ABS, Traction and Stability Control Flashcards

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?

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.

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?

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.

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?

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.

After replacing an ABS hydraulic control unit (modulator), what is the recommended way to remove air trapped inside the unit?

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.

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?

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.

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?

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.

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?

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.

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?

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.

Before disconnecting a brake line on an integral ABS with a high-pressure accumulator, what must be done first to relieve stored pressure?

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.

When checking the air gap of an adjustable wheel speed sensor, which tool should the technician use?

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.