UK Construction Equipment Shows Weak Hydraulic Response Only Under Combined Heavy Functions as Load-Sensing Signal-Line Restriction Is Investigated

Case Background

Construction equipment operating in the United Kingdom showed normal hydraulic behavior during light boom, arm and attachment movement.

The complaint appeared only when several functions were combined or when the machine attempted a heavier lift. At that point, hydraulic response became noticeably slower even though the diesel engine maintained normal speed and the main hydraulic pump showed no obvious mechanical damage.

Because the pump could still create pressure under simpler conditions, the investigation moved toward the load-sensing control circuit rather than immediately condemning the pump.

The Pump Needs to Know How Much Load the Actuator Is Seeing

In a load-sensing hydraulic system, pump output is not controlled only by engine speed.

A simplified control relationship is:

Actuator encounters load → Load-sensing pressure is generated → Signal returns to pump controller → Pump increases displacement/pressure margin

The load-sensing signal may travel through:

• small-bore pipes;
• drilled passages;
• shuttle valves;
• compensators;
• or control blocks.

If this signal is restricted or lost, the actuator may demand more hydraulic power while the pump controller receives an incomplete picture of that demand.

Why Light Functions Appeared Normal

During a light function, only limited pump displacement may be required.

Even a weakened load-sensing signal can be sufficient for:

• unloaded boom movement;
• slow arm movement;
• or low-resistance attachment operation.

Under combined UK construction work:

• multiple functions demanded flow;
• load pressure increased;
• the LS circuit needed to transmit that higher demand;
• a restriction reduced signal accuracy;
• pump displacement did not increase as required;
• hydraulic motion became slow.

This created the impression of a weak pump even though the pump itself remained capable of producing more output.

The Investigation Followed the Signal Rather Than the Main Flow

Technicians compared:

• actuator working pressure;
• load-sensing pressure;
• pump-control response;
• shuttle-valve condition;
• signal-line restriction;
• small control orifices;
• and contamination history.

Because LS circuits can use very small passages, contamination that has limited effect on a main hydraulic hose can have a significant effect on a control signal.

Exact pressure margins and control specifications remain machine-specific.

Why Pump Replacement Could Miss the Fault

A replacement pump connected to the same restricted load-sensing circuit may receive the same incomplete control signal.

The new pump can therefore behave similarly.

The diagnostic distinction is:

pump cannot create required flow

versus

pump is not being instructed to create required flow.

UK Combined-Function Operation Helped Isolate the Problem

Construction work involving simultaneous boom, arm and attachment movement created a useful test because it increased hydraulic demand without necessarily changing the mechanical condition of the pump.

The machine’s strong response during single light functions but weak response during combined heavy functions supported investigation of the control signal.

Technical Lesson

Load-sensing systems should be diagnosed as both a power circuit and a control-information circuit.

A healthy hydraulic pump cannot respond correctly if the load signal reaching its controller is restricted.

FAQ

Can a load-sensing fault make a good hydraulic pump appear weak?

Yes. If the pump receives an incorrect load signal, it may not increase output appropriately.

Does normal main pressure rule out an LS problem?

No. Main pressure may be available under some conditions while dynamic pump control remains incorrect.