Solving Transmission Seal and Cooling Hose Issues on the Caterpillar 935CII Loader

[MikePhua]

The 935CII and Its Powertrain Design
The Caterpillar 935CII track loader, produced in the early 1990s, was part of Cat’s mid-size loader lineup designed for rugged earthmoving and site preparation. Equipped with a 3-speed powershift transmission and torque converter, the 935CII offered smooth directional changes and reliable hydraulic performance. Its compact footprint and robust undercarriage made it popular for clearing, grading, and utility work. Though no longer in production, thousands of units remain in service, especially in rural and owner-operated fleets.
The transmission system on the 935CII relies on a sealed filter housing and a dedicated cooling circuit for the torque converter. When these components fail—whether due to aging seals or damaged hoses—fluid loss and overheating can quickly compromise performance.
Transmission Filter Cap Seal Fitment Issues
One common frustration involves replacing the transmission filter cap seal. The original seal, typically part number 5K1770, is designed to fit snugly into the groove of the filter housing. However, aftermarket versions of this seal often differ slightly in diameter or material flexibility. Even a minor size discrepancy can cause the seal to pinch or extrude during installation, leading to leaks and premature failure.
In one case, an operator installed an aftermarket seal that appeared to match the part number but was slightly oversized. Upon tightening the cap, the seal deformed and disintegrated, leaving a trail of fluid and a damaged groove. The solution came in the form of a genuine Caterpillar-branded seal, which fit precisely and held under pressure when installed with a thin layer of grease to seat it properly.
This highlights a broader lesson: while aftermarket parts can offer savings, seals and gaskets—especially those under pressure—are best sourced from OEM suppliers to ensure dimensional accuracy and material compatibility.
Terminology Clarification
- Transmission Filter Cap Seal: A rubber or synthetic ring that prevents fluid leakage at the filter housing
- O-Ring: A circular sealing element used in hydraulic and fluid systems
- Groove: The recessed channel in a housing where the seal sits
- OEM: Original Equipment Manufacturer, referring to parts made to factory specifications
- Aftermarket: Parts produced by third-party suppliers, often with slight design variations
Torque Converter Cooling Hose Replacement Challenges
The torque converter on the 935CII uses a pair of hydraulic hoses to circulate cooling fluid. These hoses are routed tightly around the transmission and engine bay, making access difficult. When one of these hoses begins to leak—either from a cut or degraded outer jacket—replacement becomes a logistical puzzle.
In a documented repair, the upper hose on the left side of the converter was leaking and nearly inaccessible due to its position above another hose. After soaking the fittings with penetrating oil, the operator attempted removal but found no room for a standard wrench. The solution involved cutting the damaged hose near the fitting with a reciprocating saw, allowing a socket to be placed directly on the nut.
For reinstallation, a 1¼-inch crowfoot wrench on a long extension was recommended to reach the fitting without disassembling surrounding components. This approach saved hours of labor and avoided disturbing other hydraulic lines.
Best Practices for Hose Replacement

• Use penetrating oil and allow time for soak-in before attempting removal
  
• Cut damaged hoses near the fitting to gain access with standard tools
  
• Use crowfoot wrenches and extensions for tight spaces
  
• Replace both hoses if one shows signs of stiffness or jacket degradation
  
• Inspect fittings for scoring or corrosion before installing new hoses
  

Field Wisdom and Operator Experience
One operator shared that after months of cosmetic and service work on his 935CII, his first field test ended abruptly when a hydraulic hose burst, spraying fluid across the cab. The failure occurred at the bucket cylinder feed line, a reminder that aged hoses can rupture under pressure even after minor use. His quick thinking—shutting down the engine and avoiding further hydraulic engagement—prevented additional damage.
Another technician emphasized the importance of checking fluid levels before restarting after a leak. Running the system dry can cause pump cavitation and internal scoring, leading to expensive repairs.
Preventive Maintenance Recommendations
To avoid similar issues:

• Replace seals and hoses with OEM parts when possible
  
• Apply grease to o-rings during installation to prevent pinching
  
• Inspect hoses annually for cracks, stiffness, or jacket separation
  
• Keep crowfoot wrenches and long extensions in your tool kit
  
• Flush hydraulic systems after major hose failures to remove contaminants
  

Final Thoughts
The Caterpillar 935CII remains a capable and serviceable machine, but its transmission and cooling systems require precision and patience. Small parts like seals and hoses may seem minor, but they play a critical role in keeping the loader operational. With the right tools, quality components, and field-tested techniques, even tight spaces and stubborn fittings can be conquered. In the world of heavy equipment, success often comes down to the details—and the willingness to get dirty solving them.