Resolving Hydraulic Weakness in the Cat 336EL Excavator

[MikePhua]

The Evolution of the Cat 336EL
The Caterpillar 336EL hydraulic excavator is part of the E-Series lineup, introduced in the early 2010s as a successor to the D-Series. Designed for heavy-duty earthmoving, demolition, and infrastructure work, the 336EL features a C9.3 ACERT engine delivering up to 303 horsepower and an operating weight of approximately 80,000 lbs. Its advanced hydraulic system, integrated with Cat’s Electronic Control Module (ECM), allows for precise multifunction operation and fuel-efficient performance.
Caterpillar Inc., founded in 1925, has consistently led the global market in construction machinery. By 2015, the 336E series had sold tens of thousands of units worldwide, particularly in North America, Europe, and Asia. Its popularity stems from its balance of power, control, and serviceability, making it a staple on large excavation sites and utility projects.
Symptoms of Hydraulic Dysfunction
Operators have reported a peculiar issue with the 336EL: the swing function and right track movement are notably weak unless another function is activated simultaneously. This phenomenon—where performance improves during multifunction operation—suggests a problem not with the hydraulic pumps themselves, but with the pressure signaling system that controls pump output.
In hydraulic terminology, “stroke up” refers to the pump increasing its displacement to meet higher flow demands. The 336EL uses a dual-pump system, each regulated by pressure feedback from the control valves. When a function demands high pressure, the system responds by increasing pump output. If this feedback is disrupted, certain functions may receive insufficient flow, resulting in sluggish or weak movement.
Understanding the Resolver Network
At the heart of the issue lies the resolver network—a series of shuttle valves that detect the highest pressure among active functions and relay that signal to the pump controller. A shuttle valve, also known as a resolver, allows fluid from multiple sources to pass through a single outlet, selecting the path with the highest pressure.
If a resolver valve is stuck, bypassing, or contaminated, the pump may not receive the correct signal to increase output. This explains why swing and right track functions are weak when used alone but recover when paired with high-pressure functions like stick or boom movement.
Diagnostic Strategy and Valve Mapping
Experienced technicians recommend a structured approach to isolate the faulty component:

• Hydraulic Schematic Review: Obtain the full hydraulic diagram and trace the resolver network. Identify which valves correspond to swing, travel, boom, and stick functions.
  
• Function-by-Function Testing: Operate each hydraulic function individually and observe pump response. Note which combinations restore swing or travel power.
  
• Pressure Monitoring: Use diagnostic tools to measure pressure at key points in the circuit. Compare expected values with actual readings during single and multifunction operations.
  
• Check Valve Inspection: Inside the main control valve, check valves regulate flow between pumps and prevent cross-contamination. A failed check valve can allow pump oil to bypass critical circuits, reducing pressure to swing and travel motors.
  
• Relief Valve Examination: The NFC2 relief valve, located at the bottom of the main valve block, has been identified as a failure point. If the relief seat is damaged, it can bleed off pressure prematurely, weakening downstream functions.
  

Field Anecdote and Resolution
One operator in British Columbia faced this exact issue. His 336EL, prefix BZY, showed weak swing and right track movement unless the stick was engaged. After ruling out electronic faults using Cat ET diagnostics, he inspected the shuttle valves and check valves manually. With dealer support, the team discovered that the NFC2 relief valve had failed, damaging its seat and allowing pressure to escape.
Replacing the relief valve restored full hydraulic performance. Interestingly, the failure occurred in the bottom section of the valve block, contrary to expectations, highlighting the importance of thorough inspection beyond common assumptions.
Additional Case and Cross-Model Comparison
A similar issue was reported on a 336F model with RKB serial. The machine would not track on the right side and had weak swing until the stick was activated. This cross-model consistency suggests a shared vulnerability in the resolver and check valve architecture across the E and F series.
Technicians noted that the check valves in the bottom section of the main valve block are designed to allow flow from each pump to the main relief while preventing crossover. If one fails, pressure from pump 2 may bypass through the stick spool, starving the swing and travel circuits unless the stick is active.
Preventative Measures and Recommendations
To avoid similar failures and ensure long-term hydraulic health, consider the following:

• Routine Valve Inspection: Include shuttle and check valves in scheduled maintenance. Look for signs of wear, contamination, or corrosion.
  
• Filter Analysis: Cut open hydraulic filters during service intervals to detect metal particles or debris that may indicate internal wear.
  
• Use Clean Hydraulic Fluid: Contaminated oil accelerates valve degradation. Maintain strict fluid cleanliness standards.
  
• Replace Valves in Pairs: When one check valve fails, its counterpart may be compromised. Replace both to ensure balanced flow.
  
• Document Serial-Specific Issues: Keep records of valve failures by serial number to identify patterns and anticipate future problems.
  

Conclusion
The Cat 336EL remains a powerful and reliable excavator, but its hydraulic system demands careful attention to pressure signaling components. Weak swing and travel functions often point to resolver or check valve failures rather than pump or electronic faults. By understanding the interplay between hydraulic circuits and pressure feedback, operators and technicians can diagnose and resolve issues efficiently, keeping these machines productive and precise in demanding environments.