Gehl 6635 Skid-Steer Hydraulic Seal Failures: Causes, Diagnosis & Fixes

[MikePhua]

Introduction to Common Hydraulic Seal Issues on the Gehl 6635
The Gehl 6635 skid-steer loader, frequently equipped with a turbocharged Deutz diesel engine and two‑speed hydraulics, is robust but known to experience seal wear over time. Hydraulic seal failures—particularly around drive or hydraulic pumps—can lead to fluid leaks, loss of power, or total hydraulic failure. Understanding root causes and repair methods helps owners maintain reliability and avoid expensive downtime.
Symptoms of Hydraulic Seal Failure

• Visible hydraulic fluid leak beneath the machine, often pooling near skid plates.
  
• Loss of hydraulic functions such as travel or loader movement.
  
• Sudden drive motor stall while bucket & boom still operate.
  
• Excessive fluid sprayed or dripping near pump housing or hoses.
  

Key Locations for Seals in the Gehl 6635

• Drive Motor Seals / Case Drain: Internal leakage in drive motors causes high case drain flow and loss of travel power. A worn seal kit (OEM part 132655) addresses this problem.
  
• Hydraulic Pump Housing Seals: Wear or damage to external seals at pump ports causes external fluid seepage.
  
• Cylinder and Valve Body Seals: Hydraulic lift/tilt cylinders and valve block assemblies include o-rings and shaft seals that degrade over time. Valve spool seals may allow pressure bypass or external seepage.
  

Common Causes of Seal Failure

• Aging seals hardened by heat and exposure to hydraulic oil.
  
• Contaminants such as dirt or metallic particles wearing against seals.
  
• Excessive hydraulic pressure from a slipping or malfunctioning priority valve or relief valve.
  
• Abrasive build-up or debris in the belly causing seed grit to cut or wear seals.
  

One owner reported sliding into a washout; afterward, while bucket/tilt functions remained, travel drive ceased. Leaking occurred beneath skid plate—indicating drive/motor seal failure.

Diagnostic Checklist

1. Visual Inspection
   Clean undercarriage and skid pans. Trace fluid from drive motors, gear pumps, couplings to identify seal failure points.
   
2. Check Hydraulic Case Drain Flow
   Disconnect the case drain line from one drive motor while machine immobilized. Engage travel; if drain flow is high compared to the other side, the motor seal is likely compromised.
   
3. Inspect Valve & Cylinder Assemblies
   Look for seepage or spray around solenoid valve packs or cylinders during operation.
   
4. Identify Wear in Wheel Mounts or Pump Interfaces
   Loose couplings or misaligned drive flanges can compromise seal alignment and pressure integrity.
   

Repair and Replacement Options

• Drive Motor Seal Kit (OEM 132655): Use genuine or high-quality aftermarket replacements for all drive motor seals.
  
• Pump Seal Replacement: Remove and replace external seals; ensure mating surfaces are clean before seal installation.
  
• Valve Block Re-sealing: Service kits for lift/tilt spool sections and O-rings are available in service parts manuals.
  

Maintenance Recommendations

• Regular cleaning of undercarriage and hydraulic reservoir helps prevent abrasive wear.
  
• Monitoring and maintaining clean hydraulic fluid and filtration prevent seal grinding.
  
• Routine functional checks—such as pressure tests or case drain observation—catch failing seals before a major hydraulic breakdown occurs.
  
• Using OEM or quality aftermarket parts ensures proper seal fit and pressure resistance.
  

Terminology Explained

• Case Drain: Low-pressure return from drive motor housing; excessive flow indicates internal leakage.
  
• Drive Motor Seal Kit: A set of internal seals for the hydrostatic tracks.
  
• Valve Spool Seal: Inside the hydraulic control valve; prevents bypass or leakage during spool operation.
  
• Cylinder Shaft Seal: Seal between piston rod and housing to prevent external fluid escape.
  

Real-World Examples & Insights

• One owner discovered that after heavy use in muddy terrain, travel functions ceased while bucket and tilt remained functional. Inspection under the belly revealed leaking hydraulic fluid from the drive motor seal. Replacing the seal kit restored drive control.
  
• Another operator cleaned accumulated oil and grit near the drive pump area before seal replacement—which avoided recontamination and accelerated wear.
  

Summary List: Key Takeaways

• Drive motor seals and case drain leakage are common failure points.
  
• High internal bleed in motor case drain hoses is diagnostic of seal wear.
  
• External leak points include couplings, pump housing, and valve sections.
  
• OEM seal kits (e.g. part 132655) recommended for reliability.
  
• Keeping undercarriage and fluid clean extends seal life.
  
• Compare both sides of machine to detect asymmetric leakage or wear.
  

Conclusion
Seal failures in the Gehl 6635 skid-steer loader are a frequent source of hydraulic issues, especially in units used in abrasive or muddy conditions. By monitoring case drain flow, performing careful visual inspections, and using proper seal kits for replacements, operators can restore hydraulic performance and prevent repeat failures. With thoughtful maintenance and quality parts, these durable machines can continue working efficiently for many more hours.