Wander at Highway Speed After Steering Gear Replacement
A 2019 conventional school bus with 168,000 miles had its power steering gear replaced after a seal leak. The driver now reports the bus wanders and needs constant small corrections above 45 mph, worse when loaded. The shop confirmed tire pressures are at placard, front tires are matched, and there is no play at the tie rod ends or kingpins. Turning the wheel with the engine off, the technician measures about 1.5 inches of free play at the rim before the pitman arm moves. With the engine running, the wheel has light on-center feel and the return-to-center after a turn is slow. The drag link and intermediate shaft joints were checked and are tight. The gear was installed with the wheels straight ahead and the steering wheel centered.
1. The technician wants to tighten the mesh preload screw on the gear to remove the free play. Evaluate whether this is the correct next step and explain what the measured free play and on-center feel suggest.
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Tightening the preload screw is the wrong move here. The free play was measured with the engine off and about 1.5 in at the rim, and the on-center feel is light with slow return. That pattern points to excess lash in the recirculating-ball gear or an improperly adjusted gear, not to a worn linkage. However, preload adjustment on a steering gear is a bench/spec procedure and must be done with the gear at center and the pitman arm disconnected; adjusting it in the vehicle to mask free play can cause binding off-center and steering lockup. The correct sequence: confirm the free play is in the gear (not the column, intermediate shaft, or wheel), verify the gear is at true center, then follow the manufacturer's adjustment procedure with the linkage disconnected. If the gear cannot be brought into spec, it is defective and should be replaced under warranty.
2. List the checks you would perform to prove the free play is inside the gear rather than in the column or intermediate shaft before any adjustment.
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- With the engine off, have an assistant hold the steering wheel while you watch the intermediate shaft and its universal joints; any relative motion between the wheel and the shaft means column/joint play. 2) Watch the pitman arm while rocking the wheel; movement of the shaft input without corresponding pitman arm movement localizes lash to the gear. 3) Check the steering shaft support bearing and tilt/telescope mechanism for looseness. 4) Verify the gear is bolted tight to the frame and the frame is not cracked. 5) Confirm the wheels are straight ahead and the steering wheel is centered so the gear is at its high point; lash measured off-center will read falsely high. Only after the play is confirmed inside the gear do you adjust or replace it.