Signs Your Excavator Hydraulic Pump Is Failing

When an excavator turns slow or starts to whine, the main pump gets the blame. It is also one of the most expensive parts on the machine. Often the fault is elsewhere: the oil, the suction side, the pilot circuit, the engine, or the valve that tells the pump how hard to work.

Start with which functions are slow. That one question rules out more than any other.

Why are my excavator’s hydraulics slow?

The main pump feeds every function, so a weak pump rarely slows just one. If everything is slow, or fine until the machine works hard, look at what they all share: the oil, the pump and its regulator, the pilot circuit and the engine.

If one function is slow and the rest are normal, the pump is an unlikely cause. Look first at that function’s own parts: the pilot signal from its lever, its section of the control valve, its relief valve, and its cylinder or motor.

Many excavators have two main pumps in one assembly, each feeding its own group of functions, with the boom and arm often taking oil from both. If every slow function shares one pump, that pump or its regulator moves up the list. The split differs between machines; the service manual’s hydraulic schematic shows it.

On many machines each pump also feeds one travel motor, so a weak pump is one reason an excavator tracks to one side.

What does a failing hydraulic pump sound like?

Every piston pump makes some noise. Listen for a change in it.

• A steady whine that rises with engine speed. This often means cavitation. The pump is starved: oil cannot fill its piston bores in time, so vapor bubbles form and then collapse as they reach the pressure side. Common causes are a clogged suction strainer, a low oil level, thick cold oil and a collapsed suction hose.
• A rattle, like gravel in the pump. This often means aeration: outside air drawn in at a loose clamp, a cracked suction hose or a worn seal on the suction side. Look for foam or fine bubbles in the sight gauge. The controls may also feel spongy or jerky.

The sound alone does not settle it. The two can sound alike and often happen together, and foam in the tank is the surer sign of air.

Both ruin a pump long before normal wear would. Collapsing vapor bubbles pit the valve plate and the face of the cylinder block. Air bubbles crushed on the pressure side overheat the oil around them, which darkens and varnishes the parts.

Neither is usually the pump’s fault. The cause stays in the circuit, and a new pump fitted to it fails the same way. Our technical paper on pump inlet conditions, cavitation and aeration sets out both mechanisms and how to tell their damage apart.

Why is my hydraulic oil running hot?

A worn pump leaks inside itself. Oil at working pressure slips back past worn parts, and the power spent pressurizing it becomes heat instead of motion. Oil that runs hotter than it used to, on the same work in the same weather, can be a pump wearing out. Your machine’s gauge and manual say what is normal.

Heat and leakage feed each other: hot oil is thinner and slips past worn parts more easily. So a tired pump is often fine on cool oil and slow on hot.

Other things heat oil too: a leaking or low-set relief valve, a cylinder held against its stop, a clogged cooler, a weak fan, a low oil level. A technician with a non-contact thermometer can usually find where the heat is made.

Why is my excavator losing power?

The main pump is the engine’s biggest load, so pump trouble shows in how the engine behaves when you dig.

• The engine does not pull down, and the machine is weak. A badly worn pump can lose its oil internally before pressure builds, so it asks little of the engine. A main relief valve stuck open does the same.
• The engine bogs down or stalls. The regulator should back the pump off as pressure rises, so it never asks for more power than the engine has. Stuck at full stroke, the pump can drag the engine down.

On many current machines a controller runs pump and engine together, through sensors and a solenoid valve on the regulator. A failed sensor or solenoid can then mimic a worn pump. A weak engine makes the controller cut pump output to prevent a stall, which feels like weak hydraulics. A fault code on the monitor is often the first clue.

What does metal in the hydraulic filter mean?

Filters show what the circuit is shedding. Visible flakes in a used return filter, or a heavy coat of metal on the tank’s magnet if it has one, usually mean something is coming apart. Brass-colored flakes often come from a piston pump or motor. An oil analysis names the metals and narrows down their source.

A tripped bypass indicator on a filter means oil has been going around it, carrying whatever it should have caught.

A failed pump spreads debris through every line it feeds, and a new pump fitted into that circuit inherits it. So the circuit is cleaned first: lines, cooler, tank, strainer and filters, with the motors and valves it fed checked for damage. The technician and the service manual decide how. Our paper on fluid cleanliness and component life explains why it matters.

How does a technician test a hydraulic pump?

Two measurements settle whether the pump is worn.

• Case drain flow, where the manual gives a figure. Oil leaking past the pump’s internal parts returns to the tank through its case drain. The technician measures that flow at the speed and oil temperature the manual states. Too much means worn internals, and a reading inside the figure sends the search elsewhere.
• Flow at pressure. A flow test shows whether the pump still puts out the flow the manual lists as pressure rises. On a two-pump machine, one falling well short of the other is telling.

Leave these tests to a qualified technician with the right gauges and the service manual. A hydraulic circuit can hold pressure after shutdown, and working oil can be hot enough to burn. Never feel for a leak with your hand. A pinhole leak can inject oil through the skin, and that is a medical emergency even when the wound looks small.

Is it the pump or its regulator?

The regulator is the control valve on the pump. It sets how far the pump strokes, and so how much oil it puts out. It works from system pressure, pilot signals and, on many current machines, a signal from the controller.

A pump blamed for weak hydraulics can be sound. Its regulator may be sticking, out of adjustment or not getting its signal, so the pump is never told to stroke. A failed solenoid, sensor or connector does the same, and these can often be replaced on their own.

Two pumps can look the same and fit the same mounting, yet carry different regulators: another control scheme, power setting or solenoid. On the wrong machine, such a pump can run slow, stall the engine or set fault codes. So the replacement has to match the regulator and control, not only the housing and ports.

What should you check before replacing the pump?

Several of these are yours to check. Anything that opens the hydraulic system is a technician’s job.

Check	What it rules out	Who
Work mode and engine speed setting	A machine slowed on purpose by an economy or lifting mode	You
Fault codes on the monitor	A failed sensor, solenoid or controller	You, then a technician
Oil level, in the manual’s checking position	A starved pump, or air drawn in from a low tank	You
Oil in the sight gauge	Water or air in the oil, if it looks milky or foamy	You
Engine air and fuel filters	An engine that cannot carry the pump’s load	You, then a technician
Hydraulic filters and any bypass indicator	A blocked filter, and metal that points to wear	A technician
Suction strainer	A starved pump inlet	A technician: the tank may be under pressure, the oil hot
Pilot pressure and the pilot filter	Weak signals to the control valve and the regulator	A technician with gauges

How do you get the right replacement pump?

If the pump is at fault, start with the number on the tag: it usually identifies the whole assembly, regulator included. Add the machine’s make, model and serial number, and say whether the regulator carries solenoids and a wiring connector. Send that to a dealer through the request form, or find a dealer near you. The main hydraulic pumps page has the short version of these checks.