator to supply very high pressure for power assist. Opening a line while charged can spray fluid and cause injury or damage painted surfaces. With the ignition off, the technician pumps the pedal, typically 25 or more times, until a significant increase in pedal effort is felt. Always follow the service information for the specific system.
Question 10: When checking the air gap of an adjustable wheel speed sensor, which tool should the technician use?
- A steel feeler gauge placed between the sensor and the tone ring
- A micrometer measuring the sensor body diameter
- A nonmagnetic (brass or plastic) feeler gauge or paper shim (Correct answer)
- A dial indicator mounted on the brake rotor
Correct answer: A nonmagnetic (brass or plastic) feeler gauge or paper shim
A steel gauge would be attracted to the sensor magnet and give a false reading, so nonmagnetic feeler gauges or specified shims are used.
Wheel speed sensor air gap affects signal strength; a gap that is too wide can cause weak or erratic readings at low speed. Because passive sensors contain a magnet, a steel feeler gauge will be pulled toward it and distort the measurement. Technicians use a nonmagnetic brass feeler gauge, a plastic gauge, or the manufacturer's paper spacer. Many modern sensors are non-adjustable and the gap is set by design, so always check service information.
ASE A5 ABS, Traction and Stability Control
A technician spins a wheel by hand with a digital multimeter on AC millivolts connected across a passive (variable-reluctance) wheel speed sensor.
Which reading is expected from a good sensor?
What Practice Test Geeks adds beyond an AI chat for ASE A5
An AI chatbot can quiz you, but it can't reproduce the exam conditions or keep your record. Here you get:
- A timed, full-length ASE A5 mock exam — 55 questions in 75 minutes with a 70% pass mark, modeled on the official exam's question count and time limit, followed by a score report broken down by exam domain.
- A fixed ASE A5 question bank: the same test every attempt, so your score is comparable from one try to the next instead of changing with every regenerated quiz.
- Progress saved on your device: completed tests, scores and missed questions stay in My Progress, so you can retake only what you got wrong.
- A printable ASE A5 cheat sheet you can print or save as a PDF.
- ASE A5 flashcards built from the same question bank.
These practice questions were written with AI tools. They are not real exam questions. Exam facts are checked against the official sources listed below. Not reviewed by a subject-matter expert. Editorial policy
Sources & references
Exam facts on this page (fees, format, dates, passing scores) are checked against these official sources; the date is our last check. Editorial policy
- ase.com — Eligibility, registration deadline, test dates, test delivery, retake policy, score release, score validity · verified Nov 2026