Replacing a pump in a hydraulic system should be simple, right? Just take the pump out of the box, install it in the machine, fill the case with oil, connect all the lines, and turn it on. It sounds simple, but an incorrectly installed hydraulic pump could have your customer buying another one before the end of the shift. Pumps usually fail because of external problems. Check these 3 things when commissioning a pump into a hydraulic system.

Check #1: Oil

The oil is the lifeblood of the hydraulic system. It transmits the hydraulic power, lubricates, cools, and carries away contaminants in the system. Clean oil is the best way to prevent breakdowns and wear. Keeping records on oil service is key to maintaining the machine. Here are some things to keep in mind when servicing the oil in the hydraulic system:

  • Check the filters. When a pump is replaced, changing the filters should be an automatic service item. Inspect pressure and return filters and replace them with correctly rated filters. Using a filter cutter to inspect the element is a good idea to find out what type of contamination the filter has captured and possible sources.


  • Check for water contamination. Water is an enemy to hydraulic systems. It causes corrosion, speeds up wear, increases oxidation rates and causes oil to break down faster than it should. Perform a visual inspection. Look at a sample of the oil. It should not be cloudy or be a milky color. Some hydraulic systems are equipped with a water drain at the bottom of the reservoir to remove water that has settled out of the oil. The best way to check for water in the oil is to send a sample to an oil testing facility for analysis. You can also use the hot plate method for checking water contamination. Set a hot plate to 250 degrees and place a drop of the sample oil on it. If it crackles and sputters, there is water in the oil.
  • ISO cleanliness charts. Oil contamination can be charted using ISO guidelines for contamination. The oil sample will be given a series of 3 numbers that can be compared to machine records and contamination limit charts to check if the oil is actually clean or if it just “looks clean.” Most contaminants are invisible to the naked eye, so even if the oil appears to be clean, it could carry contaminants that will cause premature pump failure.

Check #2: Connections

Check the connections from the pump to the machine when installing a pump. Here are some things to inspect:

  • Mounting bracket and drive coupling. This is where the prime mover transmits mechanical power to the pump, which turns it into hydraulic power. Check the mounting surface for dents, dings, burrs or uneven surfaces. These things will cause misalignment of the pump to the prime mover and cause uneven force and vibration on the drive shaft. These can lead to premature bearing failure inside the pump. Check the coupling set on both the prime mover and pump.  Wear, corrosion, or damage can cause vibration. Replace the drive coupling if necessary.


  • Fittings and flanges. Inspect the conductor connections when replacing a pump. Replace O-rings on fittings and flanges to prevent leakage. Inlet lines are generally low-pressure hose. Check the inlet line for collapse, deterioration or leaks that can cause aeration.


  • Hydraulic lines. Sometimes mechanics will place rags or plugs inside the hydraulic lines to prevent leakage while removing and replacing the pump. Before connecting the new pump, check the lines for any type of rag, plug, or metal debris that might interfere with pump flow.


  • The case drain line. The case drain line is very important to any pump that needs one. Any type of obstruction can cause high case pressure, which will result in severe pump damage. Removing and visually checking the case drain line is important, especially if the previous pump had a catastrophic failure. Debris can be lodged inside of fittings and conductors which will restrict case flow and cause pressure to build up. Avoid using compressed air to check for obstructions.

Check #3: Reservoir

Although its primary function is to hold the oil, the reservoir has several other key jobs in relation to the pump. The reservoir allows entrained air in the oil to diffuse, it cools oil, it allows contamination particles to settle to the bottom of the tank as well as other accessories incorporated into it that are important to the hydraulic system. Some things to check on the reservoir include:

Some things to check in the reservoir include:

  • Open the clean-out cover. Doing a reservoir clean-out is important when changing the pump. It allows maintenance personnel to remove all the sludge and heavy contamination that has settled to the bottom of the tank.


  • Check the inlet strainer. Of all the maintenance items when replacing a pump, this item is probably the most important and most overlooked. An inlet strainer is a coarse filter installed on the suction line of the pump. It is generally inside the reservoir and can only be serviced by opening the clean-out cover so a reservoir clean-out and inlet strainer inspection should be done at the same time. Some pump manufacturers do not install inlet strainers so it might not be there. Inlet strainers can be replaced or cleaned and reinstalled.


  • Air breather. This is another often-forgotten-about service item. As the hydraulic system operates, the level of oil in the reservoir will increase or decrease, depending on if cylinders are extended or retracted. The air breather allows air to go into and out of the reservoir while maintaining the cleanliness of the air by filtering it. Air breathers can be hard to find on the reservoir and can sometimes be incorporated into other devices on the reservoir such as the oil filler cap. If the air breather becomes plugged, it can cause cavitation if the inlet pressure of the pump gets too low. This can greatly damage the pump. Replace the air breather as a periodic maintenance item to prevent these issues.


Failing to perform routine checks when commissioning open loop hydraulic pumps can create serious hydraulic system problems. Close inspections of the system’s oil, connections and reservoir will help avoid costly and time-consuming fixes.

Disclaimer: The above information is based on general pump manufacturer’s recommendations and more than 30 years of my own pump repair and troubleshooting experience. Machine maintenance manuals should always be followed, and situations and problems exist that are beyond the scope of this document. This information is not an all-encompassing troubleshooting guide but serves as often overlooked practical advice for maintenance people. HPS is always available to help if you have any questions on pump installation.

Steve Downey
Steve Downey

Steve Downey is an Adjunct Fluid Power Instructor at Henry Ford Community College and Macomb Community College. He has worked in the Fluid Power Industry for 30 years in both Industrial and Mobile Hydraulics. Steve Holds 11 International Fluid Power Society certifications.