Hydraulic Pump Basics for Hydraulic Excavators: How They Work, How to Spot Trouble, and What Repairs Cost

Have you noticed your hydraulic excavator slowing down during operation or leaking oil from the machine? If the hydraulic pump—the component at the heart of a construction machine—fails, it can lead to project delays and costly repairs.

This article explains the basic operating principles of a hydraulic pump, five warning signs that can help you detect trouble early on site, and typical repair costs by component.
Use it as a reference for proper hydraulic pump maintenance and prompt parts replacement.

The Role of the Hydraulic Pump and an Overview of the Hydraulic Circuit

The hydraulic pump converts power from the engine into hydraulic energy and serves as the heart of the hydraulic excavator.
To deal effectively with problems in the hydraulic system, it is essential to understand the role of the pump and how the hydraulic circuit works.

Here, we explain how the pump transmits power to each part of the machine from three perspectives.

• The difference between the main pump and the pilot pump
• The overall structure of the hydraulic circuit and the flow of power
• Types of pump control systems and how they work

Let’s take a closer look at each one.

The Difference Between the Main Pump and the Pilot Pump

Hydraulic excavators are equipped with two types of pumps, each serving a different purpose.
The first is the main pump, which supplies a large volume of pressurized fluid to the attachments and the travel motors.
A variable-displacement piston pump is commonly used for this purpose, allowing it to generate substantial power.

The second is the pilot pump, which converts movement of the control levers into hydraulic signals.
This is often a small gear pump and plays a supporting role, using the small force applied to the levers in the cab to actuate the large main control valve.
By working together, these two pumps make it possible to combine the power needed to excavate heavy soil with the precise control expected by the operator.

The Overall Structure of the Hydraulic Circuit and the Flow of Power

The power in a hydraulic excavator is generated within a circulating system that begins at the engine and returns to the hydraulic fluid tank.
First, the engine drives the main pump, which draws hydraulic fluid from the tank and pressurizes it.

The pressurized fluid is then sent to the main control valve, which governs the machine’s movements.
When the operator moves a control lever, the valve opens and directs the appropriate amount of pressurized fluid to the required circuit.

The fluid then flows to the hydraulic cylinders that move the boom and arm, as well as to the travel motors that drive the machine.
Finally, it is cooled by the oil cooler and returned to the hydraulic fluid tank.

Types of Pump Control Systems and How They Work

The systems used to control the main pump and regulate fluid volume can be broadly divided into two functions: pump horsepower control and pump flow control.
Pump horsepower control automatically distributes engine output among multiple hydraulic pumps so that the total demand does not exceed the engine’s maximum capacity and stall it.

Pump flow control, on the other hand, reduces losses by preventing unnecessary hydraulic fluid from being discharged and returned to the tank when the machine is idling or the controls are not being operated.
To achieve this, systems such as negative control, positive control, and load-sensing control are used, helping improve fuel efficiency and overall operating performance.

Five Warning Signs of Hydraulic Trouble You Can Spot on Site

When a problem develops in the hydraulic pump or circuit, the machine’s behavior usually changes in noticeable ways.
If you catch these early warning signs and respond quickly, you can prevent major failures and costly repairs.

There are five signs in particular to watch for during operation on site.

• Reduced output and loss of power
• Unusual noise during operation
• Oil leaks around the machine
• Slower operating speed
• Rising hydraulic fluid temperature

Let’s look at each symptom one by one.

Reduced Output and Loss of Power

If the machine can no longer lift heavy soil or feels weak when traveling, there may be a problem in the hydraulic system.
When components inside the pump wear, the pump can no longer generate sufficient pressure.

A faulty main relief valve or an insufficient hydraulic fluid level can also cause a loss of power.
If power drops sharply when the oil temperature rises, there is a strong possibility that internal leakage inside the pump is allowing pressure to escape.

Unusual Noise During Operation

If you hear unfamiliar noises around the pump, stop the machine immediately and inspect it.
A rattling, metal-on-metal sound can indicate damaged bearings or pistons inside the pump.

A gritty, grinding sound often points to cavitation.
This is a serious condition in which countless bubbles form in the hydraulic fluid and collapse, eroding the metal surfaces inside the pump.
Common causes include a clogged suction filter and a low hydraulic fluid level.

Oil Leaks Around the Machine

If hydraulic fluid is dripping from beneath the machine or around the arm, there is likely a gap or leak somewhere in the hydraulic circuit.
Common causes include deteriorated or cracked hydraulic hoses and loose pipe connections.

If oil is seeping from the connections on the hydraulic pump or around the cylinder rod, rubber sealing components such as oil seals or O-rings may have reached the end of their service life.
If left unaddressed, oil leaks can lead to a low fluid level and even catastrophic damage such as pump seizure, so prompt replacement is essential.

Slower Operating Speed

If the boom rises or the upperstructure swings more slowly than before even when the control lever is fully engaged, that is another typical warning sign.
It indicates that the volume of fluid being delivered by the pump is insufficient.

Possible causes include a clogged hydraulic filter preventing the pump from drawing in enough fluid, or wear inside the pump allowing fluid to escape through increased clearances.
A sticking control valve can also restrict the flow path and slow down operation.

Rising Hydraulic Fluid Temperature

Under normal conditions, the hydraulic fluid in a hydraulic excavator is kept at an appropriate temperature—around 55°C (135℉)—by the oil cooler.
However, if the hydraulic oil temperature warning light comes on or the machine becomes too hot to touch, the system is in a dangerous condition.

Once the fluid temperature exceeds 82°C (180℉), deterioration accelerates rapidly and rubber seals are more likely to harden and fail.
Possible causes include a clogged oil cooler or a malfunctioning cooling fan.
Excessive internal friction in the pump, or heat generated by a relief valve stuck open, can also be responsible.

Typical Costs of Hydraulic System Repair or Replacement and Key Points for Selecting Parts

When a problem occurs in the hydraulic system, one of the first concerns is how much the repair will cost.
Understanding the total cost—including labor and travel charges as well as the parts themselves—can help with budgeting.

In this section, we explain four key points related to repair costs.

• Typical repair and replacement costs for hydraulic pumps
• Factors that affect repair costs
• The features of genuine, remanufactured, and used parts, and how to choose among them
• How to reliably identify genuine parts using an app

Let’s take a closer look.

Typical Repair and Replacement Costs for Hydraulic Pumps

The cost of repairing the hydraulic system varies depending on the failed component.
For a simple hydraulic hose replacement, the typical cost is around JPY 10,000 to 30,000 (USD 60 to 190) per hose, including parts and labor.

Disassembly and repair of a hydraulic cylinder or seal replacement can cost anywhere from several tens of thousands of yen to around JPY 150,000 (USD 940) per cylinder, depending on the machine model and size.
If the hydraulic pump itself has to be replaced as a complete unit, the cost may range from several hundred thousand yen to over JPY 1,000,000 (USD 6,280), depending on the size of the machine.
Replacing a swing motor or travel motor can also cost JPY 300,000 (USD 1,880) or more on some machines.
Actual prices vary by service provider, so be sure to confirm the exact cost through a quotation.

Factors That Affect Repair Costs

The total repair cost is not determined by parts alone.
In general, the larger the machine, the heavier and more robust the parts, and the higher the cost.
There is also usually a travel charge for having a repair technician come to the site, which can range from several thousand yen to several tens of thousands of yen.

Labor is typically calculated at around JPY 7,000 to 10,000 (USD 45 to 60) per hour.
Jobs such as hydraulic pump replacement involve major disassembly, so labor costs can add up quickly.
If metal particles are found in the hydraulic fluid, the entire circuit may also need to be flushed and the fluid replaced completely, which can add several tens of thousands of yen more.

The Features of Genuine, Remanufactured, and Used Parts, and How to Choose Among Them

Replacement parts include genuine parts, remanufactured parts, used parts, and low-cost non-genuine aftermarket parts.
Genuine parts offer the reassurance of manufacturer quality assurance and are designed to meet the original specifications.
They are the best choice for core machines that you plan to keep in service for the long term.

Non-genuine and used parts, on the other hand, may carry risks such as premature failure, damage to surrounding components, and loss of warranty coverage, which can ultimately lead to higher costs and more downtime.
Remanufactured parts are overhauled by specialist companies and can help reduce costs, but you should evaluate them carefully by checking their quality and whether they come with a warranty.

How to Reliably Identify Genuine Parts Using an App

When arranging replacement parts, accurate identification is essential because even the same type of construction machine may have different hydraulic specifications.
Installing an incompatible part can result not only in improper operation but also in new failures.

To avoid ordering mistakes and save time, a manufacturer-provided smartphone app can be extremely useful.
For example, if you are using pumps or motors made by Kawasaki Heavy Industries, you can simply enter the serial number shown on the nameplate into “Kawasaki Hydraulic Finder” to quickly identify the correct genuine parts.
For more reliable and efficient maintenance, it is worth considering digital tools like this.

Summary: Early Detection and Day-to-Day Management Help Extend Hydraulic Pump Life

Continuing to operate a machine with a problem in the hydraulic system can expand the scope of the repair and drive up costs.
When considering repairs or parts replacement, the first step is to confirm whether the installed components are genuine parts.

By entering the serial number shown on the nameplate into “Kawasaki Hydraulic Finder,” you can identify genuine parts, confirm accurate information, and contact authorized dealers more smoothly.
To maintain the performance and durability of your valuable machinery, be sure to make use of it.

Reviewed by

Ryoji Irie

Manager
After-Sales Promotion Office, Marketing & Sales Group, Precision Machinery & Robot Company, Kawasaki Heavy Industries, Ltd.

《Career History》

After joining Kawasaki Heavy Industries, I spent ten years developing the new K7V pump series as a design engineer, followed by five years at KPM (UK) as an application engineer supporting European OEMs and distributors. I am currently responsible for promoting the after-sales business globally.