A quarry operation experienced a terrifying handling defect on a Cat 950GC wheel loader. At road speeds above 15 mph, the steering wheel would suddenly develop a violent, back-and-forth oscillation-a "shimmy"-that required the operator to grip the wheel with both hands to maintain control. At low speeds, the steering felt overly sensitive, with the slightest input causing the machine to dart left or right. The front tires showed severe, uneven scalloping on the outer lugs, indicating rapid, erratic yaw movements.
Initial suspicion fell on mechanical components: the steer axle kingpins and tie rod ends were checked for play, and the front tires were replaced and balanced. The shimmy persisted. The focus then shifted to the hydraulic pilot steering system. The 950GC uses a steering metering valve (orbital unit) that acts as a fluid meter and a mechanical link between the steering column and the main steering cylinders.
A hydraulic flow meter was plumbed into the pilot circuit. When the steering wheel was held steady, the flow to the main steer spool should have been zero. However, the gauge showed erratic, pulsing bursts of pilot fluid being sent to the left-turn port, even when the wheel was stationary. The steering metering valve was removed and disassembled. The internal metering spool and sleeve, which must rotate in precise sync, were found to be heavily scored with bronze contamination. The scoring allowed pressurized pilot fluid to bypass the metering lands, causing the spool to hunt back and forth uncontrollably. This sent rapid, alternating pressure spikes to the main steer cylinder, causing the front axle to violently jerk.
The source of the bronze shavings was discovered in the steering priority valve. The priority valve spool was caked in a hard, varnish-like residue and was sticking in the fully extended position. This restriction starved the metering valve of the steady, low-pressure pilot flow it required for smooth operation, exacerbating the spool hunting issue. The varnish was a result of severely oxidized hydraulic oil that had been operated well past its service interval.
The repair required replacing the entire steering metering valve assembly, as the scoring was too deep to hone. The priority valve was disassembled, chemically cleaned to remove the varnish, and fitted with a new spool. The hydraulic tank was drained, flushed with a solvent cycle, and refilled with fresh ISO 68 hydraulic oil. After bleeding the air from the pilot lines, the loader was road-tested; the steering tracked straight and true with zero oscillation, saving the axle components from further destruction.
