ASE A5 ABS, Traction and Stability Control

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?

Accepted Answer

An AC voltage that increases as the wheel is spun faster

Answer

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.

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?

Accepted Answer

A square wave of constant amplitude whose frequency increases

Answer

Active sensors are powered by the ABS module and output a digital on/off signal. Speed is carried by frequency, not amplitude.

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?

Accepted Answer

A cracked or damaged tone ring or debris packed on its teeth

Answer

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.

Question 4

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

Accepted Answer

Use a scan tool automated bleed procedure to cycle the valves and pump

Answer

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.

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?

Accepted Answer

The ABS is operating normally and the pedal pulsation is expected

Answer

During ABS activation the modulator rapidly cycles its valves and pump, which produces pedal pulsation and noise. Low-traction surfaces trigger ABS easily.

ASE A5 Practice Test

ASE A5 Practice Test

ASE A5 ABS, Traction and Stability Control