Diagnosing Power and Drive Issues on the 2006 Tigercat 625C Skidder

[MikePhua]

The Tigercat 625C and Its Role in Steep Terrain Logging
The Tigercat 625C is a six-wheel-drive articulated skidder designed for high-traction forestry operations. Released in the mid-2000s, it was part of Tigercat’s push to offer more versatile machines for challenging ground conditions, especially in frozen or steep terrain. With a Cummins 5.9L turbocharged diesel engine and hydrostatic drive, the 625C was engineered for durability and control. Its popularity grew in regions with difficult access, where traditional 4x4 skidders struggled to maintain traction or stability.
Tigercat, founded in Canada in 1992, quickly became a respected name in forestry equipment, known for its purpose-built machines and strong dealer support in logging-heavy provinces. However, in areas with less forestry infrastructure, such as southern Alberta, service access can be limited, making diagnostics and repairs more complex.
Terminology Annotation
- VP44 injection pump: An electronically controlled rotary diesel pump used in many Cummins engines, sensitive to fuel pressure and voltage fluctuations.
- Supply pump: A low-pressure electric pump that feeds fuel to the injection pump; failure can cause engine derate or internal damage.
- Derate condition: A protective mode triggered by the engine control unit (ECU) that limits power output due to detected faults.
- Servo piston: A hydraulic actuator within the drive pump that controls flow direction and volume; worn seals can cause drive slippage.
Engine Power Loss and Fuel System Suspicions
Operators reported that the 625C bogged down under load, especially when climbing hills or operating the grapple. Engine torque peaked at 75%, and turbo boost reached 19 psi—within acceptable range but not optimal. Blue smoke under load suggested incomplete combustion, often linked to poor fuel delivery or air intrusion.
The VP44 injection pump used in this model is highly dependent on consistent supply pressure. If the electric supply pump fails or voltage drops, the injection pump may continue running but at reduced output, slowly damaging itself internally. This can result in:

• Reduced RPM and sluggish throttle response
  
• Inconsistent torque under load
  
• Increased exhaust smoke
  
• Premature wear of pump internals
  

Recommendations:

• Install a fuel pressure monitoring kit with a cab-mounted warning light
  
• Tap into the inlet banjo bolt using a 12x1.5mm or 14x1.5mm fitting to check pressure
  
• Replace the supply pump if pressure falls below manufacturer spec (typically 10–15 psi for VP44 systems)
  
• Check voltage at the pump during operation to rule out electrical faults
  

One technician noted that supply pump failures were so common that aftermarket kits were developed to alert operators before damage occurred.
Cooling System and Airflow Direction
Another issue involved excessive heat buildup under the hood, especially near the pre-cleaner. The fan was found to be pulling air through the radiator rather than pushing it out, contrary to standard configuration. This reversed airflow caused hot air to recirculate around the intake, reducing engine efficiency and increasing under-hood temperatures.
Solutions:

• Reverse fan orientation to create a pusher setup
  
• Relocate the pre-cleaner outside the hood, as seen on newer Tigercat models
  
• Seal gaps around the radiator to prevent hot air recirculation
  
• Inspect air-to-air intercooler connections for leaks or misalignment
  

Improper airflow can lead to heat soak, where components remain hot even after shutdown, accelerating wear and reducing performance.
Drive System and Reverse Slippage
The 625C also exhibited weak reverse drive, especially under incline or load. Increasing the speed dial did not improve performance, suggesting hydraulic bypass or servo piston wear. The machine uses either a Linde or Rexroth hydrostatic drive pump, depending on build spec. Worn packing on the servo piston can cause internal leakage, reducing drive pressure and causing slippage.
Diagnostic steps:

• Use onboard diagnostics to measure pedal input voltage and confirm full travel
  
• Inspect servo piston seals and packing for wear
  
• Check for directional control valve faults or contamination
  
• Verify that no engine codes are triggering a derate condition
  

If the coolant level sensor is faulty, it may falsely trigger a derate, limiting drive output even when no warning lights are present.
Operational Context and Terrain Demands
The machine was purchased at auction and had logged over 8,000 hours. Despite its age, it remained structurally sound and had performed well for 700 hours in steep terrain. All six wheels were chained, providing excellent traction in frozen ground where 4x4 skidders failed. The operator had previously used a Cat 527 swing boom forwarder, which was slower and more physically demanding.
This context highlights the importance of choosing the right machine for terrain. The 625C’s six-wheel drive and stability make it ideal for alpine logging, but its complex fuel and hydraulic systems require proactive maintenance.
Conclusion
The 2006 Tigercat 625C is a capable skidder for extreme terrain, but its performance hinges on fuel pressure integrity, proper cooling airflow, and healthy hydrostatic drive components. Power loss and reverse slippage often stem from overlooked supply pump issues or servo wear. By upgrading monitoring systems, correcting fan orientation, and inspecting hydraulic internals, operators can restore full functionality and extend the life of this forestry workhorse. In remote regions with limited dealer support, mechanical diligence becomes the key to keeping timber moving.