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   This is the second part of the short IH series devoted to its majesty charge pressure (read part 1 here). In this bit I would like to talk about peculiarities of closed loop charge pressure measuring, namely how, where and what for.

    As a rule, a technician may require to read charge pressure of a closed loop in one of the two cases - either he is adjusting its level, or he is troubleshooting a malfunctioning transmission. Despite of the charge circuit being relatively simple, there are more than one ways, or better, places to measure charge pressure in a "standard" closed loop. A good technician should understand the differences between the measuring points, because the information these readings can provide varies greatly with the point you are reading it at.

    Let us take a look at a typical closed loop charging circuit. Here we have the charge pump ( I put the suction filter on the schematics, as well as the suction line pressure measuring port just to show that this option can exist, and that this port can be used for verifying suction line condition), the charge pressure relief valve, oil conditioning element/elements (a filter, a cooler, or both), and the check-valve. Note that on the diagram I put two relief valves. The actual position of the relief valve (whether it is before or after the oil conditioning elements) will depend on the manufacturer, some brands have, actually, both of the relief valves installed, one of them being a safety valve (installed before the filter or cooler to protect them during cold starts, often called a "cold start valve", and is normally set around 5-10 bars above the "normal" relief valve setting), and the other one being the charge pressure relief valve.

   On the diagram you can see that there are three points in the charge system where we can install our pressure gauge and take a reading - directly at the charge pump outlet (before the oil conditioning elements), after the oil conditioning elements (before the check valve), and after the check vale (in the low pressure side of the loop). Let us consider them one at a time.

   1. Directly at the charge pump outlet, before the filter. This is a good place to measure charge pressure for "adjusting" purposes,   and often there will be a conveniently placed test port, but there's a catch - in pumps that have charge pressure relief valves installed before the filter (cooler, or both) - clogging of the filter can result in insufficient oil supply to the loop and the consequent cavitation, which will occur without the charge pressure drop at this measuring point - a situation that can lead a technician to wrong conclusions. Sauer Danfoss series 90 is a good example of such pump design.

   2. After the oil conditioning elements. Again a good place to measure charge pressure, without the above described drawback of leading into an error in case of clogging of the oil conditioning system, and most of the pumps have a test port in this point. There's still a drawback , however, and quite similar to the above one - in case of a check valve malfunction (blocked valve) the loop will cavitate, without the charge pressure drop at this measuring port - once again making it possible to come to an incorrect conclusion.

    Despite of what some might think, it is possible for such a simple thing as a check valve to malfunction. For example, Sauer Danfoss series 90 - multifunction valves can easily fail due to particle contamination. In some small pumps, the check valves are actually small ball valves, that can through time damage the housing and literally fall into the oil passage blocking the loop feeding gallery (example). Want another example?  - Take a look at these two check valve/relief valve assemblies from different size Rexroth A4VG pumps, as you can see - the threads are equal, while the travel of the poppets is not. If the smaller pump gets the "bigger pump valve", the poppet will have very little travel, resulting in partial blockage of the charge oil passage and the consequent loop cavitation.

    It must also be noted that the charge pressure at this measuring point will be affected by the oil conditioning system pressure drop (will be lower than at the point 1) - something that can be used for filter condition monitoring, for example, or for verifying if the cooler is obstructed.

   3. And finally - the reading taken after the check valve, or directly in the low pressure side of  the loop. This is the best place to see if the loop is cavitating, because any volumetric or charge feeding problem will be immediately indicated  by the pressure drop. One thing you don't want to do is to use a low scale pressure gauge at this point, like the one you used in the above described points (unless, of course, you are fond of gambling or are collecting burst pressure gauges). The best way.to take the reading at this point is with a high range digital pressure gauge. An analogue one will also do, but it may be difficult to read with precision low pressure values on a high range scale.

    Although this point is the best spot to check for cavitation symptoms, it's not always the best place to read the charge pressure for adjustment purposes, especially on old equipment, which can have lines' pressure fluctuations due to the drifting null condition (IH articles on hydrostatic transmission null adjustment techniques are here).

     So, if you are simply adjusting charge pressure of an operational closed loop, the most convenient points to read it are points one or two. Reading and comparing pressures at both points will give you an idea of the pressure drop, caused by the oil conditioning system, and will help you come to a conclusion whether the filter/radiator needs to be changed/cleaned. In this case it will be OK to use a low scale pressure gauge.

    If you are troubleshooting a closed loop and are suspecting excessive leakage - you should first measure the pressure in the low pressure side of the loop during its operation, and then, if the cavitation condition (charge pressure drop) is confirmed, you should measure the charge pressure at the point one, to see if it drops too. If it's not dropping, the malfunction is being caused by the charge oil flow interruption either due to the filter/cooler clogging or due to a check valve malfunction.

    Understanding these simple differences is very important for correct charge pressure adjustment and efficient closed loop  troubleshooting, and can literally save you many an hour of downtime.