Hydraulic pump failure symptoms at a glance
The hydraulic pump is the heart of an excavator's hydraulics. It converts the diesel engine's power into oil flow, which drives every cylinder and hydraulic motor on the machine. When its output drops, the whole machine suffers. One point matters for diagnosis: not every symptom means the pump itself has failed. Use the table below to narrow down the cause before any parts are replaced.
| Symptom | Likely cause | Next check |
|---|---|---|
| All functions slow, lacking power | Internal pump wear, faulty pump regulator, low pilot pressure, incorrectly set relief valve | Check engine speed, measure working and pilot pressure at the test ports and compare with specified values |
| Whining or screaming, louder under load | Cavitation: restricted suction side, low oil level, oil too cold or too viscous | Check oil level, suction strainer, suction line and tank breather |
| Knocking, rattling, rough running | Air in the oil, bearing damage inside the pump | Check suction side for leaks, look for foam in the oil, locate the noise |
| Oil temperature rises unusually fast | High internal leakage, dirty oil cooler, relief valve opening constantly | Check cooler and fan, measure the pump's case drain flow |
| Oil foaming or milky | Air drawn in on the suction side, water in the oil | Check hose clamps, suction line and shaft seal, take an oil sample |
| Metal particles in filter or oil | Wear in pump, hydraulic motor or cylinder | Open the filter, assess the particles, arrange an oil analysis |
| Oil leaking at the pump shaft seal | Worn seal, excessive case pressure from a restricted drain line | Check the case drain line is clear, measure case pressure |
| Jerky movements, fluctuating pressure | Air in the system, regulator fault, sticking valve spool | Watch the pressure on a gauge, test functions one at a time |
The binding values for pressures, engine speeds and case drain flow are in the machine manufacturer's service manual. Without them, no measurement can be judged reliably.
Slow or weak hydraulics: is the pump to blame?
The most common complaint is hydraulics that respond sluggishly or give way under load. A pump produces flow. Pressure only builds up through resistance in the system. As a pump wears, its internal leakage increases: some of the oil slips back inside the pump instead of reaching the actuators. This leakage grows with pressure and with temperature.
That produces a typical pattern. The machine works reasonably well first thing in the morning, then becomes noticeably weaker once the oil is warm and the machine is under load. This behaviour points strongly to internal wear in the pump.
Most excavators use variable displacement piston pumps with power control. If the regulator misbehaves, the pump does not swivel out fully and delivers too little flow, even though the rotating group itself is fine. Other causes outside the pump include:
- low engine speed or falling engine output
- low pilot pressure, for example from a worn pilot pump
- a main relief valve that is set too low or leaking
- blocked filters or a restricted suction strainer
So always check engine speed and pilot pressure first, before suspecting the main pump.
Whining and cavitation: reading pump noises
A new, unusual noise is often the earliest warning. Two different mechanisms are worth separating:
- Cavitation: excessive vacuum develops on the suction side. Vapour bubbles form and collapse violently as they reach the pressure side. The result is a steady, high-pitched whine, and the collapsing bubbles erode surfaces inside the pump.
- Aeration (air in the oil): air is drawn in through a leak. The noise tends to be irregular, rattling or knocking, often with foam visible in the tank.
Typical causes are a low oil level, a clogged suction strainer, a kinked suction line or one with a collapsed inner lining, a blocked tank breather, loose clamps on the suction side and oil of the wrong viscosity. A cold start with very thick oil can also cause brief cavitation, so in cold weather let the hydraulics warm up without load.
Bearing damage inside the pump, by contrast, usually sounds like grinding or rumbling that changes with engine speed. Carrying on with a persistent whine risks follow-on damage in a short time.
Overheating and foaming hydraulic oil
Every leak in the system turns hydraulic energy into heat. A worn pump with high internal leakage therefore heats the oil more. Hot oil becomes thinner and leakage rises further: a cycle that accelerates wear. Before blaming the pump, check the oil cooler, fan and oil level. A cooler clogged with dust is a frequent and easily fixed cause.
Foaming oil points to air in the system. Foam is compressible, movements feel spongy and the oil ages faster. Milky, cloudy oil indicates water, for example from condensation or from washing the machine. In either case, remove the cause and change the oil as the manufacturer specifies, otherwise the pump and every other component will suffer.
Metal particles in the oil or filter: what they mean
A small amount of fine wear debris is caught by the filter and is normal over the machine's working life. Visible chips, glittering particles or metal flakes in the filter element are a serious warning. Colour gives a clue to the source: yellowish particles suggest bronze or brass parts, which are used in many piston pumps and hydraulic motors. Steel particles can come from bearings, gears or pistons.
A laboratory oil analysis shows the type and quantity of particles. What matters most is the consequence: debris spreads through the whole circuit. After a pump failure, the system needs flushing, all filters need changing and other components need checking. If only the pump is replaced, the remaining contamination can damage the new one too.
Pump, valve or motor? Narrowing down the fault
The key diagnostic question is whether the problem affects all functions or just one. A great deal follows from the answer:
- All functions weak: suspect the supply side, meaning pump, pump regulator, pilot pressure, main relief valve or engine speed.
- Only the functions of one pump circuit weak: many excavators have two main pumps with the functions split between them. If only one circuit is affected, the pump feeding it becomes the focus.
- Only one function weak: more likely the valve section, port relief valve, cylinder or motor for that function.
- Only slewing weak: swing motor, swing gearbox or associated valves, see swing drives and swing motors.
- Only one track side weak: travel motor, rotary joint or travel valve on that side.
For measurements on the hydraulic system, this sequence has proven itself:
- Make safe: park the machine on level, firm ground, lower the attachments to the ground, stop the engine and remove the key. Depressurise the hydraulics as the manufacturer specifies and release tank pressure. Let hot oil cool down.
- Visual check: inspect oil level, hoses, fittings and the suction line. Never feel for leaks with your hand: pressurised oil can penetrate the skin.
- Assess oil and filter: check colour, foam, water and particles.
- Check engine speed: compare idle and rated speed with the specified values.
- Measure pilot pressure: at the designated test port with a suitable gauge.
- Measure working pressure: for each pump circuit, including the opening pressure of the main relief valve.
- Assess pump output: measure flow with a flow meter or, on piston pumps, measure the case drain flow.
- Compare and record: check all readings against the service manual and write them down.
Increased case drain flow on a piston pump is a clear sign of internal wear. Pressure measurements should only be carried out by qualified staff with the right test equipment. Pumps and motors of similar design wear in similar ways, so it pays to measure before ordering either a hydraulic motor or a pump.
Repair or replace the hydraulic pump?
Many hydraulic pumps can be overhauled. Whether that makes sense depends on several factors:
- Extent of damage: a leaking shaft seal is a very different matter from scored control surfaces or a destroyed rotating group.
- Parts availability: seal kits and internal parts are not always available for older pumps.
- Downtime: an overhaul involves stripping, inspection and a test bench run. Fitting a new pump can shorten the time the machine stands idle.
- Knock-on damage: if debris has spread, flushing and filter changes are needed either way.
For an overhaul, a specialist hydraulic workshop is the right contact. GAM supplies new hydraulic pumps. Fitting should be carried out by a specialist workshop. Whatever you decide, the root cause must be dealt with first. Contaminated oil or a suction problem will put a new or overhauled pump at risk too. When commissioning, follow the manufacturer's instructions, in particular filling the pump housing before the first start and bleeding the system.
Finding the right hydraulic pump
Hydraulic pumps vary from machine to machine in design, regulator, shaft profile, flange, ports and direction of rotation, often even within one model series. To identify the correct pump reliably, we need:
- machine make and model
- machine serial number
- the pump's part number, if available
- a photo of the machine's ID plate and, if legible, the plate on the pump
- photos of the pump from several sides if needed
Our guide finding the part number and serial number shows where to look. For an overview of the pump types we supply, see hydraulic pumps. Send us the details via Request a part and we will check which pump fits and come back to you with a quote.
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