650/750 scv
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Thread: 650/750 scv

  1. Top | #1
    AlKozak's Avatar
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    650/750 scv

    I've noticed that the loader will not keep the tractor off the ground. The FEL stay raised OK with a load, but the tractor will drop to the ground in a few seconds if I lift it with the loader.

    After looking at the parts diagram for the SCV, it appears that all the O-rings in the unit are there just to keep fluid inside the valve and leakage within the circuits of the spool valve is contained only by the clearance between the valve and its bore. I am going to rebuild the lift cylinders soon since the parts were cheap and I doubt they've ever been rebuilt.

    But assuming the issue is the valve, it looks like there is no fix for this problem beyond replacing it. At $600 or so for a new one, if I can find it, I'm inclined to live with the problem.

    So 2 questions:

    1. Am I correct in that there no way to fix the valve?
    2. Hydraulic parts are pretty standardized. Has anyone ever adapted another valve to this application? At least, one that doesn't cost $600?

    Al
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    Ray_PA's Avatar
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    I would say there is a lift cylinder with a leaking piston seal. A leaking piston seal will allow a cylinder to extend under load but a leaking piston seal will not necessarily allow a cylinder to retract under laod. This is due to the piston side volume of the cylinder being much smaller than the barrel side of the cylinder. For the same reason, cylinders generate more force when extending than they do retracting.

    And yes, valves that are leaking internally cannot be fixed. This happens due to worn spools or spool bores.

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    AlKozak(04-04-2017)

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    AlKozak's Avatar
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    Thank you, Ray.

    I did pull the cylinders off the loader today, disassembled them and started prepping the tubes for paint. While I found nothing that looked like complete failure inside, the piston "rings" do seem pretty worn and the pistons came out of the cylinder easier than I expected. We'll see what a new set of seals does.

    The boss at the base of the cylinder (with the mounting hole) looked like it was horribly corroded when I removed it, but on closer inspection it looks like someone built it up (not very well) with weld. Why, I do not know, and if it was not done carefully the heat could have damaged the internals. Based on comments the guy I bought it from made, it sounds like the FEL was not original to the tractor and had been sitting around for decades unused. The bucket cutting edge looks like it had never been used. The dump cylinders had been replaced with aftermarket Dayton units that are brand new. A plate on the FEL indicates that it came from Broward county FL and thus may have had some salt air exposure.

    Al
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    Ray_PA's Avatar
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    Quote Originally Posted by AlKozak View Post
    Thank you, Ray.

    I did pull the cylinders off the loader today, disassembled them and started prepping the tubes for paint. While I found nothing that looked like complete failure inside, the piston "rings" do seem pretty worn and the pistons came out of the cylinder easier than I expected. We'll see what a new set of seals does.

    The boss at the base of the cylinder (with the mounting hole) looked like it was horribly corroded when I removed it, but on closer inspection it looks like someone built it up (not very well) with weld. Why, I do not know, and if it was not done carefully the heat could have damaged the internals. Based on comments the guy I bought it from made, it sounds like the FEL was not original to the tractor and had been sitting around for decades unused. The bucket cutting edge looks like it had never been used. The dump cylinders had been replaced with aftermarket Dayton units that are brand new. A plate on the FEL indicates that it came from Broward county FL and thus may have had some salt air exposure.

    Al
    It makes sense that you would not have found complete failure, as the lift and lower still worked. The lift cylinders leaked out slowly under load when lifting the front of the tractor. This would have been caused by slow oil leakage passed the piston seals, not profuse leakage. Sounds like just normal seal wear.

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    AlKozak(04-04-2017)

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    AlKozak's Avatar
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    Here is something interesting.

    I have the cylinders apart and doing cosmetic work on the exterior - sandblast; drill out, tap and replace the destroyed zerk fittings; paint. I put the rams in a bench vise to disassemble them and compare the old parts with the new. The kit has a small O-ring that does not seem to belong anywhere. It is the same size as the threaded end of the ram and looks like it would keep fluid from leaking past the piston on its inside.

    Took the piston off the other ram and, lo and behold, there is an O-ring inside sitting in a groove in the ram. The first ram I disassembled has no groove. WTF?

    I'm going to take the ram to a machine shop and have them cut an O-ring groove in the grooveless ram.

    Al
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    Captain Hook Kennyd's Avatar
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    The cylinders did change a bit over the years with different serial numbers, perhaps one was replaced.
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    Kenny

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    AlKozak's Avatar
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    If you are curious to see what I'm talking about, here is a picture.

    Click image for larger version.

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    AlKozak's Avatar
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    I love to laugh at my own mistakes. But only after I curse like a longshoreman.

    I rebuilt and reassembled the lift cylinders. I know you don't have to bleed the hydraulics but I do not know why. So I left the retract hoses loose and raised the bucket all the way, then tightened the retract fittings and lowered the bucket under power. The tractor stayed off the ground with the lifts fully retracted. Ran the cylinders through a few cycles to make sure the air was fully incorporated into the fluid and then noticed that I couldn't lift the tractor anymore. Of course I continued to work the SCV. After a few WTF moments, I noticed a LARGE puddle of hydraulic fluid working its way under the tractor to the side I was standing on.

    I apparently hadn't made sure the snap ring on one of the cylinders was fully seated and I had pushed the seal/stop out of the top of it and was pumping oil onto the floor. Broke my cheap snap ring pliers (I wanted new ones anyway) getting it all back together. Problem solved!

    I am glad that I keep large bags of kitty litter on hand just for misadventures like this...

    Al
    Last edited by AlKozak; 04-12-2017 at 07:53 PM.

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    Yup.. It happens. Wish I could say it never happened to me! I've done a few operational check bleeding and fluid replacements. Lol

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