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I HAVE TRIED EVERY SENDING UNIT/ HYDROLIC TEST THAT THE LOCAL JD DEALER KNOWS ANY THING ABOUT. THERE IS NOTHING THEY CAN FIND OR I CAN FIND TO FIX IT.
Well it could be your charge pump is worn, or something could be stuck in the charge pressure relief valve preventing it regulating the pressure......
It supprises me that your dealer can't work it out! they should have all the gear to track it down.
skidsteer.ca
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This is a very basic system (if its related to the NH skidsteer from the 90s) Open center system. Oil from from the reservoir is sucked throught the pump and pressured to the valve (s) that run the boom, bucket and aux hydraulics (3rd valve) Usually a 3 spool valve (sometime a 2 spool with a power beyound and a 2nd single spool valve down stream.
The return flow is routed through a restrictor valve. It won't let the oil return to the tank until the return flow is raised to the charge pressure used to run the hydros (wheels/ drives)
What my best guess is that someone has put a "power beyond" valve in the system. This type of valve (or the addition of a insert to convert your valve to power beyond) splits the return flow from the valve into two circuits. One circuit is the flow from when your valves spools are centered (not using the foot pedals) it appears this flow is routed through the restrictor as it should be and you have charge presure under this cituation.
However if the valve is power beyound when you move a spool (to lift your boom, or tilt the bucket) this oil comes out a separate port on the valve. it would seem the charge pressure from this port is not being restricted, but just returns to the reservoir, and this cause your charge pressure to colapse, turning on the warning and messing up the drive.
Could you take some pictures of the valve showing the hoses coming and going from it and email it to me?
If it is a two spool valve it should have 6 hoses if it is not power beyoung. Or if it has 3 spools, then there should be 8 hoses. If yours has 9 hoses (on a 3 spool or 7 on a 2 pools, then it is PB.
This is my guess with out seeing it, but it seems to fit your senario.
http://www.mediafire.com/?ov9zqt7xwtl here is a link to more info on the charge pressure circuit.Download the file and see page 264 and 265 in the hydrostatic section for info on how the circuit works.
Ken
skidsteer.ca
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Aaron
To further our conversation
Your loader has 2 valves in the circuit. A two spool one controls your boom and bucket and downstream from this valve is a single spool that runs you Aux hydraulics. (or perhaps its opposite, but not as likely) Anyway, which ever valve is 1st in the series must be a power beyond valve. That means it has 2 flow return lines coming out of it. If its your 2 spool valve that is 1st then it will have 7 hoses total instead of only 6.
If the single spool valve is first, it will have 5 hoses instead of only 4.
Either way, the charge pressure is created with the return flow from these valves. Since you have power beyond you now have 2 return flows. One only sees flow when the power beyond valve is in neutral (no spools are being used, spools are in the center)
And the second return line only sees flow when the pb valve spools are being used. It is the flow from this 2nd return line that is not being restricted to maintain the 170 psi charge pressure the hydro drives need to function properly. When you use the foot pedals, the charge pressure colapses and the drives begin to chatter (which is a bad thing, speaking from a wear stand point)
In order to supply the charge pressure to the hydros, both of these lines need to be T'd back together into the charge supply line to the hydros, but each would also requires a check valve, so the flow can ONLY come out of the return. This check valve prevents the flow from one return line being pumped back into the second return line. If the check valve is not functioning you could be losing you pressure there too.
Either way there is either not a restrictor to create charge pressure on the 2nd return line. Or the second return line is not plumbed to the hydros, or there is a bad check valve.
I can't hardly believe you have talked to all those folks and tried changing everything you mentioned without this (pb flow thing) coming up.
If it will push hard so long as you do not use the boom or bucket, your hydros are fine.
You say there is only one pump, (not a separate charge pump, which is generally the case) it does its job under one scenario, and the hydro do their job under a certain scenario. To me it has to just be a matter of the flow not being deliver to the hydros under the second scenario. Find out why and it should work.
If you get your manual back look up the hydro section and see if the charge pressure circuit works different then the manual I have.
See if you can find the information on adding the Aux hydraulic valve and how they re plumb the system for the 2 return lines. I can picture in my mind how I think it should be done, but that does not mean some crafty engineer did not come up with a different way and maybe for a good reason to. I'm not familiar with you machine, just with the theory of how this should work.
I would also recommend posting the power beyond question in the hydraulic forum at www.tractorbynet.com There are some good hydraulic guys there also.
If you do let me know and I'll try to follow that thread as well.
Ken
This is a very old thread but I have a related problem with the same machine. I think there may be a problem with the pressure relief valve on my front drive pump. The pump case is getting pressurized and forcing the fluid out the little cover on the side of the pump. i removed the pump and sent it to a shop to be checked/rebuilt. They didnt find anything wrong with it. So I want to know the theory on how this is supposed to work. What is going on when the pump is running but the drive is not being used? Is the excess pressure fed back into the charge system via a relief valve? if this valve did not open would this pressurize the pump case? The lift hyraulics work fine and there is no issue with the rear drive pump.
When the motor is turning and you are not moving the machine, the hydrostatic pump does not pump any fluid. They are vairable displacement pumps, the more you move the steering lever, the more fluid it pumps, the more angle that the pumping parts (rotating group) run, the more fluid moves, in neutral, they simply spin but do not pump anything.
I'm not sure where the extra fluid is coming from that you are talking about, there should be pressure in the case at all times, it can be from 100-400 psi depending on the model
Could the leak just be a bad seal? Does the machine run well otherwise?
