Hydraulic Troubleshooting: Getting the Correct Diagnosis - The First Time

Brendan Casey
Tags: hydraulics

Troubleshooting hydraulic systems can be a complex exercise. It involves a lot of science and sometimes, a bit of art. Incorrect diagnosis prolongs downtime and can result in the unnecessary repair or replacement of serviceable components. Avoiding these costly mistakes requires the correct equipment and a logical approach.

The process of troubleshooting should always begin with checking and eliminating the easy things first. The benefits of this approach are clearly illustrated by a recent troubleshooting situation. The machine in question had a complex hydraulic system, the heart of which comprised two engines driving 10 pumps. Six of the pumps were variable displacement units and four of these had electronic horsepower control.

The symptoms of the problem were slow cycle times combined with engine lug-down (loss of engine rpm). The machine had just been fitted with a new set of pumps.

The mechanic in charge diagnosed that the hydraulic system was tuned above the power curve of the engines, that is, the hydraulic system was demanding more power than the engines could produce. This resulted in engine lug-down and therefore slow cycle times. The other possible explanation was that the engines were not producing their rated horsepower.

In order to eliminate the easy things first (the complexity of the hydraulic system meant that it would take approximately four hours to run a complete system check and tune-up), I inquired about the condition of the engines and their service history. The mechanic not only assured me that the engines were in top shape, he was adamant that this was a hydraulic problem.

Four hours later, after running a complete check of the hydraulic system without finding anything significant, I was not surprised to find that the problem remained unchanged. After a lengthy discussion, I convinced the mechanic to change the fuel filters and air cleaner elements on both engines.

This fixed the problem. It turned out that a bad batch of fuel had caused premature clogging of the engine fuel filters, which prevented the engines from developing their rated horsepower.

Assess The Problem and Eliminate The Obvious
Before you incur the expense of hiring a technician, assess the problem and eliminate all of the obvious, possible causes. In the previous example, if the relatively simple task of changing the engine fuel filters had been carried out when the problem was first noticed, an expensive service call and four hours downtime could have been avoided.

I have lost count of the number of times that I’ve been called to a problem and found that the cause was something quite simple. A wire that had broken off a solenoid valve, a pin that had fallen out of a mechanical linkage, an isolation valve that had vibrated closed, a blocked heat exchanger . . . and the list goes on.

Your oversight won’t bother the technician, because his hourly rate is the same, regardless of how easy or difficult it is to find the problem. But you may be annoyed with yourself for not checking something so obvious, knowing that you could have easily saved a couple hundred dollars.

Why Quality is More Important Than Quantity
Paying for a technician’s time when it is not required is certainly not desirable. But it is nowhere near as costly as paying for the unnecessary repair or replacement of serviceable components, as a result of incorrect diagnosis of a problem. Incorrect diagnosis in a troubleshooting situation is usually the result of the technician’s incompetence, insufficient investigation of the problem or a combination of both.

Unfortunately, it is not possible to determine a technician’s competency from the badge on his shirt or his charge-out rate. While charge-out rates may be a factor in deciding which technician you hire, from an overall cost perspective it is far more important to evaluate the technician and his diagnosis, so that you don’t end up paying for his mistakes.

Consider this example: Several years ago, I was asked for a second opinion on the condition of a set of pumps operating a processing plant. The customer had called in a technician to check the performance of these pumps and was alarmed when the technician advised that all four pumps needed repair.

The pumps in question were variable-displacement units fitted with constant power control. The power required to drive a hydraulic pump is a product of flow and pressure. A constant power or power-limiting control operates by reducing the displacement, and therefore flow, from the pump as pressure increases, so that the power rating of the prime mover is not exceeded. The advantage of this control is that more flow is available at lower pressures, so the machine operates faster under light loads. This results in better utilization of the power available from the prime mover.

Pump performance is checked using a flow-tester to load the pump and measure its flow rate. As resistance to flow is increased, pressure increases and the flow available from the pump to do useful work decreases because of internal leakage. The difference in the measured flow rate between no load and full load determines the volume of internal leakage and therefore pump performance.

I tested all four pumps, recording flow against pressure from no load through to maximum working pressure. In my report, I explained to the customer how the tests revealed that pump flow did decrease significantly as pressure increased, but that this is a normal characteristic of a pump fitted with constant power control. I further advised that apart from the constant power control requiring adjustment on two of the pumps, the performance of all four pumps was acceptable.

The first technician’s assessment can be explained only by fraud or incompetence. I suspect it was the latter, with the technician failing to either establish or understand that the pumps he was testing were fitted with constant power control. This ignorance led to an incorrect interpretation of the test results. Whatever the explanation, the customer could have paid thousands of dollars for unnecessary repairs, if he had not sought a second opinion.

When you have a problem with your hydraulic equipment, carry out an informed assessment of the problem and eliminate the obvious before you call a technician. If you do need to hire a technician, be sure to evaluate the technician and his diagnosis so you don’t end up paying for his on-the-job-training or worse, his mistakes!

About the Author

Brendan Casey has more than 20 years experience in the maintenance, repair and overhaul of mobile and industrial equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his Web site, www.InsiderSecretsToHydraulics.com.

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