Uncovering the Leak: Diagnosing and Repairing Torque Converter Check Plate Issues on the Case 850B Dozer

[MikePhua]

Introduction
The Case 850B crawler dozer, a staple in mid-size earthmoving operations, is known for its rugged reliability. Yet, like many aging machines, it can develop hydraulic and transmission fluid leaks that compromise performance and safety. One particularly elusive issue involves fluid leaking from the torque converter check plate—an area that demands both mechanical insight and patience. This article explores the symptoms, causes, and repair strategies for this problem, enriched with terminology, field anecdotes, and historical context.
Understanding the Torque Converter and Check Plate Assembly
The torque converter in the Case 850B serves as a fluid coupling between the engine and transmission, allowing smooth power transfer under varying loads. The check plate is a removable cover that provides access to the flywheel and torque converter internals for inspection and maintenance.
Key components include:

• Torque Converter: A hydraulic device that multiplies torque and allows slippage between engine and transmission.
  
• Check Plate: A bolted cover that seals the converter housing and provides visual access to the flywheel.
  
• Seal #7: A critical internal seal within the converter assembly, often the culprit in major leaks.
  
• Skid Plate: Protective metal shielding beneath the converter, often collecting leaked fluid.
  

Terminology Explained

• Hydraulic Coupling: A fluid-based mechanism for transferring rotational power.
  
• Flywheel Housing: The casing surrounding the flywheel, often adjacent to the converter.
  
• Seal Assembly: A set of gaskets and rings designed to prevent fluid migration between compartments.
  
• Converter Pull: The process of removing the torque converter from the machine for service.
  

Symptoms and Initial Diagnosis
Operators typically notice:

• Accumulated oil in the skid plate beneath the converter
  
• Fluid visibly leaking from the check plate area
  
• Rapid fluid loss during operation (up to 5 gallons in 4 hours in one case)
  
• Transmission slipping or inconsistent power delivery
  

Diagnosis involves:

• Removing skid plates to inspect fluid accumulation
  
• Starting the machine and observing leak origin
  
• Identifying whether the leak is from the engine or transmission side of the flywheel
  

Field Anecdote: Kentucky Corn and Converter Seals
A farmer in Gravel Switch, Kentucky noticed his Case 850B was hemorrhaging fluid during spring tilling. After losing gallons of transmission fluid, he traced the leak to Seal #7 inside the torque converter. With planting season looming, he pulled the converter himself, replaced all seals, and had the machine back in service within a week—just in time to get corn in the ground.
Repair Strategy: Pulling and Rebuilding the Converter
Repairing the leak requires removing the torque converter, inspecting seals, and replacing worn components. While daunting, the job is manageable with preparation.
Steps include:

• Step 1: Preparation
  • Drain transmission fluid
    
  • Remove skid plates and check plate
    
  • Disconnect linkages and hoses around the converter
    
• Step 2: Converter Removal
  • Unbolt the converter from the flywheel housing
    
  • Use lifting equipment to extract the unit
    
  • Note that the engine can remain in place during removal
    
• Step 3: Seal Inspection and Replacement
  • Disassemble the converter housing
    
  • Inspect all seals, especially Seal #7
    
  • Replace with OEM or high-quality aftermarket seals
    
• Step 4: Reinstallation
  

• Reassemble the converter
  
• Reinstall into the machine, aligning with the flywheel
  
• Reconnect hoses and linkages
  
• Refill fluid and test under load
  

Historical Perspective: The Rise of Torque Converter Technology
Torque converters became standard in heavy equipment during the 1960s, replacing clutch-based systems for smoother operation. Case’s adoption of hydraulic converters in the 850 series reflected a broader industry shift toward operator comfort and mechanical simplicity. While modern machines use electronically controlled transmissions, the mechanical converters of the 850B remain serviceable and effective decades later.
Related Case: Australian Rebuild Inquiry
An operator in New South Wales, Australia asked whether the converter could be removed without pulling the engine. The answer: yes. This design feature makes the Case 850B more field-serviceable than many competitors. Rebuilt converters vary in cost depending on region and supplier, but DIY seal replacement remains a cost-effective option.
Best Practices and Preventive Measures

• Monitor fluid levels regularly and inspect skid plates for pooling
  
• Replace all seals during converter service, not just the leaking one
  
• Use proper lifting tools to avoid injury or damage
  
• Keep a service log to track fluid changes and leak history
  
• Consider preventive seal replacement during major maintenance intervals
  

Conclusion
A leaking torque converter check plate on the Case 850B is more than a nuisance—it’s a call to action. With methodical diagnosis and careful repair, operators can restore their machines to full functionality. These dozers were built to last, and with a bit of grease and grit, they’ll keep pushing dirt for years to come.