Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairMedium
A vehicle exhibits a brake pulsation felt through the pedal and steering wheel during moderate braking. The technician measures the rotor runout and finds it to be within manufacturer specifications. What is the MOST likely cause of this pulsation?
- AAir in the hydraulic system.
- BA faulty brake booster.
- CWorn wheel bearings.
- DExcessive rotor parallelism (thickness variation).
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive rotor parallelism (thickness variation).
Brake pulsation is commonly caused by rotor issues. While excessive runout is a frequent culprit, if runout is within spec, the next most likely cause is excessive thickness variation (parallelism). This means the rotor's thickness varies around its circumference, causing the pads to be pushed back and forth unevenly, resulting in pulsation.
Why the other options are wrong
- A. Air in the hydraulic system causes a spongy pedal, not pulsation.
- B. A faulty brake booster typically causes a hard pedal, not pulsation.
- C. Worn wheel bearings can cause noise or looseness, but typically don't cause brake pulsation unless they lead to excessive rotor runout, which was already ruled out.
Rotor Thickness Variation
Also known as parallelism, it describes the inconsistency in a brake rotor's thickness around its circumference, which can cause brake pulsation even if rotor runout is within specification.
- Causes pedal and steering wheel pulsation.
- Rotor is not uniformly thick.
- Often a result of improper lug nut torque or overheating.
Memory trick: Pulsation is a wobble, check the rotor, then the hub.