Why Does the Case Poclain 788 Run Poorly After Repairs

[MikePhua]

Intermittent power loss in the Case Poclain 788 excavator—especially after pump inspection and relief valve replacement—often points to hydraulic control instability, electrical signal drift, or contamination in pilot circuits. These issues can mimic pump failure but originate elsewhere in the system.
Case Poclain 788 Background and Hydraulic Architecture
The Case Poclain 788 was introduced in the mid-1990s as a wheeled excavator designed for urban utility work, road maintenance, and light demolition. It featured a robust hydraulic system powered by a variable displacement pump and controlled via pilot-operated valves. The machine was part of the Poclain legacy, which merged into Case Construction Equipment during the 1980s, blending French hydraulic innovation with American manufacturing.
The 788’s hydraulic system relies on precise pilot pressure and clean fluid pathways to modulate boom, stick, and travel functions. Unlike newer models with electronic load-sensing, the 788 uses mechanical feedback and pressure relief logic to maintain flow balance.
Terminology Note

• Pilot Circuit: A low-pressure hydraulic system that sends control signals to main valves.
  
• Main Relief Valve: A pressure-limiting valve that protects the system from overload.
  
• Hydraulic Drift: Unintended movement or loss of pressure due to internal leakage or valve instability.
  
• Load-Sensing Pump: A pump that adjusts output based on demand, common in newer machines.
  
• Signal Lag: Delay or fluctuation in control response due to contamination or wear.
  

Symptoms and Observations

• The machine runs normally for short periods, then loses power for several seconds before recovering.
  
• The hydraulic pump was removed and bench-tested with no faults found.
  
• A new main relief valve was installed, but the issue persists.
  
• No visible leaks or overheating were reported.
  
• The problem returned after six months of stable operation.
  

These symptoms suggest that the issue is not with the pump itself but with control signals, pilot pressure stability, or contamination affecting valve response.
Root Causes and Diagnostic Strategy

• Pilot pressure fluctuation: Dirty pilot filters or worn pilot pumps can cause erratic control signals, leading to temporary power loss.
  
• Electrical signal drift: If the machine uses electro-hydraulic controls, poor grounding or sensor failure can disrupt valve actuation.
  
• Contaminated fluid: Microscopic debris can clog or restrict pilot orifices, causing delayed response or valve sticking.
  
• Relief valve miscalibration: Even new valves can be improperly set or mismatched to system specs.
  
• Thermal expansion effects: After warm-up, seals may expand and restrict flow in marginal components.
  

Recommended Inspection and Solutions

• Check pilot pressure at multiple points using a gauge. Look for drops during operation.
  
• Inspect and replace pilot filters—these are often overlooked and critical to signal clarity.
  
• Flush the hydraulic system with compatible fluid and inspect for metallic particles or water contamination.
  
• Verify relief valve settings against factory specifications. Use a calibrated test bench if available.
  
• Inspect electrical connectors and grounds if electro-hydraulic controls are present.
  
• Monitor temperature behavior—some failures only occur after thermal expansion.
  

Field Experience and Anecdotes
In Denmark, a contractor struggled with a 788 that lost power intermittently. After pump testing and valve replacement, the issue persisted. A technician later discovered that the pilot filter was partially blocked, causing signal lag. After replacement and fluid flush, the machine returned to stable operation.
In Quebec, a fleet manager noted similar behavior in a 788 used for sewer trenching. The root cause was a cracked pilot hose that only leaked under pressure, introducing air and disrupting control. Replacing the hose resolved the issue.
Recommendations for Long-Term Reliability

• Replace pilot filters every 500 hours, even if not listed in standard service intervals.
  
• Use hydraulic fluid with anti-foam and anti-wear additives.
  
• Keep a log of temperature-related behavior to identify thermal faults.
  
• Train operators to recognize early signs of drift or lag in control response.
  
• Maintain a clean hydraulic reservoir and inspect suction screens annually.
  

Conclusion
The Case Poclain 788’s intermittent power loss is rarely due to pump failure alone. Pilot pressure instability, contamination, and signal drift are more likely culprits. With targeted diagnostics and preventive maintenance, these issues can be resolved, restoring full performance to a machine that still holds value in urban excavation and utility work.