Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairMedium
A technician is diagnosing a vehicle with an electric vacuum pump that supplies vacuum to the brake booster. The customer reports a hard brake pedal on cold starts and occasional extended stopping distances. The technician finds no vacuum leaks in the lines or booster, and the engine vacuum is normal when measured directly at the intake manifold. What is the MOST likely cause?
- AA leaking power steering hose.
- BA malfunctioning electric vacuum pump.
- CA faulty brake pedal position sensor.
- DA clogged PCV valve.
Show answer & explanationAnswer & explanation
Correct answer: B. A malfunctioning electric vacuum pump.
If the electric vacuum pump is malfunctioning, it won't be able to adequately supply vacuum to the booster, especially on cold starts when engine vacuum might be lower or during conditions where the engine isn't producing enough vacuum (e.g., modern GDI engines). This leads to a hard pedal and extended stopping distances.
Why the other options are wrong
- A. A leaking power steering hose would cause fluid loss and reduced power steering assist, but does not affect the brake booster's vacuum supply.
- C. A faulty brake pedal position sensor would typically affect cruise control, ABS, or other electronic systems, not directly cause a hard pedal due to lack of vacuum.
- D. A clogged PCV valve can affect engine idle quality and oil consumption but does not directly cause a hard brake pedal by reducing booster vacuum, especially if engine vacuum is otherwise normal.
Electric Vacuum Pump Failure
A malfunction of an electrically operated pump designed to augment or provide vacuum for the brake booster, especially in vehicles with low engine vacuum.
- Leads to a hard brake pedal due to insufficient vacuum assist.
- Symptoms are more pronounced on cold starts or under specific driving conditions.
- Requires checking pump operation and its electrical circuit.
Memory trick: Electric pump is the 'vacuum maker'; if it stops, vacuum 'starves'.