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    When I was writing the article on the new pump control invention for a John Deere forester, I remembered the following troubleshooting episode, which happened a long time ago, but still has a good point.

    A client appeared in the workshop with an open circuit pump from a forest crane. I do not remember what model of crane it was, all I remember that it was Timberjack (at the time), and the pump was Hydromatik A10VO something, maybe 140, good old series 3. The client was convinced that the pump was damaged. The main symptom was the crane movements becoming slow as the oil heated up. The man had worked with hydraulics for a long time, and was pretty much sure that the problem was being caused by the pump's low efficiency, with the internal leakage aggravating with hotter and, therefore, more fluid oil.

    The pump was opened and no serious damage or wear was detected, so it was reassembled and tested, presenting good efficiency and controllability on the test stand. However, when the pump was installed on the machine the next day, the crane displayed the same symptom - movements slowing down as the oil was heating up, becoming very slow eventually and making work with the crane impossible.

   The pump was dismounted, brought to our workshop, striped down, and yet again nothing wrong beyond normal wear was detected. Once again, the pump was reassembled, retested and showed good efficiency and correct control function (pressure limiter and load sensing). When the pump was remounted on the machine nothing changed, the crane movements kept on slowing down as the temperature rose. This time the man asked if I could "take a look", so I grabbed my gear and went to see what was going on.

    The hydraulic circuit of the crane was of a closed center load sensing type. The main distributor valve was electric proportional. Comparing the pressures at the pump's outlet and the load sensing line showed steady delta P of 30 bars when the movements were getting slow, even when the joystick was "pedal to the metal". Clearly, something was throttling the oil between the pump's outlet and the load sensing signal port. The most obvious explanation for this would be partial stroke of the distributor spools. Further digging into showed that the pilot pressure reducing valve, supplying oil for the proportional solenoids, was malfunctioning, causing the pilot pressure to drop significantly with the rise of the oil temperature. There was simply not enough pilot pressure to make the spools go full stroke, and the pump's load sensing system was sensing the pressure drop through the partially opened spool, as it should, and compensating for it with the reduced flow. To tell the truth, I don't remember what exactly was causing the valve to malfunction, but I do remember that the problem was solved "on the spot" and the crane's normal operation was restored.

     If you look more carefully into this case, you'll see that there is a major flaw in the troubleshooting conclusion that the problem was caused by the pump's low efficiency. Indeed, it is absolutely plausible for an worn or damaged open circuit pump to be able to perform much better with cold and more viscous oil, and fail to produce sufficient outlet flow with hot and more fluid oil, BUT, in such situations another major symptom would have been detected - SEVERE OIL OVERHEATING! When we loose significant part of flow to leakage, all the energy, spent to produce it, will be transformed into heat. So, if the pump indeed were worn or damaged, and the SIGNIFICANT loss of speed were connected to leakage losses, the oil temperature would have been much higher than usual.

     In this case the problem was starting to occur when the oil temperature was only getting close to the normal operating temperature, without further overheating, thus putting the low efficiency theory under question. I know it because the machine was equipped with an oil temperature gauge and an alarm, going off at 70 C, which is common for Timberjack/John Deere foresters, and is something I consider a "must" for any piece of hydraulic equipment.

    This is just another example of how important it is to, at least, know what a normal operating oil temperature of your hydraulics is. It is, by far, the most important troubleshooting reference point.

    This is also a good example of how precipitated troubleshooting conclusions can easily cause unnecessary downtime. It is always a good practice to double-check your theories whenever it is possible.