Power Assist Units, ABS, and Related Electrical/Electronic Components Diagnosis and Repair
What a school-bus technician must know
Power assist may be vacuum (hydro-boost) or hydraulic (engine-driven pump). Understand that a hydro-boost uses power steering pressure to assist the master cylinder; a failed pump or low fluid causes a hard pedal. Know the vacuum booster check: pump the pedal to exhaust assist, hold, start engine-pedal should drop slightly.
For ABS, know the wheel speed sensors, ECU, modulator valve, and warning lamp. The ABS lamp must bulb-check at key-on and go out after the self-test; a lamp that stays on means a fault stored. Understand that ABS does not reduce base braking and that a sensor gap or tone ring fault is common. Know ATC (automatic traction control) if equipped, and that ABS on air buses uses modulator relay valves.
Traps in questions
- Blaming ABS for a low pedal-ABS does not cause a low pedal.
- Forgetting that a wheel speed sensor must be properly gapped and the tone ring clean.
- Assuming the ABS lamp is just a bulb when it indicates a stored fault.
- Confusing hydro-boost (hydraulic assist) with vacuum boost.
Rule of thumb
If the brake pedal is hard with the engine running on a hydro-boost bus, check power steering fluid and pump pressure before condemning the booster; if ABS lamp is on, retrieve blink or scan codes before parts replacement.