Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A vehicle with an Anti-lock Braking System (ABS) has a 'hard pedal' symptom, but only when the ABS activates during aggressive braking. Normal braking feels fine. Which of the following components is most likely causing this issue?
- AA faulty wheel speed sensor
- BA contaminated master cylinder
- CA failing ABS hydraulic control unit (HCU) pump motor
- DA restricted brake hose
Show answer & explanationAnswer & explanation
Correct answer: C. A failing ABS hydraulic control unit (HCU) pump motor
A hard pedal specifically during ABS activation, with otherwise normal braking, points to an issue with the ABS HCU's ability to modulate pressure. The pump motor maintains pressure during ABS operation; if it's failing, the system struggles to release and reapply pressure smoothly, resulting in a hard pedal feel when it tries to engage.
Why the other options are wrong
- A. A faulty wheel speed sensor would typically cause incorrect ABS activation or a warning light, not a hard pedal during activation.
- B. A contaminated master cylinder would affect normal braking as well, likely causing a spongy or sinking pedal.
- D. A restricted brake hose would cause a pull or drag, affecting normal braking, not just ABS activation.
ABS HCU Pump Motor Failure
A malfunction of the electric pump motor within the ABS Hydraulic Control Unit, which is responsible for maintaining and modulating brake fluid pressure during ABS events.
- Causes issues specifically during ABS activation
- Can result in a hard pedal or poor ABS performance
- Often accompanied by ABS warning light
Memory trick: ABS pedal's hard, pump motor's tired, pressure control is now misfired!