Steering Systems Diagnosis and Repair
Architecture and hydraulics
Medium- and heavy-duty trucks use an integral recirculating-ball power steering gear with a rotary control valve built into the gear, or a linkage-type assist cylinder. The pump does two separate jobs: its flow control valve sets steering response (how fast the wheels move for a given steering wheel input), and its relief valve caps maximum assist pressure. Learn to separate complaints by what the hydraulics can and cannot do. Hard steering in both directions at all speeds points to low pressure, a slipping belt, or a failed pump. Hard steering only at low engine speed in parking-lot maneuvers points to low flow or aeration. A whine that rises with load, foamy fluid, and a fluid level that climbs after shutdown point to a suction-side air leak, not a bad gear.
Mechanical checks and gear adjustment
Check tie rod ends, drag link ends, kingpins, and ball joints with the tires loaded on the ground, with a helper rocking the steering wheel while you watch for relative motion. Jacking the front end unloads the joints and hides play. Gear adjustment has a strict order: worm bearing preload first, then sector shaft lash, and lash is set only with the gear at its high point (wheels straight ahead). Setting lash off center creates a tight spot and poor returnability; over-tightening lash also causes wander.
Rule of thumb
Hard steering in one direction only is almost never a pump problem; suspect a mechanical bind, a collapsed hose, or a blocked internal passage. If a proper purge does not clear aeration, inspect the suction hose and reservoir return before condemning the pump.