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Komatsu PC250 slow bucket dump

typ4

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These spacers and housing are definately on the bottom

I did not, my co worker was working on it. What are these I assume, double checks for?
I really appreciate your help with this
 

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John C.

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I believe those are the LS check valves. These machines are closed center load sensing. What that means is that the spools are always closed off to the pump flow and the pumps are set up to only put out a little bit of flow when all the spools are centered. Oil comes out of the pumps and into the valve bodies and the flows down the center of the valves which when nothing is being used stops flowing at that point. There is a second passage that goes to the LS circuits for all those functions. One of the functions of that LS circuit is to tell the pumps to destroke and not put out any more than minimum flow. Those checks are part of that LS function. Your pumps are stroking up when something is used so I figured that there was nothing wrong with those check valves.
 

John C.

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Did you say you have a service manual for this machine?
 

typ4

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I think, ill check with Dan my coworker.
We were wondering if the spool is upside down also. And the dealer is NO help, and they worked on it. They just want to sell a valve. Not happening on a machine of this age. Our thing is if the valve isnt cracked in half ,we can fix it.
 

typ4

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Dan said the engine is loaded all the time. Anyway his next step is to pull those on that block and inspect them.
Ill let you know what we find.
Again thanks.
 

John C.

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Here is a photo of the valve block. Item number 6 is the bucket spool. This diagram has three extra spools that you might not have on your machine. The book photo is sideways so if you rotate to the left you will see the actual representation of the valve as it sits in the machine.


IMG_1162.JPG
 

typ4

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oregon
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The trouble shooting chart is pointing us to the suction and suction safety valves, but our book doesnt call them out.
Thanks
 

John C.

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In English those are called circuit relief valves and they wouldn't have anything to do with restricting the flow into or out of the circuit. They only relieve high pressure or prevent a vacuum in the circuit.
 

typ4

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Ok, that makes sense. This is a very strange issue. Check balls in spool are ok, LS checks under the cover I sent a pic of are ok. We can flip the spool over unless we move the pilot housings also because of those pistons on the spool. Seriously thinking about taking them off the spool and trying that.
 

John C.

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I'd flip the spool and sway the housings as well. It wouldn't be the first time I've seen someone put a valve back together and get the spools in the wrong holes plus in the wrong direction.
 

typ4

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It appears we have found and solved the problem. I dont know if Dan took pics , but unscrewed the spool ends to check the check balls and they were there, on one of the stems there was a machined groove that looked like it accepted an oring and backup. So he put one on it, before reinstalling the bucket spool he pulled the arm spool and took it apart, sure enough the groove had a seal on it.
Assembled the valve, oiled it up and lo and behold the bucket works like it should and it doesnt load the engine as much as it used to.
We suspect the seal was for a sensing port of some type and causing the loading and back pressure.
John, thank you for everything.
 

centerline

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Ive seen this problem many times with hydraulics, and after so many costly troubleshooting procedures, I have found what generally causes it is a bad hydraulic hose....
when we get a machine in that has the symptoms, after the pressures all check out good, we go directly to looking for a bad hose.... sometimes its just a failed seal somewhere in the system.

on hoses, when the fittings are installed, if they get over crimped, the hose liner will be damaged and a "flap" can develop on the inside of the hose at the end of the fitting on the inside, creating a flapper valve.... when it gets big enough it causes a restriction and sometimes it can cause a complete blockage, but the function always works smooth and proper in one direction, but in the other direction it can be either erratically or fail dependably..... and about the only way to find it is to systematically swap the hydraulic hoses in that circuit end for end, one at a time and test running it before swapping the next hose....
the other issue we sometimes find when this happens (usually on pilot operated valves), is the spool valve is sticking, due to a spool that has came loose and is unscrewing itself inside (depending on the design), a broken spring/adjustment came loose or a bit of contamination that is not letting it travel full in one direction....

there is little else that can cause the issue...
 

typ4

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centerline, everything you described was not the issue.
Its the first things we checked, we have over 40 years each in this business.
It was a part left out by the dealer as it turns out.
 
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