Troubleshooting electro-hydraulic pumps: a 12-step program

A logical, reasoned approach to diagnosing production problems is a key to effective maintenance practices.

By Scott Follett, Robert Bosch Fluid Power

The art of troubleshooting is not really an art at all. Instead, it is more of a science. When a system malfunctions, you gather information and evidence, form a hypothesis about what caused the problem, test the hypothesis, fix the problem, then continuously verify repair. Modern industry demands tremendous control of pressure and flow during very complicated, dynamic processes. Many hydraulic pump manufacturers meet this demand by integrating advanced electronic technology with the latest hydraulic technology. As a result, manufacturers enjoy shorter response times, precise pump control, and increased pump efficiency. However, when something goes wrong, troubleshooting the pump can be a challenge. There are few maintenance people with the experience and training in both electronics <I>and<I> hydraulics to diagnose and repair electro-hydraulic pumps. The following twelve steps provide a foundation for your troubleshooting.

Step 1: Make safety your first priority

Hydraulic pressure and electricity are inherently dangerous. I personally saw a mechanic maim a finger as he was running his hand along a high pressure pipe looking for a pinhole leak. He found it. Use common sense. Unless you are absolutely positive the system is "dead", locked out, and tagged, assume the system is under pressure and electrical components are live. Do not take shortcuts if it means sacrificing safety.

Step 2: Understand the system

The electro-hydraulic pump is only one component in a larger system. It is important to know how the pump and every other component interact with each other. Review and understand the system schematics, read manufacturer specifications, know the capabilities of every component; understand the purpose of each component and how it contributes to the function of the entire system.

Your system is only as good as its weakest component. For example, if you use a pressure transducer with a tolerance 5 percent, the best tolerance you can expect your system pressure to have is 5 percent. Five percent of hydraulic pressure means a lot of psi in the deadband.

Step 3: Talk to the operator

If the system has been in operation, talk to the operator. Ask what the symptoms are, when the problem occurs, where the problem manifests itself, and the magnitude of the problem in the system. In addition to describing symptoms, the operator can often give you some history. It is not unusual for the operator to remember how a previous maintenance crew fixed the same problem before.

Step 4: Test the system

Run the system yourself to get the full experience of the malfunction. If you are not familiar wnough with it to safely operate it yourself, observe as someone else runs the system. It is usually a good idea to have the operator <I>show<I> you the problem. By doing this step, you do not have to rely on other people's opinions.

Step 5: Verify that the components are properly connected

Spread out the system schematic and compare the drawing to the actual system. Follow the flow path through the circuit. Insure that the electrical connections are correct. Look for discrepancies between the schematic and the actual system. Focus on those discrepancies.

As just one example, contamination caused two pump shaft seal failures during a distributor's pump test. Initially, we thought the shaft seal was overly sensitive to contamination and we asked for a redesign of the seal. However, after asking some very pointed questions, the distributor discovered the operator used a thirty micron filter instead of the three micron filter specified for the system.

Step 6: Compare the problem machine with one that is working

If you have the luxury of another machine with a similar pump, look for differences between the machine that works and the one that doesn't. Look for differences in components, location of transducers, the age of the system, temperatures, the size of the pump, pressure and flow differences, and anything else that helps to answer the question, "Why does one system work, while the other one doesn't?"

For example, we had a valve test stand experiencing severe flow oscillations. We were sure the pump was broken. After ordering a replacement pump, an alert maintenance mechanic pointed out that this system had about seven gallons less fluid in the reservoir than systems with similar sized reservoirs. Before changing the pump, he added fluid to the reservoir. The problem went away as soon as the pump saw a flooded suction.

Step 7: Brainstorm possible causes of the malfunction

Consider everything that could cause the malfunction. Use maintenance manuals and textbooks. Ask co-workers who previously used or maintained the machine. Pump vendors have troubleshooting guides; use them. A little time spent during this step can save a lot of time later.

You may want to consider forming an impromptu team to try to determine what went wrong. If you want to see a good, effective team in action, watch the movie Apollo 13.

Step 8: Test the pump electronics

By now you should be reasonably sure that the pump is causing or contributing to the problem. It's time to focus on finding out what's wrong with it. Use a multimeter to check the continuity of the pump leads. Check the wiring diagram and make sure the pump is wired correctly. Check the voltage from the transducers to ensure the transducers are working. If your pump has DIP or DIL switches, double check the position of each switch. Measure the voltage output of the electronics portion of the pump.

Electricity flowing through a solenoid creates a magnetic field. Hold a magnetic compass or ferrous metal object near the solenoid to verify the solenoid is energized when it is supposed to be energized. Don't get anywhere near the electrical feed to the solenoid and use properly insulated tools to hold the compass or piece of metal.

If a computer or PLC controls your system, there are several more things to check. Verify the programming, make sure the parameters are correct, look for error messages from the computer, and check that the software is loaded correctly. Debugging computer systems is tedious and time consuming and worthy of its own article.

Step 9: Test the pump mechanically

Once you are reasonably confident that the pump's electronics are working, check the mechanical functioning of the pump. Look at the suction pipe and check for restrictions or air leaks. Measure the flow and pressure coming at the pump's pressure port. Check for excessive flow from the case drain on the pump. Check the pump's speed and direction of rotation. Verify that the operating temperature is normal. If possible, check the pilot pressure to make sure there is pressure when there's supposed to be pressure. Equally important is to make sure there is no pressure when there should be none.

Step 10: Replace suspected problem components

Based on what you learned in the above steps, you should now have an idea about what is causing the problem. You may want to take a little time to brainstorm again, this time focusing on the pump. If you think a transducer is bad, replace it and retest the system. If you suspect contamination in the pilot system, open the system, look for evidence of contamination damage, clean the pieces, then retest the system.

It is important that you do only one corrective action at a time. Fixing the pump by simultaneously replacing a transducer, spool, and electronic card gives you no way of determining what exactly caused the failure. These components are expensive and few companies can afford to waste money replacing parts that are still good. 

Step 11: Call for help

Most distributors and vendors have experts who are ready to help. While you may be dealing with only one pump, they have experience with hundreds of pumps. After you have given the troubleshooting your best shot, call the distributor who sold you the pump. If the distributor is unable to help, ask for the telephone number to the factory. Be prepared to describe the application and symptoms in detail. You may want to fax a schematic of the system to the technician helping you. The more information you can give, the better will be the help you receive.

Step 12: Verify the system works

This is the final step. Monitor the system and make sure it is operating correctly. Make sure the operator knows how to monitor the system. Also, describe and show others what the problem was and how you fixed it. Another set of eyes increases confidence in the repair. As a former maintenance test pilot, I never flew an aircraft until a technical inspector and I inspected the work of a repairman. Ten thousand feet off the ground is not the safest place to discover a faulty repair job.

Another advantage to showing others the repairs is the training value. A breakdown is a chance for several people to learn. This makes the company stronger because it does not rely on only a few people to fix similar breakdowns in the future. This helps avoid the painful situation where the one person who knows how to fix a breakdown is on vacation and can't be reached.

Troubleshooting improves with practice. Every breakdown is an opportunity to hone your skills. Soon the troubleshooting process will be second nature. You will see your efficiency and skill increase. Someday, after a tricky but successful repair, you may even say to yourself, "It was beautiful."

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