Hydraulic Leaks in the Swing Motor Brake Assembly of the John Deere 690E-LC

[MikePhua]

The John Deere 690E-LC and Its Excavation Heritage
The John Deere 690E-LC hydraulic excavator was part of Deere’s E-series lineup introduced in the early 1990s, designed to compete in the 40,000–50,000 lb class. With an operating weight around 47,000 lbs and powered by a turbocharged diesel engine, the 690E-LC was built for general excavation, trenching, and heavy utility work. Deere’s LC designation refers to “long carriage,” indicating extended track length for improved stability during digging and lifting.
The swing system on the 690E-LC includes a hydraulic motor mounted atop a reduction gearbox, which drives the upper structure rotation. Integrated into this assembly is a hydraulic brake mechanism designed to hold the upper frame stationary when not swinging. Over time, seals within this system can degrade, leading to internal leaks that manifest externally—often misdiagnosed or overlooked until performance suffers.
Identifying the Source of Hydraulic Fluid Leakage
Operators encountering hydraulic fluid dripping from the bottom of the swing gearbox often suspect a failed external seal. However, the root cause may be internal contamination from the swing motor or brake assembly. When the shaft seal on the swing motor fails, pressurized hydraulic oil can migrate into the reduction gearbox, mixing with gear grease and overfilling the housing. This diluted mixture then escapes through the lowest point—typically a weep hole or failed gasket.
In one inspection, the cover plug near the rotary manifold revealed 1.5 inches of grease heavily thinned by hydraulic oil. This finding strongly suggests internal migration rather than external intrusion.
Likely sources include:

• Swing motor shaft seal failure
  
• Brake piston seal leakage
  
• Common seal interface between motor and gearbox
  
• Bearing wear allowing shaft misalignment and seal distortion
  

Understanding the Swing Brake Function and Its Vulnerabilities
The swing brake on the 690E-LC is a spring-applied, hydraulically released mechanism housed within the reduction gearbox. When hydraulic pressure is applied, pistons compress the brake springs, allowing the upper structure to rotate. When pressure is removed, the springs engage the brake, locking the swing.
This system relies on piston seals to contain hydraulic fluid. If these seals fail, fluid leaks into the gearbox cavity, contaminating the gear oil and eventually pushing out through the housing. Because the brake is internal, leaks may go unnoticed until the gearbox is overfilled or the swing becomes erratic.
Symptoms of brake seal failure:

• Hydraulic fluid in gear oil
  
• Swing hesitation or brake drag
  
• Increased leakage during swing operation
  
• Brake not holding on slopes or during shutdown
  

Recommended Diagnostic and Repair Procedures
To confirm the source of leakage, technicians should:

• Drain the swing gearbox and inspect fluid composition
  
• Remove the swing motor and inspect shaft seals and bearings
  
• Disassemble the brake assembly and inspect piston seals
  
• Check for scoring or wear on the motor shaft and brake bore
  
• Replace all seals with OEM-grade components and reassemble with proper torque
  

If the swing motor shaft shows wear, it may need machining or replacement. Bearings should be checked for play, as excessive movement can compromise seal integrity.
Preventive steps:

• Replace gear oil annually and monitor for contamination
  
• Use high-viscosity gear grease rated for hydraulic contamination resistance
  
• Install a magnetic drain plug to capture metal particles
  
• Inspect swing motor mounting bolts and gasket surfaces for leaks
  

Field Anecdotes and Operator Experience
A mechanic in Texas reported that his 690E-LC developed a swing leak after years of reliable service. Upon inspection, the gearbox was filled with a slurry of grease and hydraulic oil. After replacing the swing motor shaft seal and brake piston seals, the leak stopped and swing performance improved noticeably.
Another operator noted that his machine’s swing brake failed to hold during shutdown on a slope. The cause was traced to a leaking piston seal that reduced brake pressure. Replacing the seal restored holding power and eliminated the leak.
Conclusion
Hydraulic leaks in the swing motor brake assembly of the John Deere 690E-LC are often caused by internal seal failures that allow fluid migration into the reduction gearbox. Diagnosing the issue requires careful inspection of the swing motor, brake pistons, and gearbox fluid. With proper sealing, maintenance, and attention to wear components, the 690E-LC can continue to deliver reliable performance in demanding excavation environments. These machines may be aging, but with the right care, their swing systems can remain tight and responsive for years to come.