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  1. #1
    Platinum Member
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    Jul 2007
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    742
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    S. Central TX Hill Country

    Default New Problem S750 auxilary hydraulics

    My s750 just turned 800 hours. In all the years I've had Bobcats (I've owned 12 high flow machines), I've never come across this.. and I've seen just about everything I think.

    When my auxiliary hydraulics are on, at high RPM on standard or high flow, a loud whistle-like sound has developed. I've traced the sound to the auxiliary lines or possibly the couplers. The sound is coming from near the couplers but as I stated, it could be the lines directly behind the auxiliary couplers. It's not a vibration, rattle, or anything like that. It's a high-pitched whistle but it's definitely coming from outside the cab and I can hear it with the A/C on and ear plugs playing music. I stepped out the machine and ran an attachment at full rpm and the sound was still there. What's odd is if I roll the tilt cylinders back the sound stops for a second or two and it only whistles when the RPM's are up and the flow is at maximum either standard or high-flow.

    Has anyone ever come across this? I've gone through plenty of couplers but never heard this sound before. Could it be a couple failing on the inside and a restriction causing the sound? Any advice/thoughts would be appreciated.

    I also have a "chirp" sound coming from under the cab. The dealer has looked at it two times and says I have tube lines rubbing together at high frequency but they couldn't find the exact culprit. I'm sending it back to the shop, soon for another problem and I'm going to request they do whatever is necessary to find that "chirp" as it's driving me crazy. It only chirps when the hydraulics bog down.. but other than that sound, have no noticeable loss of performance.
    Again, any help in the right direction would be appreciated.
    Brush Chipping and Tree Mulching Texas Style!

  2. #2
    Veteran Member SSdoxie's Avatar
    Join Date
    Jun 2010
    Posts
    1,911
    Location
    Cumberland Plateau, TN
    Tractor
    Bobcat CT225

    Default Re: New Problem S750 auxilary hydraulics

    The whistle may be a small air leak in suction lines somewhere. When you retract the cylinders this puts more oil into the return/suction side than what goes into the cylinder, momentarily increasing pressure in the reservoir. Check hose clamps on suction lines, and put a torque wrench on the 2 bolts fastening the gear pump assembly to the hydrostat pump, should be 55 -60 ft/lb. If those bolts start to work loose it could draw air into the suction side around the O-ring seal before it is loose enough to start leaking.

    Chirping - Has dealer tried replacing the belt or set/readjusted the deflection stop?
    Dennis

    CT225 w/7TL QA FEL, 60"tooth dirt bucket, 68" smooth bucket, Pallet Forks, Bale Spear. Hydrualic top llin, QH, 48 RC, 60" tiller, Angle blade, Landscape rake, Carry-all, Post hole auger.

  3. #3
    Platinum Member
    Join Date
    Jul 2007
    Posts
    742
    Location
    S. Central TX Hill Country

    Default Re: New Problem S750 auxilary hydraulics

    Dennis. The dealer has looked at the belt twice and told me it wasn't the belt. I can ask about the deflection stop. It's going back in for an oil leak in a couple of weeks and I am going to tell them to look at that deflection stop

    I will check out the suction lines. The odd thing is that the sound appears to be coming from the auxiliary coupler block at the front of the machine. I even got out and that's where I seemed to pinpoint the sound. Could a coupler make that sound and not be leaking fluid? I go through a lot of couplers running a high flow mulcher but in 12 years of running mulchers, this is the first time I've heard that sound with auxiliary hydraulics turned on. It doesn't make it when I'm just using the tilt or lift circuit.
    Brush Chipping and Tree Mulching Texas Style!

  4. #4
    Platinum Member
    Join Date
    Jul 2007
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    742
    Location
    S. Central TX Hill Country

    Default Re: New Problem S750 auxilary hydraulics

    I hooked up a different high flow attachment (stump grinder) and still seemed to trace that whistle to the couplers. I believe it was the female coupler. It wasn't on there very long but the male coupler on my mulcher isn't closing all the way and may have damaged the female. I replaced the coupler and ran the stump grinder and the whistle was gone. I'm going to get a replacement for the mulcher before I attempt to connect it again. Will keep the forum posted on what I find.
    Brush Chipping and Tree Mulching Texas Style!

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