Forklift Mast Would Bog the Engine but Wouldn't Move
Hydraulic control-valve problems do not always produce one clear symptom. A recent Williston-area forklift had visible hydraulic leakage, but it also experienced unusual mast operation.
Sometimes, activating a mast function would bog the engine as if the hydraulic system were heavily loaded, but the mast would not move. At other times, a function would operate in one direction but not the other.
Those symptoms showed that this was more than a simple external leak.
Why Failed Control-Valve Seals Cause Strange Symptoms
A forklift's hydraulic control valve directs pressurized oil to the lift, tilt and other hydraulic functions. Internal seals and O-rings help keep the valve's pressure and return passages separated.
When those seals become hardened, damaged or displaced, oil may leak externally or bypass internally. Depending on where the failure occurs, the operator may experience:
- Hydraulic oil leaking around the valve
- An engine bogging when a control is activated
- A mast function that will not move under load
- A function that works in one direction but not the other
- Slow, weak or inconsistent operation
- Cylinders that drift
- One hydraulic function affecting another
An engine bogging when the mast control is activated can make it appear that the hydraulic pump is producing pressure while something prevents the function from responding correctly. That does not automatically prove that the pump, cylinder or complete control valve needs replacement. The complete hydraulic circuit must be inspected.
Repairing the Three-Section Control Valve
Accessing this forklift's control valve required removing the control knobs, levers, linkage bar and protective cover.
The repair included:
- Removing and cleaning the control covers and rods
- Accessing all three control-valve sections
- Replacing the O-rings with the correct main-control-valve seal kit
- Reinstalling the control rods, levers and protective components
- Greasing the pins, rods and pivot points
- Restoring the hydraulic-fluid level
- Operating the mast controls in both directions
- Inspecting the valve and connections for leakage
Cleaning the components was an important part of the repair. Dirt and accumulated hydraulic oil can hide the source of a leak and introduce contamination while the valve is open.
After the valve was resealed and the controls were reassembled, the forklift was operated and tested. The hydraulic functions returned to normal operation, and no leaks were found during the final inspection.
This is equipment-level troubleshooting: document the symptoms, inspect the complete system, repair the actual failure and verify every affected function afterward.
Mondak Equipment & Hose handles forklift and material-handling equipment repairs involving hydraulic, mechanical, electrical and control-system problems. Contact our team when your equipment has an intermittent or unusual problem that requires more than replacing parts.