Resolving Coolant Seal Failure on the CAT 953 Track Loader

[MikePhua]

A Common Leak Point in Aging CAT 953 Machines
The Caterpillar 953 track loader, particularly models from the 20Z serial series, has proven itself as a durable earthmoving machine across decades of service. With a robust 3204 diesel engine and hydrostatic drive, the 953 was built to handle grading, loading, and site prep with minimal downtime. However, as these machines age, coolant system leaks become increasingly common—especially around the water pump and engine interface.
One recurring issue involves a rubber seal located between the water pump and the engine block. This seal compresses between the cylinder head and the front housing, forming a critical barrier against coolant loss. When the seal fails, operators may notice coolant dripping from the front of the engine, gradual reservoir depletion, or overheating under load.
Terminology Clarification

• Water Pump: A belt-driven or gear-driven component that circulates coolant through the engine and radiator.
  
• Ferrule: A metal sleeve that reinforces the seal and ensures proper compression.
  
• RTV (Room Temperature Vulcanizing) Sealant: A silicone-based compound used to enhance sealing surfaces and prevent leaks.
  
• SCA (Supplemental Coolant Additive): A chemical additive blended into antifreeze to protect against corrosion and cavitation, especially in engines with wet liners.
  

Seal Replacement Strategy and Practical Advice
Replacing the seal between the water pump and engine requires careful disassembly. The seal itself is part of a three-component system:

• Rubber compression ring
  
• Ferrule sleeve
  
• Clamp or retainer
  

In one repair case, the operator removed the failed seal and cleaned the mating surfaces thoroughly. Although the cylinder head remained installed, RTV sealant was applied sparingly to ensure a tight seal. The new components were torqued evenly, and the system was monitored for several days to confirm integrity.
If the cylinder head is removed during the repair, cleaning and alignment become easier. However, with the head in place, access is limited, and surface prep must be done with precision tools and patience.
Coolant Selection and System Flushing
After resolving the leak, flushing the coolant system is recommended. The CAT 953 has a coolant capacity of approximately 5.8 gallons. Operators should use:

• CAT-branded antifreeze with pre-blended SCA
  
• A 50/50 mix with distilled or deionized water
  
• Coolant test strips to verify additive levels and freeze protection
  

In one case, the operator had previously added automotive antifreeze without knowing its compatibility. After consulting with a dealer, he switched to CAT full-strength coolant and mixed it properly. The counter staff confirmed that modern CAT antifreeze includes SCA, eliminating the need for separate conditioners.
Preventive Measures and Long-Term Monitoring

• Inspect coolant seals annually, especially in machines over 5,000 hours
  
• Use distilled water to prevent mineral buildup and electrolysis
  
• Monitor coolant levels weekly and check for signs of airlock or pressure loss
  
• Replace coolant every 2–3 years or as indicated by test strip results
  

One technician noted that a small bleed line near the water pump can be loosened to purge air during refilling—a useful tip for avoiding trapped air pockets that lead to overheating.
Conclusion
Coolant leaks on the CAT 953, particularly at the water pump seal, are a manageable issue with the right parts and preparation. By replacing the seal assembly, using compatible coolant, and flushing the system properly, operators can restore cooling performance and prevent future failures. As with all aging equipment, proactive maintenance and attention to detail are the keys to keeping these machines productive.