Backhoe hydraulic problem

[Rowski]

My New Holland 759C backhoe is giving me some trouble. Basically all the funtions work fine. The problem lies when you trying to pick up something heavy and hold it there. You can see it better and more controled when you try to pick up the rear of the tractor, or at least unweight it. The only circut effected is the boom cylinder. The curl and dipper work fine, at least they seem too. When you try to unweight the rear of the tractor and go as far a the relief valve lets you and put the control stick in its neutral position you can see the cylinder "leaking" down. I have checked for external hydraulic leaks and could not find any. So it seems that the possibilities are: the cylinder has an internal leak, the spool valve is leaaking by, or the relief valves are not fully seated (small peice of dirt). Does anybody have any ideas as to what might be going on? Are there any steps that can be taken to further diagnose the problem? The backhoe is only 2 years old. Attached is a diagram that I have with the operators manual. The spool valve that controls the boom is #39 which is on the right side of the diagram.

As a note this problem may have been present ever since day one.

Thanks for your help!

 

[Soundguy]

A couple things come to mind. If you were getting oil by the piston on a single acting cylinder, it would be easier to detect, as you would most likely have external seepage. I'm guessing this is a double acting cylinder. It is possible that the piston packing is blown.

Also possible as you suggest, is the relief could be sticking, or the spool could be leaking. Both the relief system and the spool system can pe presurized to test the valve seals. I'm also guessing that you are not a hydraulic mechanic, and probably dont have a hand oil pump setup to test pressure on the relief or the spool. If you did, you could also test at what pressure the relief pops at. If you have an air compressor, ( even a cheap tire pump style may work )pressure gauge, and feel like rigging up the proper fittings, the test should be telltale if either of the valves leaks <font color=blue>significantly<font color=black>.
Disconnect enough lines to isolate that section of the spool, and pressurize one side of it, gauge inline.. see if it holds pressure. Try the other side as well. Dissconnect the input to the relief and pressurize it.

Use due caution, etc, etc ( standard liability warnings ) when using compressed air, etc. And keep in mid that you are introducing air into sysstems that will need to be bled, etc, etc

You may even be able to test the cylinder the same way.. but only if it has a fairly decent leak... I'm not sure what pressure that it is taking to make the seal fail.. but you probably don't want to try to develop it with air pressure, if it is very high. ( If it won't leak with <50psi of air, I'd take the cylinder to a hydro shop for a test.

Might try disconnecting the oposed line to the double acting cylinder to see if you are getting any flow from it while engaging the other side, and vice versa. Be carefull around presurized fluids as well. If you don't feel 'safe' about these ideas, it might be a service call thing.

Pay close attention to your plumbing, possibly label the lines if it helps. Keep in mind how the flow diverter works in those spool valves... many are constant pressure, and will make it difficult to check while live and attatched.

Just a few ideas, I'm sure others with more experience will chime in with other goodies.

Soundguy

 

[SteveV]

My approach would be to examine in the hydraulic fluid for foam and or water. Then I'd full stroke that cylinder a few times to vacate any air or other problems (those cylinders get hot and can evaporate water leaving an air pocket). Since you're loading the cylinder from two directions with the same result, I'd say the leaky spool or packing is the most likely outcome. Swap hoses to another spool and see how it behaves, then it's either the spool or the cylinder! Or if you're real lucky, loose fittings!/w3tcompact/icons/smile.gifSteveV

 

[Kodiak]

Actually the water doesn't evaporate. It turns to steam that can then saturate a large potion of your system. It shows up as a foam or milky solution after it cools and condenses again.

 

[Rowski]

Called the dealer today and told them about my problem, again. It's going on the third summer now, but that another issue and post. Basically the dealer told me to put the boom cylinder at its full length extended (piston out --> boom down). Then the told me to undo the hose at fitting closest to the extended piston. He did warn me about the safety warnings of working with hydraulic fluid, I am also aware of safety and how dangerous it can be. After removing the hose try to extend the piston. He said you would obviously wouldn't be able to extend the piston any more and working against the pressure relief valve. but if you have fluid coming out of the cylinder fitting in which the hose is disconnected the packing in the cylinder is leaking by. He claims that they have been field testing cylinder seal packings this way fro a long time. Sound logical to me. I will give it a try this weekend when the chipper comes off. I will post my results. If the packing in the cylinder is leaking they will rebuild the cylinder. Should I ask for a complete new cylinder or rebuilding good enough?

I'm not to sure the packing is bad, but that is strictly a guess. Dealer claims that most problems similar to this are the packing leaking.The dealer mentioned that if the packing is good then the valve bank needs to be looked at which is more complicated to diagnose and harder to get full warranty.

Any other thoughts or ideas?

Thanks again for everyones help.

 

[jinman]

Derek, I used to have an old Ford 4000 with a loader (mid-60s vintage). When I got it, the bucket would curl up, but as soon as I let the control go to neutral, the bucket would roll down. I became very suspicious of the control valve and went to the Ford dealer to order parts, but the counter-man in the parts department was smart enough (kind enough?) to tell me I should talk to the service manager before replacing such an expensive part. When I talked to the service mgr., he told me he had repaired 100s of hydraulic systems and only one of them was the control valve. He explained that instead of using o-ring seals, they used close-tolerance machined surfaces in the valve bodies and spools and he had never seen one of those go bad. He said that valve body problems were mostly cracks in the metal rather than leaking around valve spools. Looking at your diagram, it appears much more complex than my old Ford 4000, but I do see that most of the seals are related to relief valves and external plumbing. Essentially what I'm saying is that the seals in the cylinders get a lot more wear than the seals in your valve-body. In my case, I got my bucket's curl cylinders rebuilt and the problem went away. Your dealer has suggested a test method that makes good sense. I hope that, like me, you find the cylinder to be the problem rather than the valve-body. /w3tcompact/icons/smile.gif

BTW: For my old loader, the Ford dealer didn't have a rebuild kit, so I had to take my cylinders down the street to hydraulic repair shop where they used standard seals to rebuild my cylinders. Cost: $180.00 total. The Ford dealer didn't make a penny off of it...but you can bet I went back there when I needed other stuff.

 

[Rowski]

Not good news /w3tcompact/icons/sad.gif. Took the fitting of this afternoon. I got fluid coming out both the fitting on the piston and the hose. The hose is pushing more fluid than the cylinder fitting. Called the dealer, he was closed (it was Sautrday at 3pm). Will call Monday moring first thing to report my results to him and find out what the next step is.

I will keep everyone posted.

Does anybody have a fluid diagram for this backhoe NH759C?

 

[Fly_Rodr]

I have a 757C diagram, not sure if it is the same. I would be glad to email it to you if you like. I would have sent it but did not see an email address in your profile. Let me know.



browningl@adelphia.net

 

[Fly_Rodr]

Here is the diagram.

 

[Rowski]

Thanks. Its not exactly what I was looking for, But it did give me an idea of where the hoses run which I did need to know. I was looking more for the flow of fluid through the valving for a given funtion. I don't know if a fluid diagram like this is available. In my profession you can get fluid diagrams of automatic transmission (for the different gears)to isolate a bad component before breaking down the transmission. I was hoping a fluid diagram would help isolate a component in valve body.

The diagram you attached does help me.

Thanks!