Diagnosing Fluid Leaks from the Swing Drive on a CAT 345BL

[MikePhua]

Understanding the CAT 345BL’s Swing System
The 2003 CAT 345BL hydraulic excavator is a heavy-duty machine built for mass excavation, demolition, and deep trenching. With an operating weight exceeding 90,000 lbs and a bucket breakout force over 50,000 lbf, it relies on a robust swing system to rotate the upper structure. This system includes two swing motors mounted on either side of the turntable, each driving a planetary gearbox that engages the swing ring. The swing drive is lubricated with gear oil, while the motors operate on high-pressure hydraulic fluid—two distinct systems that must remain isolated.
Symptoms of Cross-Contamination and Leakage
A recurring issue on older 345BL units involves fluid leaking from the turntable area, often appearing as a mix of hydraulic oil and gear oil. In one case, the operator discovered fluid pooling under the swing bearing after a pond excavation job. Upon inspection, the swing motor closest to the fuel tank—marked with a CAT remanufactured tag—was suspected to be the source.
The leak reappeared days after draining and resealing the hatch, suggesting internal contamination rather than external seepage. The gearbox oil level fluctuated, sometimes overflowing from the fill plug, while hydraulic fluid levels dropped—indicating a breach between systems.
Terminology Clarification

• Swing Motor: A hydraulic motor that drives the swing gearbox, enabling upper structure rotation.
  
• Planetary Gearbox: A gear system that multiplies torque from the motor to rotate the swing ring.
  
• Swing Ring Grease Bath: A sealed cavity around the swing bearing filled with grease to reduce friction.
  
• Dipstick/Inspection Plug: Used to check oil level and condition in the gearbox.
  

Root Cause and Diagnostic Strategy
The most likely cause is a failed internal seal within the swing motor, allowing hydraulic oil to leak into the gearbox. As pressure builds, the contaminated gear oil is forced past the gearbox seal into the swing ring cavity, eventually leaking externally.
To confirm this:

• Check gearbox oil levels in both swing drives. Sudden overfill suggests hydraulic intrusion.
  
• Inspect oil color and viscosity. Gear oil is typically darker and thicker; hydraulic oil is lighter and more fluid.
  
• Monitor hydraulic reservoir levels. A slow drop without external leaks points to internal loss.
  
• Use UV dye in hydraulic fluid to trace migration into the gearbox.
  

Repair Options and Parts Availability
If the swing motor is leaking internally, it must be rebuilt or replaced. CAT offers remanufactured swing motors with updated seals and pressure-tested housings. A rebuild kit typically includes:

• Shaft seals
  
• O-rings
  
• Bearing sets
  
• Pressure relief valves
  

Rebuilding requires removal of the swing motor, which may involve lifting the fuel tank side panel. Some technicians have completed this without full cab removal, depending on access and tooling.
Preventive Measures and Long-Term Solutions

• Flush contaminated gear oil and refill with OEM-spec 80W-90 gear lubricant.
  
• Replace both swing motor seals if one has failed—symmetrical wear is common.
  
• Install a magnetic drain plug to monitor future contamination.
  
• Log fluid levels weekly to detect early signs of cross-leakage.
  

One operator in Michigan reported that after replacing the swing motor closest to the fuel tank, the issue resolved permanently. The remanufactured unit had a 2020 tag, but the original repair was likely incomplete or improperly sealed.
Conclusion
Fluid leaks from the turntable area on a CAT 345BL often stem from internal seal failure in the swing motor, allowing hydraulic oil to contaminate the gearbox. By monitoring fluid levels, inspecting oil characteristics, and rebuilding the affected motor, operators can restore system integrity and prevent further damage. With proper diagnosis and OEM parts, this heavy excavator can continue performing reliably in demanding conditions.