Field diagnosis
Boom or arm drifts down under load: cylinder, spool or holding valve
Drift is a leakage question. These checks tell you whether oil is passing the piston seal, the control spool or the holding valve before you remove anything.
Check these in order
| Check | What it looks like | What it tells you |
|---|---|---|
| Test with the engine off | With the lever in neutral and the engine stopped the circuit is isolated from the pump, so drift can only come from the cylinder, the spool or the load-holding device. | Run the test with the machine level and a known load on the implement. |
| Piston seal leakage | Oil passing the piston seal drifts the rod when the oil is warm, and a badly failed seal heats the cylinder barrel. | Disconnect the rod-end line and measure leakage from the port with the cylinder loaded - the standard on-machine piston seal test. |
| Control valve spool clearance | A worn spool leaks to tank; the symptom appears on one function only and worsens as the oil heats up. | Repeat the same test on a function whose spool is known good. |
| Load-holding (load-lock) valve | This valve exists to trap the load; a scored seat, trapped dirt or a weak spring lets it creep. | Open it and inspect seat, poppet and spring, then fit a cartridge kit rather than cleaning only. |
| Counterbalance setting | A counterbalance valve set below its specification opens early and lets the load run. | Confirm the setting against the machine data before ordering parts. |
| Hot and cold behaviour | Drift only when hot points at seal or spool clearance; drift from cold usually points at a setting. | Record both conditions, then decide what to open first. |
Units and parts this usually leads to
Pump builds no pressure
Related check: pressure at the pump outlet.
Parts by number
Catalogue lines for pumps, motors and seals.
Field notes on this complaint
The notes our own team works to on this symptom. Each one is a principle you can check on the machine, a step from the case-drain and cleanliness procedure we follow, or a figure transcribed from our catalogue - not advice copied from a video or a forum. Anything we could not check against a catalogue, a manufacturer document or a working machine is left out.
- Drift has two possible sources. Cylinder piston seal or the control valve spool. Hold the boom with the engine off: if the rod creeps with the valve centred, the leak is normally in the cylinder or its holding valve rather than in the pump. (circuit principle, checkable on the machine)
- Over-centre lowering can cavitate the cap end. Gravity can pull the rod faster than the pump supplies oil when the boom goes over centre. Machines guard that with anti-cavitation (make-up) check valves and work-port reliefs, so judder there is a circuit question before it is a cylinder question. (circuit principle, checkable on the machine)
- Drift with the spool centred is inside the cylinder or the valve, not the pump With the valve in its centre position the pump is not feeding the cylinder at all, so any movement comes from oil crossing from the rod side to the cap side - past the piston seal, past the spool, or through a load-holding valve that is not seating. Ask which way it drifts and how fast: that is what separates a cylinder bypass from a leaking hold, and it decides whether the answer is a seal kit or a valve. (circuit principle, checkable on the machine)
- Pressure intensification: the rod end can see several times the system pressure When a load pulls the rod out with the cap side closed - a check valve holding, a spool centred - the rod end becomes a pump: pressure there is multiplied by the ratio of the two piston areas. Textbook circuits show values well above 5,000 psi in a 2,000 psi system, which is why a cylinder keeps blowing its rod seal while every gauge on the machine reads normal. If the rod seal fails repeatedly, suspect intensification on that circuit before the seal itself. (circuit principle, checkable on the machine)
Common questions
How much drift is normal?
Machine specifications give the permitted creep for a warm machine under a defined load. Compare against your service data rather than a general figure, and always test at working temperature.
Can I stop drift by tightening the relief valve?
No. Relief pressure does not stop oil leaking past a piston seal or a worn spool. It only changes the pressure at which the circuit opens.
Do you supply piston seals and spool parts?
Seals, O-rings, back-up rings and rotary-group kits are catalogue lines. Send the cylinder or valve nameplate and photos of the wear so the right line is identified.
Why it happens
The mechanism behind this symptom, from our technical articles: reading the principle first usually shortens the checklist.
Why extend and retract are not the same: valve control of a differential cylinder
Systems and control
A single-rod cylinder is asymmetric, so speed and force differ in each direction. That explains one-direction weakness - and why a...
Force against speed: matching the power element to the load, and why oversizing cooks the oil
Systems and control
Matching means pressure times flow on the hydraulic side equals force times speed on the load side. The envelope and the duty poin...
Why a hydraulic function overshoots and hunts: damping, correction and what you can change
Systems and control
Every hydraulic loop needs correcting: gain buys speed but costs stability, and the damping ratio moves with temperature and air. ...
More in Boom, arm and cylinder functions: All functions slow · Movement stutters or crawls when the lever is barely open · all areas