Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairMedium
A customer complains of a grinding noise coming from the front brakes. During inspection, the technician finds that the outer brake pad on the passenger side is completely worn down to the backing plate, while the inner pad has significant friction material remaining. Which of the following is the most probable cause?
- AA seized caliper piston.
- BAn incorrectly installed brake rotor.
- CA damaged or missing caliper guide pin.
- DAir in the brake line to that caliper.
Show answer & explanationAnswer & explanation
Correct answer: C. A damaged or missing caliper guide pin.
If the outer pad is completely worn while the inner pad has material remaining, it indicates that the caliper is not floating properly. A damaged or missing caliper guide pin would prevent the caliper body from sliding inward, causing the outer pad to drag and wear excessively while the inner pad is not fully engaged.
Why the other options are wrong
- A. A seized caliper piston would typically cause the inner pad (piston side) to wear excessively, not the outer pad.
- B. An incorrectly installed brake rotor (e.g., warped) would cause pulsation or uneven wear across its surface, but not typically a significant inner-outer pad wear difference on a single caliper.
- D. Air in the brake line would result in reduced braking effectiveness and a spongy pedal, generally affecting both pads on the caliper equally (or none at all).
Outer Pad Wear Cause
Excessive outer brake pad wear with inner pad remaining indicates a caliper's inability to float, often due to seized or missing guide pins.
- Common in floating caliper designs.
- Caliper body cannot move freely on its mounting.
- Results in uneven pressure application.
Memory trick: Outer pad worn thin, guide pin's where the problem begins.