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  1. #1
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    Default Intermittant hydraulic shudder and failure

    Hello, I have been lurking on this site for quite a while and decided to finally register. I have an older small omc mustang bobcat that has recently started having a problem. The lift and dump cylinders are run by a seperate gear pump from the drive motor pumps. This pump has started to shudder for no better word approximately 10-15 minutes after I run the tractor, the shudder will progressively get worse and louder until the lift/dump system will almost not work and react very slowly. Even when the problem is occurring the pump seems to be producing adequate pressure, just not flow or possibly cavitating. I do not see any external leaks, but cannot check the pickup strainer without dismantling the entire drive bay which is a pain. What is very weird is the fact that the system will work fine again after I let the tractor sit for a while(the fluid doesn't even need to cool). It doesn't look like the fluid is getting aerated. My question is whether anyone has a troubleshooting plan that may help me diagnose the problem. As of now I don't know if it is a pump or valve or pickup problem. I have ruled out the cylinder circuits since they hold pressure and position. BTW the pump was replaced maybe a few hundred hours ago and there is no filter on this circuit only a in tank pickup strainer.
    Any help would be appreciated.

  2. #2
    Super Star Member J_J's Avatar
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    Power-Trac 1445, KUBOTA B-9200HST

    Default Re: Intermittant hydraulic shudder and failure

    Quote Originally Posted by greenerdreams View Post
    Hello, I have been lurking on this site for quite a while and decided to finally register. I have an older small omc mustang bobcat that has recently started having a problem. The lift and dump cylinders are run by a seperate gear pump from the drive motor pumps. This pump has started to shudder for no better word approximately 10-15 minutes after I run the tractor, the shudder will progressively get worse and louder until the lift/dump system will almost not work and react very slowly. Even when the problem is occurring the pump seems to be producing adequate pressure, just not flow or possibly cavitating. I do not see any external leaks, but cannot check the pickup strainer without dismantling the entire drive bay which is a pain. What is very weird is the fact that the system will work fine again after I let the tractor sit for a while(the fluid doesn't even need to cool). It doesn't look like the fluid is getting aerated. My question is whether anyone has a troubleshooting plan that may help me diagnose the problem. As of now I don't know if it is a pump or valve or pickup problem. I have ruled out the cylinder circuits since they hold pressure and position. BTW the pump was replaced maybe a few hundred hours ago and there is no filter on this circuit only a in tank pickup strainer.
    Any help would be appreciated.
    I would first purchase a hyd pressure gage, a short hose and a long hose, and put a female QD on the end of the hose. The long hose is so you can sit in the loader and see the gage and perhaps determine what is happening in your hydraulic system.

    If you tee in a pressure test point at the output of the pump, then you will know if you have pressure. I would install the tee fitting with a male QD, so you can plug in the pressure gage with a female QD. If you installed a tee and QD at each pump, you should be able to test your complete system. If you think your pump is cavitating, you should shut it down and find the cause. A cavitating pump will destroy itself. Check for low fluid level, strainer clogged, air ingestion. They do make high pressure in-line filters. If all controls, valves, pumps etc, seem to work fine, then those items probably are not the problem, unless they start to develop trouble when hot.

    You say that it seems to have pressure, but then say the lift arms quit working. Pressure is what does the work, no lift, then no pressure.

    Is the hydraulic fluid level correct. You should have more than a tank strainer for your system. Are you sure you don't have something like a 10 micron filter on the input of one of the pumps. Most filters are located where they can be easily checked and replaced.
    J.J.

    When I works, I works hard. When I sits and thinks, I goes to sleep.

    Git er done.

  3. #3
    Veteran Member
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    Massey Ferguson model 85, Allis-Chalmers WD-45

    Default Re: Intermittant hydraulic shudder and failure

    I like J_J's answer. In addition to that, I suspect the fluid is aerating. I would check the suction lines. After the unit sits, the air bubbles escape and it works fine again. It could also be the suction strainer. Have you changed the fluid yet? If the suction strainer is that much of a bear to get to and you don't want to do it, guess what? Neither did the previous owners. Guess what you are going to learn how to do.
    Knowing is not enough, you must apply.
    Willing is not enough, you must do.
    Bruce Lee

  4. #4
    Advertiser kennyd's Avatar
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    Westminster, MD
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    John Deere 4110, 455AWS

    Default Re: Intermittant hydraulic shudder and failure

    I agree with WCH. A leak on a return line so you won't see an external leak because it's sucking air. Keep running it like that and you will be shopping for a new pump.
    KennyD
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  5. #5
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    318 John Deere, 4200 John Deere, 1947 John Deere "M"

    Default Re: Intermittant hydraulic shudder and failure

    Do you have separate reserviors for the drive circuit, and the lift/dump circuit? Sounds like it's starving for fluid. Separate tanks, might lead you to only check the hydro level in one. Just a newbie in this 'hydro' game, but my un-educated guess! When the lift arms fail, can you still drive the unit? How am I doing Kenny/J_J? ~Scotty

  6. #6
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    Default Re: Intermittant hydraulic shudder and failure

    thanks for the replies. I agree that I probably should do a pressure test so I will get the stuff for that. No the pump doesn't have a filter just a strainer. It is a very small bobcat skidsteer and the pump is very small. There are three pumps 2 for the drives and one small one for the cylinders, the drives are working fine, but they have spin on filters. The fluid level is right at the top of the line and I changed it a little while ago (it looks fine and I don't really want to change it again since there is about 20 gallons of fluid )when I rebuilt a drive motor. I made the mistake of not cleaning the strainer when I had everything apart. It does really act like cavitation or air intake but what is confusing me is the fact that it will work fine for quite a while then start doing it all of a sudden . Thanks for the input I will start checking some o fthe ideas you had.

  7. #7
    Super Member Scooby074's Avatar
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    BX 25, ZD 326

    Default Re: Intermittant hydraulic shudder and failure

    A pressure check is good and easy. But im betting on the suction side. You can get debris agitated in the tank that get collected on the suction screen. When you turn it off these debris can go back to the bottom of the tank, and the pump works again.

    If it was mine,, thats where id look. Pull the screen and clean the tank.

  8. #8
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    Default Re: Intermittant hydraulic shudder and failure

    What you are saying really makes sense, I have a bulldozer that had a very similar issue but with the diesel fuel tank instead of the hydraulics, it turned out that the screen would clog run the filter dry and kill the engine. Sounds like I am going to have to pull the hydraulic compartment apart. Thanks for the input to everyone.

  9. #9
    Super Star Member J_J's Avatar
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    Default Re: Intermittant hydraulic shudder and failure

    Quote Originally Posted by greenerdreams View Post
    What you are saying really makes sense, I have a bulldozer that had a very similar issue but with the diesel fuel tank instead of the hydraulics, it turned out that the screen would clog run the filter dry and kill the engine. Sounds like I am going to have to pull the hydraulic compartment apart. Thanks for the input to everyone.
    If your other pumps are working OK, then there is no need to mess with the tank. Just check the suction hose going to the pump, or replace it. If there is enough flex in the suction hose, move it around and see if you can see any cracks. You might have a loose fitting somewhere.
    J.J.

    When I works, I works hard. When I sits and thinks, I goes to sleep.

    Git er done.

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