Diagnosing Intermittent Steering Stiffness on the CAT TH357D Telehandler

[MikePhua]

Steering Behavior Linked to Hydraulic Load Sharing
The CAT TH357D telehandler, introduced around 2018, is a mid-range telescopic handler designed for construction, agriculture, and industrial material handling. It features selectable steering modes—two-wheel, four-wheel, and crab steer—powered by a load-sensing hydraulic system. In one reported case, the machine exhibited intermittent stiffness in four-wheel steering mode, while steering remained smooth in two-wheel mode. Interestingly, lowering the boom temporarily restored normal steering, but raising it had no effect.
This behavior suggests a hydraulic prioritization issue, where steering flow is compromised under certain load conditions. The TH357D uses a priority valve to allocate hydraulic flow between steering, boom, and auxiliary functions. If the valve sticks or misroutes flow, steering may become sluggish or unresponsive.
Understanding the Priority Valve Function
The priority valve ensures that steering receives hydraulic flow before other functions. It operates based on pressure differentials and internal spool movement. When the boom is lowered, system pressure drops, potentially freeing a sticky spool and restoring steering flow. However, if the valve is partially seized or contaminated, it may fail to prioritize steering under normal operating loads.
Symptoms of a faulty priority valve include:

• Steering stiffness during multi-function operation
  
• Temporary relief when boom is lowered
  
• No improvement when boom is raised
  
• Normal behavior in two-wheel mode due to reduced flow demand
  

Other Potential Causes to Investigate
While the priority valve is a prime suspect, other components may contribute:

• Steering orbitrol (hydraulic steering control unit): If worn or internally leaking, it may fail to direct flow properly
  
• Hydraulic pump wear: Reduced output under load can starve the steering circuit
  
• Contaminated fluid or clogged filters: Debris can restrict flow or cause spool sticking
  
• Steering cylinder seals: Internal leakage may reduce responsiveness
  

One technician noted that a similar issue on a TH414C was resolved by replacing the priority valve and flushing the hydraulic system. The valve had scoring on the spool surface, likely caused by contaminated fluid.
Recommended Diagnostic Steps

• Check hydraulic fluid level and condition
  
• Inspect and clean or replace hydraulic filters
  
• Test system pressure at the steering circuit under load
  
• Remove and inspect the priority valve for spool movement and contamination
  
• Verify orbitrol function and steering cylinder integrity
  

If the valve is removed, it should be disassembled and cleaned with lint-free cloths and inspected for scoring or burrs. Replacement valves should match OEM specifications, and all seals must be replaced during reinstallation.
Preventive Measures and Long-Term Strategy

• Use high-quality hydraulic fluid and change it at recommended intervals
  
• Replace filters every 500 hours or sooner in dusty environments
  
• Avoid simultaneous operation of boom and steering when possible
  
• Monitor steering response and address stiffness early to prevent wear
  

Conclusion
Stiff steering in the CAT TH357D telehandler—especially in four-wheel mode and under boom load—is most likely caused by a sticky or malfunctioning priority valve. By inspecting hydraulic flow paths, cleaning or replacing the valve, and verifying related components, operators can restore smooth steering and prevent long-term damage. In machines with complex hydraulic sharing, understanding flow prioritization is key to diagnosing intermittent control issues.