10-26-2025, 07:00 PM
The bucket curls when unrelated functions are activated
A CAT 336ELH excavator exhibited an unusual hydraulic fault: when the operator extended the stick, raised the boom, or even moved the thumb, the bucket would curl involuntarily. This behavior occurred across multiple functions, except during digging, and persisted even after removing load check valves and verifying relief pressures. The issue pointed toward internal valve body interference or signal crossover.
Terminology Clarification
The operator suspected a broken spring in the spool valve, preventing it from returning to center. After inspecting all four bucket and boom spools, no mechanical damage was found. Relief valves were swapped between circuits, and shuttle valves were replaced. Despite these efforts, the bucket continued to curl during unrelated operations.
A deeper inspection revealed that pressure was building on the rod end of the bucket cylinder when activating other functions. This suggested that the valve body itself was allowing cross-function pressure migration—likely through a shared signal passage or faulty internal seal.
Makeup Valve Spring Failure and Revised Components
One technician noted prior experience with a makeup valve spring unspiraling and jamming the valve seat open. This caused the cylinder to drift toward neutral. CAT later revised the relief valve design to prevent spring unspiraling. In this case, however, the bucket was not drifting passively—it was actively curling, indicating powered movement rather than gravity-induced motion.
ACS Calibration and Electronic Interference
The operator ruled out joystick faults by unplugging all solenoids at the control valve. The bucket still curled when other functions were activated, eliminating electronic misfire as the root cause. Nonetheless, ACS calibration was performed using CAT’s Electronic Technician (ET) software, including IVM (Integrated Valve Module) tests. No change was observed.
Valve Body Signal Contamination
The most likely cause was internal signal contamination within the valve block. Shuttle valves, which resolve directional signals to load checks, may have allowed pressure to bleed across circuits. A technician identified a Rexroth-style cartridge shuttle valve beneath the load check spring cap. If the internal ball or O-rings fail, oil can pass from one function to another, triggering unintended movement.
Recommended Resolution Steps
One field tech recalled a similar issue on a 349E where the boom raise function caused the stick to retract. After weeks of chasing electrical faults, the problem was traced to a cracked internal passage in the valve body. A remanufactured block resolved the issue instantly.
Conclusion
The CAT 336ELH’s hydraulic anomaly—bucket curling during unrelated functions—was not caused by joystick error, relief valve failure, or ACS miscalibration. Instead, it pointed to internal valve body interference, likely through compromised shuttle valves or signal passages. Resolving such issues requires methodical component swaps, pressure tracing, and ultimately valve block disassembly. In modern excavators, hydraulic logic is as complex as electronic control—and just as prone to hidden faults.
A CAT 336ELH excavator exhibited an unusual hydraulic fault: when the operator extended the stick, raised the boom, or even moved the thumb, the bucket would curl involuntarily. This behavior occurred across multiple functions, except during digging, and persisted even after removing load check valves and verifying relief pressures. The issue pointed toward internal valve body interference or signal crossover.
Terminology Clarification
- Spool Valve: A sliding valve element that directs hydraulic flow to specific actuators based on joystick input.
- Load Check Valve: Prevents backflow and maintains pressure in hydraulic cylinders during operation.
- Makeup Valve: Allows fluid into a circuit to prevent vacuum conditions, often integrated into relief valves.
- Shuttle Valve: A hydraulic component that selects between two pressure sources, often used to resolve control signals.
- ACS (Advanced Control System): CAT’s electronic system managing joystick inputs, valve timing, and calibration.
The operator suspected a broken spring in the spool valve, preventing it from returning to center. After inspecting all four bucket and boom spools, no mechanical damage was found. Relief valves were swapped between circuits, and shuttle valves were replaced. Despite these efforts, the bucket continued to curl during unrelated operations.
A deeper inspection revealed that pressure was building on the rod end of the bucket cylinder when activating other functions. This suggested that the valve body itself was allowing cross-function pressure migration—likely through a shared signal passage or faulty internal seal.
Makeup Valve Spring Failure and Revised Components
One technician noted prior experience with a makeup valve spring unspiraling and jamming the valve seat open. This caused the cylinder to drift toward neutral. CAT later revised the relief valve design to prevent spring unspiraling. In this case, however, the bucket was not drifting passively—it was actively curling, indicating powered movement rather than gravity-induced motion.
ACS Calibration and Electronic Interference
The operator ruled out joystick faults by unplugging all solenoids at the control valve. The bucket still curled when other functions were activated, eliminating electronic misfire as the root cause. Nonetheless, ACS calibration was performed using CAT’s Electronic Technician (ET) software, including IVM (Integrated Valve Module) tests. No change was observed.
Valve Body Signal Contamination
The most likely cause was internal signal contamination within the valve block. Shuttle valves, which resolve directional signals to load checks, may have allowed pressure to bleed across circuits. A technician identified a Rexroth-style cartridge shuttle valve beneath the load check spring cap. If the internal ball or O-rings fail, oil can pass from one function to another, triggering unintended movement.
Recommended Resolution Steps
- Remove and inspect all shuttle valves for wear, debris, or seal failure.
- Replace O-rings and verify shuttle ball integrity.
- Pressure test the valve body for internal leakage between adjacent circuits.
- If unresolved, remove the entire valve block for bench inspection and rebuild.
One field tech recalled a similar issue on a 349E where the boom raise function caused the stick to retract. After weeks of chasing electrical faults, the problem was traced to a cracked internal passage in the valve body. A remanufactured block resolved the issue instantly.
Conclusion
The CAT 336ELH’s hydraulic anomaly—bucket curling during unrelated functions—was not caused by joystick error, relief valve failure, or ACS miscalibration. Instead, it pointed to internal valve body interference, likely through compromised shuttle valves or signal passages. Resolving such issues requires methodical component swaps, pressure tracing, and ultimately valve block disassembly. In modern excavators, hydraulic logic is as complex as electronic control—and just as prone to hidden faults.
