The main hydraulic pump is the one component every function on an excavator depends on. When it starts to fail, the boom slows, the slew drags and travel loses power at the same time, usually with a noise from the pump compartment that was not there last month.
At that point the question is whether to send the pump away for repair or fit a replacement. This guide sets out the warning signs, the simpler faults to rule out before you condemn the pump, and how to weigh repair against replacement without guessing.
A failing main pump affects every hydraulic function at once, not just one. Rule out low oil, a blocked filter, a suction-side air leak and a low relief setting before blaming the pump. Repair means stripping the unit at a specialist and waiting for the verdict. Replacement with a Terrapart pump manufactured to OEM specification, bench-tested before despatch and covered by a 12-month warranty gets the machine back to work on a known timescale.
Signs of a failing hydraulic pump
The main pump feeds the whole circuit. If one function is slow and the rest are fine, look at that function's valve, cylinder or motor first. If everything has slowed together, the pump moves to the top of the list.
| Symptom | What it usually means |
|---|---|
| All functions slow or weak | Falling pump output. Internal wear lets oil bypass the pistons, so flow drops, particularly when the oil is warm |
| Loss of power under load, engine not labouring | The pump cannot build pressure. Check the relief valve setting before condemning the pump |
| Whining or screaming from the pump | Cavitation or air in the oil. Often caused by low oil, a blocked suction strainer or a loose suction fitting rather than the pump itself |
| Knocking or rattling from the pump | Loose or broken internal components. Stop the machine and investigate |
| Oil running hotter than normal | Oil bypassing worn internals turns pressure into heat instead of work. Rule out a blocked cooler or low oil first |
| Metal in the hydraulic filter | Something in the circuit is shedding material. A failing pump is a common source, so inspect the filter element before deciding |
| Foamy or milky hydraulic oil | Air or water in the system. Air usually enters on the suction side; water degrades the oil and corrodes components |
| Pressure below specification on a gauge | The clearest single test. Compare against the manufacturer's figure, with the engine at rated speed and the oil at working temperature |
Several low-cost faults produce the same symptoms as a worn pump:
- Low hydraulic oil level, or oil of the wrong grade for the temperature
- A blocked suction strainer or an overdue filter change
- An air leak on the suction side: a perished hose, a loose clip or a cracked fitting
- A main relief valve set low or sticking, which limits pressure however healthy the pump is
- A pilot pressure fault, which makes the controls feel lazy even though the main pump is sound
- An engine that is not reaching rated speed under load
Each of these has been misdiagnosed as a dead pump.
Why contamination decides what happens next
A pump that has been running worn does not fail quietly. Metal from the pistons, shoes and valve plate passes into the hydraulic oil and travels to every component downstream: the control valve, the swing motor, the final drives and the cylinders. The return filter catches the larger debris and then blocks.
The rule in our own final drive fitting instructions applies to any hydraulic component. Before installation, make sure the hydraulic oil is not contaminated with metal particles from the previous unit's breakdown. We strongly recommend a complete change of hydraulic oil and a new hydraulic filter when a failed unit is changed. Ingress of dirt will cause early failure, so clean the installation area and the hose ends before anything is disconnected.
This matters for the repair decision as well. If the oil is full of metal, a repaired pump goes back into a contaminated system and the same debris starts work on it.
Repair or replacement?
The right route depends on the condition of the pump, how long the machine can stand, and what the final bill looks like once the unit has been stripped.
Strip, inspect, rebuild
A hydraulics specialist strips the pump, measures the rotating group and replaces worn pistons, shoes, bearings and seals. The verdict only comes after the strip, so the cost and the lead time are unknown until the unit is on the bench. Scoring on the cylinder block or valve plate often pushes a repair close to the cost of a new unit. The machine stands while the pump is away.
New unit from stock
A Terrapart pump manufactured to OEM specification, fully inspected and bench-tested before despatch, with a 12-month warranty. Fitment is confirmed from the machine make, model and serial number before the order is placed. Stocked units ordered before 4:30pm are delivered the next working day in the UK, so the downtime is measured in days rather than weeks.
Repair tends to make sense on a pump with light wear and a sound cylinder block, where the machine can be spared for the turnaround. Replacement tends to win when the oil is contaminated, when the machine is on hire or on a programme, or when a repair quote arrives with a long list of replaced parts. In every case, do the pressure test and the filter inspection before you spend anything.
Identifying the right replacement pump
Excavator pumps are not interchangeable between models, and one machine can carry a main pump, a pilot pump and a gear pump. To confirm fitment we need:
- The machine make, model and serial number
- Which pump has failed: the main pump, the auxiliary or pilot pump, a tandem assembly or a gear pump
- A clear photograph of the identification plate on the pump, showing the part number
Send those over and our hydraulics team will confirm the unit and current availability before you order. Our hydraulic pump range covers main pumps, auxiliary and pilot pumps, tandem pump assemblies and gear pumps for all major makes.
All of the hydraulic pumps that we provide come with a comprehensive one-year warranty. Every Terrapart component is manufactured to OEM specification, tested to original performance standards and bench-tested before despatch.
Our pumps are manufactured by industry-leading companies such as Rexroth, Uchida, Hydromatik, Tong Myung and Mottrol. We also provide Kawasaki K3V and K5V units, which have been specifically designed to meet the demands of hydraulic excavators and other mobile machinery. All products are manufactured in ISO 9001 approved production facilities with rigorous quality control procedures.
Fitting the replacement
Three things protect the new pump and the warranty on it.
- Change the oil and filters. If the old pump shed metal, a new pump in old oil will follow it. Change the hydraulic oil and fit a new filter at the same time as the pump.
- Keep dirt out. Clean around every hose and fitting before disconnecting it, and cap open ports. Ingress of dirt will cause early failure.
- Test the pressure. After installation, attach a pressure gauge to the hydraulic circuit. Pressure should conform to the machine manufacturer's specification. A reading that is too high loads every component downstream; one that is too low means something in the circuit still needs attention.
If the failure also took out a travel motor, our guides to the signs of failure in a final drive and how excavator final drives work cover the drive side of the same problem.
The rest of the Terrapart range
Final drives
Excavators and tracked equipment from 1 to 50 tonnes, with up to 24 months warranty. View final drives.
Hydraulic pumps
Main, pilot, tandem and gear pumps for machines from 1 to 50 tonnes, 12-month warranty. View pumps.
Swing motors
Stocked for 6 tonne and larger excavators, 12-month warranty. View swing motors.
Minimum 12-month warranty across the range. See Terrapart Hydraulics for the full offer, or the make-specific pages such as JCB hydraulics and Komatsu hydraulics.
Frequently asked questions
What are the signs of a failing excavator hydraulic pump?
All hydraulic functions slowing or weakening together, loss of power under load while the engine runs freely, a whining or screaming noise from the pump, oil running hotter than normal, metal in the hydraulic filter, foamy or milky oil, and system pressure below the manufacturer's specification on a gauge test.
Should I repair or replace a hydraulic pump?
Repair suits a pump with light wear and a sound cylinder block, where the machine can be spared while the unit is stripped and rebuilt. Replacement suits a contaminated system, a machine that cannot stand, or a repair quote with a long parts list. Do a pressure test and inspect the filter before deciding either way.
Why does my excavator whine when the hydraulics are working?
A whine or scream from the pump usually means cavitation or air in the oil. Check the oil level, the suction strainer and the suction hose and fittings before blaming the pump itself. If the noise continues with the oil level and suction side confirmed good, the pump needs a closer look.
Do I need to change the hydraulic oil when fitting a new pump?
Yes. A failing pump sheds metal into the oil, and that debris will damage the replacement. We strongly recommend a complete change of hydraulic oil and a new hydraulic filter when a failed unit is changed.
What information do you need to supply a replacement pump?
The machine make, model and serial number, which pump has failed, and a clear photograph of the identification plate on the pump showing the part number. With those we can confirm the correct Terrapart unit and current availability before you order.
What warranty comes with a Terrapart hydraulic pump?
All of the hydraulic pumps that we provide come with a comprehensive one-year warranty. Every unit is manufactured to OEM specification and bench-tested before despatch.
Need a hydraulic pump?
Send us the machine model and serial number, or a photograph of the pump identification plate, and we will confirm the right unit and current availability.
Astrak Group develops and manufactures products intended to be compatible with machinery and equipment from various manufacturers. Unless indicated otherwise, Astrak Group has no relationship, commercial or otherwise, with the owners of the trademarks.
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