Power Steering Assist Systems
What the candidate must know
Most school buses use hydraulic power steering (engine-driven vane pump, reservoir, pressure and return lines, integral gear with rotary spool valve and piston). Some use electro-hydraulic or electric power steering on newer chassis. Know the function of the flow control valve, pressure relief valve, and the rotary/torsion-bar spool valve that proportions assist to steering effort. Understand fluid type and contamination (ATF vs. dedicated PS fluid), air entrainment causing whine and foaming, belt condition and tension, and line restriction or kinked return line causing hard steering and pump noise.
Traps in questions
- Assuming low assist is always the pump - a restricted return line, clogged reservoir filter, or failed relief valve produces the same symptom.
- Forgetting that air in the system (foamy fluid, whine) is bled by turning lock-to-lock with the engine running and the reservoir cap off, then rechecking level.
- Mixing fluid types, which can swell seals and cause leaks.
- Ignoring that a sticking spool valve can cause a self-steer or pull condition independent of alignment.
Rule of thumb / worked example
For a hard-steer complaint with a noisy pump, check in this order: fluid level and condition, belt tension/condition, return-line restriction, then pressure test at the pump port against the manufacturer's spec at idle and at relief. If pressure is within spec but assist is poor, suspect the gear's internal valve or piston seal, not the pump. Replace contaminated fluid and bleed before condemning components.