Diagnosing Hydraulic Drift and Power Loss on the Deere 590D

[MikePhua]

The Deere 590D and Its Hydraulic Architecture
The John Deere 590D excavator was introduced in the early 1990s as part of Deere’s mid-size lineup, designed to compete with offerings from Komatsu, Case, and Caterpillar. With an operating weight of roughly 30,000 lbs and powered by a reliable 4-cylinder diesel engine, the 590D was built for general excavation, trenching, and utility work. Its hydraulic system was designed around open-center flow logic, using a gear-type pump and spool valve bank to distribute fluid to the boom, stick, bucket, and travel motors.
Though Deere has since moved to closed-center load-sensing hydraulics in newer models, the 590D remains a workhorse in many fleets. However, as these machines age past 10,000 hours, hydraulic issues—especially drift, bogging, and erratic cylinder behavior—become increasingly common.
Terminology Clarification

• Drift: Unintended movement of a hydraulic cylinder when the control lever is in neutral.
  
• Spool Valve: A sliding valve inside the control block that directs hydraulic flow to specific functions.
  
• Relief Valve: A pressure-limiting valve that protects the system from overload by diverting excess fluid.
  
• Pilot Circuit: A low-pressure control system that actuates the main hydraulic valves.
  
• Contamination: Presence of metal flakes, dirt, or degraded fluid in the hydraulic system, which can block valves or damage seals.
  

Symptoms of Hydraulic Dysfunction
In one case, a 590D exhibited a troubling behavior: the dig stick would extend fully, but halfway through its stroke, the machine would bog down. Upon releasing the control lever, the stick would retract on its own—suggesting internal leakage or valve malfunction. The operator had recently replaced the hydraulic pump, cleaned the reservoir, and installed new filters, yet the issue persisted.
This combination of bogging and retraction points to a loss of pressure control and possible contamination in the valve body or cylinder.
Root Causes and Diagnostic Pathways
Several potential culprits can cause this type of hydraulic misbehavior:

1. Contaminated Relief Valve
   
   1. Metal flakes from a failed pump or cylinder can lodge in the relief valve, causing it to open prematurely or fail to seat properly. This results in pressure loss and erratic cylinder movement. Cleaning the valve and inspecting the seat surface is essential.
      
   2. Damaged Arm Cylinder
      
   3. If the cylinder seals are compromised, fluid can bypass internally, causing drift and loss of holding power. Disassembly and inspection of the piston, barrel, and seal stack will reveal scoring or wear.
      
   4. Spool Valve Obstruction
      
   5. Debris lodged in the spool valve can prevent full engagement or cause partial flow in neutral. Pulling the valve bank and inspecting each spool for smooth travel and debris is a critical step.
      
   6. Pilot Valve Malfunction
      
   7. A weak or blocked pilot signal can cause the main valve to misfire or remain partially open. Checking pilot pressure and cleaning pilot screens can restore proper function.
      
   8. Swing Motor Contamination
      
2. Metallic debris often migrates to the swing motor head, where anti-cavitation valves draw in unfiltered oil. Cleaning these valves and inspecting the motor head can prevent further system degradation.
   

Field Anecdote and Operator Insight
In Alberta, a farmer-turned-contractor restored a 590D for ditching and culvert work. After replacing the hydraulic pump, he noticed the stick would retract without input. Suspecting contamination, he flushed the system and installed temporary cleanup filters—high-micron filters designed to trap residual debris. After a week of operation and filter changes, the issue resolved. He later discovered a flake lodged in the pilot spool, confirming the importance of deep cleaning beyond the reservoir.
Recommended Solutions and Preventive Measures
To address and prevent hydraulic drift and bogging:

• Flush all lines with high-flow fluid and temporary cleanup filters.
  
• Disassemble and inspect arm cylinder for internal bypass.
  
• Pull and clean spool valves, checking for scoring or sticking.
  
• Replace pilot filters and verify pilot pressure with a gauge.
  
• Inspect swing motor head and anti-cavitation valves for debris.
  
• Use OEM-spec hydraulic fluid and change filters every 500 hours.
  

For machines with pump failure history, consider a full teardown of the valve bank and cylinder inspection. Partial cleaning often leaves behind contaminants that resurface later.
Modern Comparisons and Lessons Learned
Newer Deere excavators use electronically controlled valves and diagnostic ports that simplify troubleshooting. However, older models like the 590D require manual testing and mechanical intuition. Investing in pressure gauges, flow meters, and training on hydraulic schematics can dramatically improve repair outcomes.
Conclusion
Hydraulic issues on the Deere 590D—especially stick drift and bogging—are often rooted in contamination and valve malfunction. While replacing pumps and filters is a good start, true resolution requires deep inspection of cylinders, spools, and pilot circuits. With methodical diagnostics and preventive care, the 590D can continue serving reliably in excavation and utility work, even decades after its release. For operators and mechanics alike, understanding the interplay of pressure, flow, and control is key to keeping these machines productive.