Troubleshooting Torque Converter Fluid Contamination in the Caterpillar 977K

[MikePhua]

The 977K and Its Mechanical Legacy
The Caterpillar 977K track loader was part of CAT’s 11K series, powered by the robust 3306 diesel engine. Introduced in the 1970s, the 977K was designed for heavy-duty excavation, loading, and site preparation. With its torque converter drive system and hydraulic transmission, it offered smoother operation and better maneuverability than earlier clutch-and-brake models. The 3306 engine itself became one of CAT’s most widely used powerplants, known for its durability and ease of service, with production continuing into the early 2000s across multiple platforms.
The 977K was widely adopted in North America and overseas, particularly in mining, forestry, and infrastructure development. Though no longer in production, thousands of units remain in operation, often maintained by owner-operators and small contractors who value its mechanical simplicity and parts availability.
Identifying the Source of Milky Transmission Fluid
A common issue in aging 977K units is the appearance of milky fluid in the transmission and torque converter housing. This condition typically indicates water contamination, often caused by a failed oil cooler. The 977K uses a section of the engine radiator to cool transmission and torque converter oil. If the internal seals of the cooler rupture, coolant can mix with transmission fluid, leading to emulsification.
Symptoms include:

• Milky or foamy fluid on the transmission dipstick
  
• Oil leaking from the torque converter yoke seal
  
• Reduced transmission performance or erratic shifting
  
• Sticky residue in sump screens and filters
  

In one case, the contamination was traced back to a failed pre-cup seal in the cylinder head, which allowed coolant to enter the combustion chamber and eventually the transmission cooler circuit. After replacing the pre-cup seals and O-rings, the coolant leak stopped, but the torque converter continued to leak due to residual contamination and scavenger pump malfunction.
Scavenger Pump and Sump Screen Inspection
The torque converter in the 977K relies on a scavenger pump to maintain proper oil levels and prevent flooding. This pump draws excess fluid from the converter sump and returns it to the transmission reservoir. If the pump fails or its screen becomes clogged, fluid can accumulate in the converter housing and leak past the output shaft seal.
Recommended inspection steps:

• Remove the converter sump cover and clean the magnetic screen
  
• Check for white sticky residue, often left by radiator sealants like Blue Devil
  
• Inspect the scavenger pump gear for wear or sheared keys
  
• Verify free rotation and clearance of the pump shaft
  
• Replace the sump gasket and reinstall with proper torque
  

In one documented repair, the operator left the sump screen and filter out temporarily to allow unrestricted flow during flushing. After circulating 30 gallons of fresh 20W transmission oil, the fluid cleared and the leak subsided, confirming that the pump was functional once debris was removed.
Yoke Seal and Spiral Knurling Considerations
The output shaft of the torque converter features reverse spiral knurling—a machined pattern designed to direct oil away from the seal. If this knurling becomes clogged with debris or sealant residue, it can fail to redirect oil properly, leading to persistent leaks.
To address this:

• Remove the drive shaft and yoke
  
• Inspect the knurling for blockage and clean thoroughly
  
• Replace the lip seal and, if necessary, the seal carrier
  
• Confirm that the bearing behind the seal is pressure-lubricated and intact
  

A similar issue was observed in marine engines with reverse-rotation crankshafts, where spiral knurling played a critical role in oil control. In those cases, improper seal installation or clogged knurling led to rear main seal failures.
Cooler Replacement and System Flushing
To eliminate the risk of recurring contamination, both the engine oil cooler and the torque converter cooler should be replaced. These components are often integrated into the radiator and may require full radiator replacement or custom retrofitting.
After cooler replacement:

• Flush the transmission and converter circuits with clean oil
  
• Replace all filters, including the magnetic strainer and hydraulic filter
  
• Run the engine at idle for 30–60 minutes to circulate fluid
  
• Monitor for leaks at the yoke seal and cooler connections
  

In one successful rebuild, the operator ran the machine over a service pit, allowing easy access to the sump and converter housing. After confirming leak-free operation, the machine was pressure washed and prepped for repainting.
Final Adjustments and Injector Maintenance
During post-repair testing, a diesel leak was discovered at injector number four. The injector body and spray nozzle were replaced, along with updated seals featuring embedded metal rings. Torque specs were verified: 150 ft-lbs for the pre-cup and 105 ft-lbs for the injector nut. After installation, the engine ran cleanly with no fuel or coolant leaks.
Operators should also:

• Check transmission fluid levels after flushing; overfilling can cause aeration
  
• Use a sandblaster attachment for pressure washing to remove grease buildup
  
• Source new decals and cosmetic parts from vintage equipment suppliers
  
• Consider installing a rear winch for added utility; compatibility depends on frame configuration and hydraulic routing
  

Conclusion
The Caterpillar 977K remains a serviceable and powerful machine when maintained properly. Torque converter leaks and fluid contamination are common in older units but can be resolved through systematic inspection, cooler replacement, and scavenger pump servicing. With attention to detail and a willingness to dig into the mechanical systems, operators can restore full functionality and extend the life of this classic track loader. Whether working in a pit or prepping for paint, the 977K continues to earn its place in the field.