A lumber yard reported that a Caterpillar DP70N (5-ton diesel) forklift's steering felt extremely heavy. When the operator tried to turn the steering wheel to the full lock (either direction), the engine would violently lug down and stall. If the operator eased off the wheel, the engine would recover.
The technician first checked the power steering pump and the Orbitrol (steering control unit). He disconnected the steering cylinder lines and found that the flow to the cylinder was weak. He then checked the main hydraulic pump pressure while turning the steering wheel. The pressure spiked instantly to 250 bar (main relief), and the engine stalled.
The Cat DP70N uses a "Priority Flow Divider Valve" mounted on the main hydraulic pump. This valve ensures the steering circuit receives a guaranteed flow of oil before the lift/tilt circuit. The technician removed the priority valve housing. The internal "Steering Priority Spool" was seized solid in the "Lift" position due to extreme varnish buildup and a lack of lubrication.
Because the priority spool was stuck, it was routing all the pump's flow to the lift/tilt control valve and completely starving the steering circuit. When the operator turned the steering wheel, the Orbitrol opened, but there was no oil flow available to the steering cylinder. Instead, the trapped oil in the dead-ended steering line was hitting the steering relief valve. The main pump was trying to push maximum flow against a completely blocked circuit, creating so much mechanical resistance that the diesel engine stalled.
The repair involved removing the priority flow divider valve and cleaning the spool bore with brake cleaner and fine lapping compound. The seized spool was carefully freed and polished. Fresh hydraulic oil and a new suction filter were installed to prevent varnish recurrence. After reassembly, the steering operated smoothly, the priority valve correctly routed flow to the Orbitrol, and the engine maintained full RPM during full-lock steering maneuvers.
