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     Last time I tried to peek through a crack of CAT's fence, I swear I felt a bullet pass by my left ear! You must understand that after putting this information here I will change my residence as well as my appearance... Just kidding, CAT folks are all right, all the information about their hydraulic pumps and motors is readily available online, all technical catalogues are free and downloadable, and all technicians are super-competent and eagerly land you any diagnostic equipment upon your request, oh yeah, and I NEVER EVER lie!!!

     The following information is ONLY what I (MYSELF) think this control is, which may be not accurate. I have nothing against Caterpillar, nor am disclosing any secret information. I just happen to have a thing about dismantling stuff to see how it works.

     Anyhow, CAT engineers seem to have an itch to over-engineer their hydraulics. They take a simple control from a well known brand and then they start to "improve" it by adding (or asking a brand engineers to add) a thousand and one unnecessary functions which all, strangely, CAN be technically justified but, well, are just like a spare wheel on a sports motrcycle... In the end they end up with an exclusive control mounted on a seemingly standard pump/motor unit which can intimidate many by its apparent complexity, protected looks and original threads, BUT not us, back-engineering connoisseurs, who consider complexity a challenge and laugh in the face of  "protected" OEM hydraulics.

     This pump is a Caterpillar/Rexroth closed circuit pump, used for boom turning on an excavator. A pretty standard AA4VG pump, on which you will easily find tons of information online, but not for this control. The pump reference is AA4VG56DWD1/32R, and you will not find the description of the DWD1 control in the standard model catalog. You know what it means, don't you? Time to back-engineer!

    Pic1 shows you the normal A4VG body. Upon closer inspection we see that there is no servo-pressure supply, so the hole which normally supplies servo pressure for, for example, proportional electrical EP control, is not connected to charge pressure, and, consequently, is used only for pressure cut-off function (pressure limiter). Let us inspect further, the control has three pilot connections, marked as Y1, Y2, and (surprisingly) Y3. It has no feedback link, two servo-cylinder and one tank connection. This pretty much tells us that this is hydraulic no-feedback control of some type. Some more digging reveals pilot orifices in Y1 and Y2 (pic5), which make sense as this control has pressure cut-off option; a shuttle valve (pic6), which connects servo pressures to the cut-off valve, as well as to the Y3; and two valves like this (pic7), which look strange and sophisticated at a first glance, but on a closer look (pic8) are nothing but a piloted two position flow restrictors. Add up a pair of anti-cavitation check valves connecting servo-cylinders to tank (pic9) and here you have it (pic10), an improved DG (hydraulic direct operated) control with remote servo pressure control option, which more then surely is used by the excavator's power management system to control boom rotation power consumption. Which may be done by some external electrical proportional pressure limiter connected to the Y3 port, for example. Not so tuff and sophisticated now, is it? 

    Pump controls, both standard and OEM, are nothing but devices to control  pump's displacement in accordance to some external input signal and internal logics. That's it! Controls are easy when you know what to expect and don't get intimidated by the absence of technical information. Assumptions that make sense are correct most of the times.

    I like CAT's pumps because they are always a protected puzzle. I hate CAT's pumps because they are always a protected puzzle.

    The same schematics in pdf is here.
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