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Air in hydraulic oil

Discussion in 'Dozers' started by shaunw7708, Oct 9, 2013.

  1. shaunw7708

    shaunw7708 Well-Known Member

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    I have a D7h 79z1723 crawler, and I am having trouble with air getting in to the hydraulic system, originally I thought it was in the tilt, so I bled the cylinders and everything worked fine, but as you work and the hydraulic oil gets hotter I lose my ability to tilt. I bled off one of the cylinders when it was hot and the oil is filled with air. I checked the sump and put in a new filter, but everything was clean. What is my next step to start to solve this problem.
     
  2. fast_st

    fast_st Senior Member

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    How do things look on the suction side of the pump? is the pump whining or crying at all at higher rpm's / hydraulic loads?
     
  3. mitch504

    mitch504 Senior Member

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    Like I think fast_st was getting at, it's usually a leak on the pump suction combined with a clogged strainer, or pump seals.
     
  4. shaunw7708

    shaunw7708 Well-Known Member

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    Had the strainer out with the filter and everything was clean. It does sound like it is whining at idle. Do I need to drain the hydraulic tank, and start tracing everything back to the pump? The hydraulic oil temp runs at about 140 F, and the front cylinders seem to work fine, although they do lift faster at higher rpm. Is the hydraulic pressure supposed to go up with the rpms or is it like a tractor where it should stay constant regardless of rpms. Because if it is my pump is probably weak.
     
  5. Construct'O

    Construct'O Senior Member

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    Hot oil is usually some what foamy when you break a hose so sounds normal there to me.I would check the tilt cyclinder hoses to see if they are getting pinched somewhere when blade is tilted.Also check the tilt linkage under the tilt lever.Ends might be getting wore out and not letting it open the valve all the way.Good luck
     
  6. shaunw7708

    shaunw7708 Well-Known Member

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    I don't think any hoses are kinked I can check them though. After I bleed a cylinder I can tilt the blade all the way both ways but the longer I run the machine the less tilt I have, after 5 or 6 hours I have almost no tilt at all. When I start the next morning cold I have about half tilt, then it tightens up until there is almost no tilt again. I will also check linkage. thanks
     
  7. theironoracle

    theironoracle Senior Member

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    shaunw7708, your machine runs a gear pump for all hydraulics, this pump is constant pressure constant displacement, the only thing that makes flow change is engine rpm. pull the floor boards, under where your left foot was is where the pump is, you can also see it a bit if you look behind the engine on the left side of the machine. anyways is there any oil residue around pump? if so change those O-rings, top and bottom, you will have to drain hydraulic tank or pull a vacuum on it. as far as the tilt not working properly I am guessing you have a packing failure in that cylinder. will the blade lift the front of the machine off the ground at idle, even slowly? if not pretty certain you have a pump failure by the sounds of it anyway. the proper way to diagnose would be to put a gauge on the system...2. Problem: Pump makes unusual noise, the cylinder rods do not move smoothly and there are air bubbles in the oil. follows is a cut and paste from sis....TIO

    PROBABLE CAUSE:

    1. Oil aeration from a loose connection or restriction of the oil line on the inlet side of pump.

    2. The oil level is too low.

    3. The cylinders and lines are not properly purged of air.

    4. Porous welds or broken baffles in hydraulic tank.
     
  8. HATCHEQUIP

    HATCHEQUIP Senior Member

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    Unhook the tilt cylinder lines and hook a gauge into each end when it quits and see if you have pressure if so TIO hit it on the head with bad cylinder packing.
     
  9. rare ss

    rare ss Senior Member

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    as TIO and HATCHEQUIP mentioned with what your describing I would think the ram seals are bypassing internally causing aeration of the oil (which would cause the pump noise), when the cylinders start to play up check the temp of them, if they are hot to touch you should look at a leak test

    if you bottom out the cylinders, remove the dead headed side (the side which should have minimal oil left in the cylinder) and place the hose end from the ram in a bucket, fire up the machine and stall the ram out (to pressurize the opposite side of the ram, there should be no oil exit the open side of the cylinder, my bet is that alot will

    I have been caught with this method of test in the past when the cylinder barrels are very worn though as the ram seals will only bypass when the ram is mid-stroke, safety way to test this is with a gauge either side and the ram locked in mid travel
     
  10. Delmer

    Delmer Senior Member

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    This makes no sense to me, so I'll take a wild guess with this suggestion.

    Does the tilt control valve lose range of movement when it's been run for hours? probably just in one direction? I'm not familiar with this valve, but in the wide world of hydraulic valves, most of them have a dust cap bolted to the bottom of each spool. If the spool is leaking that cap gets filled up with enough oil to block the spool from moving all the way, over night that oil seeps out and it works better the next day for a while.
     
  11. kshansen

    kshansen Senior Member

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    Another possibly less messy test than what rare ss suggested.

    I'm not familiar with this exact machine so descriptions are a little generic.

    Disconnect the line that goes to the bottom (opposite end from the ram) of the cylinder and cap off both the line and the fitting at the cylinder. Be sure to use caps that will take the full pressure of the system. Now run machine and operate controls to put pressure to the head end of cylinder. The rod should try to move in to cylinder under this pressure, but with the other end blocked it should not move other than a slight amount at first. If it is instead moving out it would mean the packings are leaking, how fast it moves is related to how bad the packings are leaking. There is more surface area on the piston opposite the ram the cylinder rod will move out. If there are two tilt cylinders it would be best to do the test on both at the same time. Or cap off both lines to the other cylinder. Then do the same test on the second cylinder.

    Probably should have asked first but do all the other hydraulic functions work good when the tilt is giving you trouble?
     
    Last edited: Oct 12, 2013
  12. shaunw7708

    shaunw7708 Well-Known Member

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    I'll pull the floor board and have a look at the pump for leaks, the front cylinders lift the machine at idle easily so probably not pump problem. Lift cylinders do move smoothly, just the loss of tilt, it has dual tilt cylinders, so would that mean they both have packing failure. It is a high hour machine so it would not suprise me.
     
  13. shaunw7708

    shaunw7708 Well-Known Member

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    Yes, it does lose range of movement, but in both directions. Eventually will only tilt about an inch either way. It has dual tilt cylinders, if that makes any difference to your diagnosis. I will try and have a look at the tilt control valve.
     
  14. shaunw7708

    shaunw7708 Well-Known Member

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    I don't have time to track down a gauge right now so I will try to check for bypass with bottomed out and I'll check to see how hot they get after a few hours.
     
  15. Nige

    Nige Senior Member

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    The hydraulic tank on this machine is designed to pressurize as the oil heats up so that the pump suction is actually "pressurized" to a certain extent to stop the pump cavitating with hot oil. After working the machine to the point where you lose hydraulic function try (gently) unscrewing the tank cap just a little - DO NOT remove it completely. You should hear a significant release of pressure. if you don't I'd suggest that the tank cap is not sealing correctly. If it was sealing correctly and you had any sort of leak in the line between the tank and the pump, or in the pump itself, the pressure created in the tank would cause you a major oil leak as the pressure relieved itself. If you have no oil leak in this area then go looking at the tank cap as a first step IMHO.

    The tank may not be the cause of all your problems (there may be issues elesewhere) but if you troubleshoot the system using the basis that the oil starts from the tank then goes via the pump, control valve, cylinders, and returns back to the tank again then the tank is the logical place to start your troubleshooting.

    To check the cap is all there, remove it when cold. You should find a loose circular steel plate secured by a spring clip in the underside of the cap. Remove the spring clip and there should a a small ball bearing behind the plate that fits in a cup in the underside of the cap. Finall there should be a circular rubber seal about 1/4" wide that seals the cap to the tube welded into the top of the tank. That seal might well be bad.
     
    Last edited: Oct 12, 2013
  16. shaunw7708

    shaunw7708 Well-Known Member

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    I've taken the cap off after the oil is hot and the tank is pressurized, so that should eliminate one thing. I will still look at the cap.
     
  17. kshansen

    kshansen Senior Member

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    On the top of your hydraulic tank there should be the cap, the filter cover and a breather. Breather is about 2 inches around and 3 inches tall. The breather has a valve in it that controls the pressure in the tank kind of like the radiator cap on a radiator. There is a snap ring holding on the cover and under this cover is a metal filter. I make it a habit of removing and cleaning this filter at each maintenance period. I was not able to find mention of this in SIS for you machine.
     
  18. Delmer

    Delmer Senior Member

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    I meant the HANDLE for the control valve. If this is a more complicated/modern system, or if the handle moves the same way hot or cold, then ignore this.
     
    Last edited: Oct 12, 2013
  19. kshansen

    kshansen Senior Member

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    Breaker Valve.gif

    Found a picture of the Valve I was referring to above.

    Parts are 2-2S4663 o-rings, figure # 1
    1-5P8132 retaining ring, figure #2
    or complete valve 3G9895

    This is the valve that controls the pressure in the tank.

    Not suggesting this is your problem more for your information.
     
    Last edited: Oct 12, 2013
  20. Nige

    Nige Senior Member

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    ksh. I know what you are referring to - the breaker relief valve. It blows off like a relief valve when the pressure inside the tank is too high and allows air to enter the tank if there is an internal vacuum. According to SIS the hydraulic tank of the 79Z-prefix D7H does not have a BRV, at least it's not listed or show in the Parts Manual.