A municipal utility department reported that a Terex TLB840 backhoe loader had developed steering faults that worsened at highway speeds. At low speed during bucket or breaker work, steering response was normal; however, when the machine was driven at approximately 20 km/h on paved roads, the steering wheel became abnormally heavy, and the front axle would shake left-and-right when the machine passed over bumps - requiring excessive operator correction to keep the machine tracking on the road.
The technician first verified the hydraulic oil level (correct) and inspected the steering pump's drive belt (correct tension). He then installed a pressure gauge at the steering pump's outlet and measured pressure at idle and at travel RPM. At idle, steering pressure read 95 bar (spec is roughly 120 bar). At travel RPM, the pressure read 110 bar - still below spec, indicating the steering pump's displacement was declining rather than increasing with RPM. This is characteristic of an internally worn steering pump, where internal leakage increases proportionally with speed.
He then tested each steering cylinder individually with the engine off and the machine parked. By disconnecting each cylinder's rod-end and cap-end hydraulic lines and applying external pressure, he measured each cylinder's "internal bypass." The left steering cylinder showed a leak rate of 8 liters per minute from cap-end to rod-end (spec maximum 0.5 l/min) - severe internal bypass. The right cylinder was within spec. This bypass would have caused asymmetric steering response, explaining the high-speed shake: as ground shocks moved the cylinder rod within the bore, the bypass allowed one cylinder to release pressure while the other held it, causing the front axle to oscillate.
He also inspected the steering system's pilot filter - the element was visibly contaminated with particulate, indicating long-neglected hydraulic oil maintenance had allowed contaminants into the steering pump and accelerated its wear.
The root cause was traced to internal oil contamination (from neglected filter changes) combined with steering pump component wear. The neglected pilot filter allowed contaminants to circulate throughout the hydraulic circuit; the contaminated oil had accelerated wear on the steering pump's gears and bushings, and had also damaged the left steering cylinder's piston seal.
The repair involved replacing the steering pump with a new unit, resealing the left steering cylinder with a honed bore and new piston seal assembly, and replacing the steering pilot filter with fresh specified hydraulic oil. The hydraulic system was flushed to remove residual contaminants, and the return filter was replaced. After reassembly, steering pressure was restored to 120 bar at idle and 135 bar at travel RPM. A full-load road test showed immediate steering response, no front-axle shimmy, and normal steering effort. Steering pilot filter maintenance was added to the machine's 500-hour checklist.
