Repairing Transmission Seal Leaks and Serial Number Recovery on the Caterpillar 955L

[MikePhua]

Caterpillar 955L Track Loader History and Design
The Caterpillar 955L was introduced in the late 1970s as part of the evolution of the 955 series, which had been a staple in Caterpillar’s track loader lineup since the 1950s. Designed for versatility in excavation, loading, and site clearing, the 955L featured a turbocharged 3304 diesel engine, torque converter drive, and a powershift transmission. With an operating weight of around 32,000 lbs and breakout force exceeding 20,000 lbs, it was widely used in construction, forestry, and mining.
The 955L was known for its mechanical simplicity and robust undercarriage, but like many machines of its era, it relied on lip seals and mechanical couplings that degrade over time. Identifying and repairing leaks around the transmission input shaft is a common maintenance task, especially on machines with decades of service.
Locating the Serial Number on Older Units
One challenge with vintage Caterpillar equipment is locating the correct serial number, especially when identification plates are obscured or missing. In the case of the 955L, the serial number plate is typically mounted low on the frame near the front firewall, often hidden behind cab modifications or welded panels.
Operators have successfully recovered serial numbers by:

• Using mirrors and flashlights to inspect recessed areas
  
• Photographing suspected tag locations and enhancing images digitally
  
• Cutting access ports in obstructing sheet metal
  
• Cross-referencing engine, transmission, and torque converter serials
  

In one instance, the engine was confirmed as a 3304 with serial number 78P4377, and the transmission arrangement was identified as 4S8564. After persistent searching, the machine’s serial number was finally located: 85J7056. This confirmed the loader’s identity and allowed the owner to source the correct service manual (REG00634) and parts catalog.
Diagnosing and Repairing the Transmission Input Shaft Leak
A visible leak around the drive shaft entering the transmission is typically caused by a failed lip seal. On the 955L, this seal is part number 5S4408 and is designed to prevent transmission fluid from escaping along the rotating shaft.
Steps to repair the leak include:

• Removing the coupling from the transmission input shaft
  
• Inspecting the seal surface for grooves or scoring
  
• Replacing the lip seal with a new OEM or equivalent part
  
• Checking the shaft for wear and polishing if necessary
  
• Reinstalling the coupling and verifying alignment
  

If the seal surface is damaged, a repair sleeve may be required. Specialty tools are minimal—typically a seal puller, alignment punch, and torque wrench. No snap ring is involved unless the front bearing is being serviced.
Tips for Disassembly and Shaft Retention
During disassembly, keeping the drive shaft from rotating while loosening the balancer bolts can be tricky. Recommended techniques include:

• Using a strap wrench or chain wrench on the shaft
  
• Locking the flywheel or coupling with a fabricated bracket
  
• Applying penetrating oil to stubborn bolts and using impact tools cautiously
  

Once the seal is replaced, the transmission should be refilled with the correct fluid type and volume. Caterpillar recommends checking fluid levels after warm-up and inspecting for leaks under load.
Anecdote from West Virginia
An operator in West Virginia inherited a 955L with no visible serial number and a persistent transmission leak. After days of searching and cutting into the cab firewall, he located the serial tag and confirmed the machine’s identity. With guidance from experienced mechanics, he replaced the input shaft seal and restored the machine to working condition. He described the process as “laborious but satisfying,” noting that access to community knowledge was more valuable than any manual.
Recommendations for Vintage Caterpillar Owners
To maintain and restore older Caterpillar loaders:

• Document all visible serial numbers on engine, transmission, and torque converter
  
• Use digital photography and image enhancement to recover faded tags
  
• Source service manuals based on confirmed serial numbers
  
• Replace seals proactively during fluid changes
  
• Inspect coupling surfaces and bearings for wear
  
• Keep a log of repairs and parts used for future reference
  

For machines with unknown history, tracing serial numbers is essential for accurate parts sourcing and service procedures.
Conclusion
Repairing a transmission seal leak on a Caterpillar 955L is a manageable task with the right tools and information. While locating the serial number may require creativity and persistence, it unlocks access to technical documentation and ensures proper maintenance. These machines were built to last—and with careful attention, they continue to serve decades after leaving the factory floor. In the world of legacy iron, every seal replaced and tag uncovered is a step toward preservation and performance.