Hydraulic Pump Identification and Replacement on a 1975 Caterpillar 988 Wheel Loader

[MikePhua]

The Evolution of the CAT 988 and Its Hydraulic System
The Caterpillar 988 wheel loader, introduced in the 1960s, quickly became a cornerstone of heavy material handling in mining, quarrying, and large-scale earthmoving. By 1975, the 988 had undergone several refinements, including updates to its hydraulic system to support higher lift capacities and faster cycle times. The machine featured a robust frame, planetary final drives, and a high-displacement diesel engine, but its hydraulic system remained the heart of its operational efficiency.
The main hydraulic pump on the 988 was mounted behind the rear cab window, accessible through the upper deck and firewall. This pump powered the lift arms, bucket tilt, and auxiliary functions. Over time, Caterpillar sourced components from various suppliers, including Vickers, a well-known manufacturer of hydraulic pumps and valves.
Terminology annotation:
- Hydraulic pump: A mechanical device that converts mechanical energy into hydraulic energy, supplying pressurized fluid to actuators.
- Axial piston pump: A type of hydraulic pump where pistons move parallel to the drive shaft, offering variable displacement and high efficiency.
- Cartridge assembly: A modular internal component of a pump that can be replaced without removing the entire housing.
- Gear pump: A simpler hydraulic pump using meshing gears to move fluid, typically used in low-pressure systems.
Identifying the Original Pump Configuration
In many 1975 CAT 988 units, the main hydraulic pump bore a small tag marked “CAT,” but the internal design and casting often matched Vickers specifications. This led to confusion during replacement, especially when sourcing parts decades later. Without a serial number—such as the 87A prefix used on early 988s—it's difficult to determine whether the pump was a Vickers vane-type, axial piston, or a later gear-type retrofit.
Operators who serviced these machines in the 1980s and 1990s often encountered difficulty sourcing direct replacements. Caterpillar eventually phased out support for older pump models, but individual cartridges and seals remained available through aftermarket suppliers and remanufacturers.
Recommendations for Pump Replacement and Sourcing
To replace or rebuild the hydraulic pump on a 1975 CAT 988:

• Locate the machine’s serial number to identify the exact hydraulic configuration
  
• Inspect the pump tag for manufacturer codes, displacement ratings, and rotation direction
  
• Determine whether the pump is a fixed or variable displacement unit
  
• If the pump is a Vickers model, cross-reference with Eaton-Vickers catalogs for cartridge kits
  
• Consider remanufactured options from hydraulic rebuilders specializing in legacy CAT equipment
  

For axial piston pumps, ensure that the swashplate angle and control mechanism match the original. Gear pumps, while simpler, may require modifications to flow rate and pressure settings.
Field Anecdotes and Practical Advice
One millwright recalled replacing the pump on a 988 in Buffalo, New York. The pump was mounted vertically behind the cab and required careful maneuvering to extract. Although the tag read “CAT,” the internal layout matched a Vickers vane pump. At the time, sourcing a direct replacement was impossible, but a remanufactured cartridge restored full function.
Another technician working in a stone quarry noted that newer 988 models transitioned to gear-type pumps for simplicity and durability. However, these lacked the fine control of axial piston units and were less efficient under variable load conditions.
Preventive Maintenance and Long-Term Reliability
To extend the life of hydraulic pumps on older CAT loaders:

• Change hydraulic fluid every 1,000 hours or annually
  
• Use fluid with anti-wear additives and proper viscosity (typically ISO 46 or ISO 68)
  
• Replace suction filters and inspect return lines for contamination
  
• Monitor pump noise and temperature during operation
  
• Check for leaks around the shaft seal and housing
  

If the pump begins to whine, chatter, or lose pressure, inspect the internal wear surfaces and consider a rebuild before catastrophic failure.
Caterpillar and Vickers Historical Context
Caterpillar, founded in 1925, has long partnered with component suppliers to streamline production. Vickers, established in the early 20th century and later acquired by Eaton Corporation, was a leading name in hydraulic technology. Their pumps powered everything from aircraft to industrial machinery, and their modular cartridge design made field repairs feasible even in remote locations.
By the 1990s, many CAT machines transitioned to in-house hydraulic components or partnered with other suppliers, but Vickers pumps remained in circulation through rebuilders and surplus dealers.
Conclusion
Replacing the hydraulic pump on a 1975 CAT 988 requires more than just part numbers—it demands an understanding of legacy systems, supplier history, and field adaptability. Whether the pump was a Vickers axial piston unit or a later gear-type retrofit, careful identification and sourcing are key to restoring performance. For those maintaining vintage iron, the challenge is part of the reward—and every successful rebuild keeps a piece of heavy equipment history alive and working.