Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairMedium
A technician is diagnosing an ABS warning light on a vehicle that also exhibits a 'hard' brake pedal and extended stopping distances, particularly during sudden stops. The scan tool shows no specific ABS fault codes, but the brake booster vacuum line and check valve have been confirmed to be in good condition. What ABS-related system component should the technician investigate NEXT?
- AThe master cylinder for internal leakage.
- BWheel speed sensors for inaccurate readings.
- CThe ABS hydraulic control unit (HCU) for internal valve issues.
- DThe power assist unit (PAU) or vacuum pump for the brake booster.
Show answer & explanationAnswer & explanation
Correct answer: D. The power assist unit (PAU) or vacuum pump for the brake booster.
A hard brake pedal and extended stopping distances, even with a good vacuum line and check valve, strongly suggest a lack of power assist. The power assist unit (PAU) or a dedicated vacuum pump (in some vehicles) provides the vacuum for the brake booster, and its malfunction would directly lead to these symptoms.
Why the other options are wrong
- A. Internal master cylinder leakage would cause a 'spongy' or sinking pedal, not a hard pedal.
- B. Inaccurate wheel speed sensors would primarily affect ABS function (pulsating pedal, warning light) but wouldn't typically cause a 'hard' pedal or extended stopping distances on their own.
- C. Internal HCU valve issues would primarily affect ABS modulation and pedal feel during ABS events, not the fundamental power assist responsible for a hard pedal.
Power Assist Unit (PAU) / Vacuum Pump
In vehicles using engine vacuum or a dedicated electric pump for brake assist, the Power Assist Unit (PAU) or vacuum pump generates the necessary vacuum for the brake booster. A failure results in a 'hard' pedal and reduced braking efficiency.
- Provides vacuum for brake booster.
- Failure causes hard pedal, extended stopping.
- Crucial for modern braking systems.
Memory trick: Hard Pedal: Think Vacuum Power!