Field diagnosis
Every function slow at once: system-level causes in check order
When travel, swing, boom and bucket are all slow, the fault sits upstream of the individual circuits. Work these system checks before ordering anything.
Check these in order
| Check | What it looks like | What it tells you |
|---|---|---|
| Main relief pressure on both circuits | If the pump sounds normal and every movement is slow and weak, system pressure is the first suspect. | Gauge at the pump outlets and compare with the machine specification. |
| Pump stroke at standby | If displacement never rises above minimum, flow can never meet demand. | Measure standby pressure and watch the regulator stroke. |
| Pilot pressure | Low pilot pressure moves every control spool less than commanded, so the whole machine feels tired. | Check accumulator charge and pilot relief setting - a flat accumulator shows up mainly at idle. |
| Suction strainer, oil level and suction hose | Starvation reduces flow as engine speed rises and adds the noise that workshops often read as pump failure. | Inspect the strainer and hose; a hose that collapses internally can look perfect from outside. |
| Oil temperature trend | Hot thin oil increases leakage everywhere, so cycle time falls through the shift. | Log temperature over two hours of duty; if it keeps climbing, treat the machine as an overheating case first. |
| Case drain flow of both pumps | Two pumps worn beyond specification slow every function together. | Measure case drain on both pumps at the same working temperature and compare pump to pump. |
Units and parts this usually leads to
K3V112DTP
20–25 t hydraulic excavator - 112 mL/r (per pump)
K5V140DTP
27–33 t hydraulic excavator - 140 mL/r (per pump)
K3V112S
10–12 t excavator and industrial open circuits - 112 mL/r
K5V80S
6–10 t hydraulic excavator and industrial open circuits - 80 mL/r
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.
- Speed is flow, force is pressure. A machine that is slow but still digs hard points at flow: regulator setting, spool travel or a blocked parallel passage. A machine that is fast but weak points at pressure: relief setting or an internal leak. Decide which of the two you have before ordering anything. (circuit principle, checkable on the machine)
- Check the engine reaches rated rpm first. Pump flow follows engine speed. Verify full throttle actually reaches rated rpm (and that the throttle linkage is not stopping short) before condemning the pump. (circuit principle, checkable on the machine)
- Load sensing works only while the pump can out-supply the valves Load sensing holds a constant pressure difference only as long as pump flow covers the sum of what the open functions demand (Q_pump >= sum Q_valve). Take the pump below that - a worn pump, a saturated function, a cold-oil flow limit - and the compensating valve on the heavy branch stands fully open, the pressure difference collapses and the lightest function quietly takes the flow. That saturation is exactly what LUDV was introduced to fix. So everything slowing down together is a flow-budget question first. (circuit principle, checkable on the machine)
- Which system is on the machine? The centre condition decides the diagnosis An open-centre circuit - the prerequisite of a negative-flow (NFC) system - unloads the pump at neutral. A closed-centre circuit - the prerequisite of load sensing / CLSS - holds pressure at neutral. The two fail in different ways and send you to different components. Before ordering anything for an all-functions-slow machine, confirm which of the two it is; on many brands it is written into the model code. (circuit principle, checkable on the machine)
- Measure the load-sense margin: outlet pressure should be load plus margin On a load-sensing machine the pump is meant to hold a fixed margin above the highest load pressure - 300 psi in textbook circuits - so the number to write down is not the outlet pressure alone but the difference between the outlet and the load signal. If the margin is right, the pump is doing its job and the restriction is downstream; if the margin is missing, test the compensator and the LS line, and check that the line is not blocked or the signal not taken from the wrong point. (our own case-drain and cleanliness procedure)
Common questions
All functions are slow - is the pump always the cause?
No. Relief pressure, pilot pressure and oil temperature slow every circuit without any pump wear. Only case drain flow together with output under load identifies a worn rotary group.
How do I separate low relief pressure from a worn pump?
A worn pump still reaches near-specification pressure at low flow but cannot hold it at high flow. A low relief setting caps pressure at every flow, hot or cold.
Can this be corrected with adjustments alone?
Sometimes. Relief, pilot and regulator settings are field-adjustable. If case drain flow is above the specification for your unit, the rotary group needs parts.
Why it happens
The mechanism behind this symptom, from our technical articles: reading the principle first usually shortens the checklist.
Same valve, different speed: what "the current controls the opening" really means
Components and valves
A proportional valve controls an opening, not a flow. That one fact explains speed droop, why identical machines differ, and what ...
Speed droop, travel drift and run-away: what a velocity loop changes on a machine
Systems and control
Open-loop throttle control droops with load and can run away downhill. A velocity loop holds speed until the supply runs out of fl...
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...
More in Boom, arm and cylinder functions: Cylinder drifts down under load · Movement stutters or crawls when the lever is barely open · all areas