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Noisy Hydraulics on Cat 955L

July 26, 2007


Jim (in photo), having recovered from his one lap of the US, was up for some Saturday afternoon projects, and dropped by to lend a hand, or a wrench as it were.  I’ve been using this loader on my building projects since late 2000, and have now begun to address some issues that have developed largely due to it’s idleness.   The hydraulics that run the bucket have become extremely noisy and sometimes stutter a bit as if the pump is starving for oil.   Having heard the tell-tale groans from most of the other hydraulic machinery around here at one time or another, I know that it’s usually a sign of low hydraulic oil, or a clogged suction screen.   This machine pumps upwards of 50 gallons per minute, so any restriction is going to make a racket.  

The week before, I replaced the stacked cartridge filters in the front of the tank.  Since the tank was full of oil, and the filters were clean, we needed to check out the plumbing, and check on the suction screen.  This meant draining the tank and removing the 44 bolts that retain the access cover.  If air tools are not available, I’d suggest use of a cordless impact wrench such as one of these.  They really speed up the job.


The design of this machine, likely on the drawing board in the late 60’s, has the the valves and all the key plumbing located inside the tank, where it stays submerged in oil.  As a result, the inside looked like it did when it rumbled off the factory line sometime 30 odd years ago.

The screen was found in the very bottom of the tank and was quite coarse by all the standards I’m used to.  It wouldn’t pass a nut or bolt, but certainly nothing smaller was going to clog it up.   This is very different from the extra fine, pleated nylon suction screens found on my excavator, which had become clogged with a teflon looking  residue.   I had solved it’s noisy, groaning ailments by cleaning those, so I thought I was on the right track here.

Alas, everything in here was immense, and quite unclogged.   As I traced the lines and tried to make sense of the plumbing, not having a factory service manual handy, I did note these two loose bolts on the pressure return line from the control valves back to the filters.   At the time,  I discounted the fact that they were under oil, and wondered if having a bad seal there could affect the pump prime.  It really can’t, but I tightened the bolts anyway.


We reassembled everything, refilled with fresh oil, and were not suprised to find the noisy operation remains.   It’s odd, that the first few cycles of the bucket are normal and quiet, but by the third, the noisy condition returns.  The suction lines are all hard pipe, so a hose hasn’t collapsed.  Unless I’m hearing a sticky relief valve opening in the control valve because the spring has fatigued, and bleeding off flow and pressure, I’m running out of ideas other than looking into the pump.  The pump is about the size of a bowling ball, and I’ll wager a good deal heavier and probably costs as much as a used Honda.   Any commentor speculation on cause of the noise? 

As it still works, albeit a bit more noisy than it should be, I’m going to haunt ebay for a parts and service manual, and investigate further before I do anything drastic and start replacing parts.

Now a small conceit.   I had several objectives for this post:

1) Test out the results of having a topical title.  My post titles are usually rather esoteric, and as a result, aren’t favored with much traffic from search engines unless I blog about stone and concrete work, in which case, the traffic flows in.  Clear message to me that topical post titles should make a difference.

2) I will be interested to see if any experienced Cat mechanics will find this post and offer any constructive words, beyond trading for a 953 / 963.

3) Test out the idea of adding text overlays, call outs or effects to pics to help communicate the story.

4)  Share a bit of insight into my character.  We all find things – different things at different points in our lives, that help us express our passions, our emotions, our sense of who we are outside of our careers and other obligations.  Some sail, some Ski, some Sky dive, some find joy and purpose in their children.  For me, I really enjoy a sense of empowerment.     I really like working on, and running these machines, because for the moments that I can sit up six feet in the air and push stuff around, I feel completely different than I do while translating abstract ideas into PowerPoint, sending emails, or otherwise struggling for relevance in cubeville. 

6 Comments leave one →
  1. July 26, 2007 4:47 pm

    You sir, amaze me. I can barely open a can let alone the hydraulics hatch on an earthmover. I am not worthy.

  2. July 30, 2007 2:55 pm

    1) If I’m writing anything “topical” as you put it, focusing on a specific item, subject, industry, product, etc, I do try to include keywords, or at the least a very direct title. My “esoteric” titles come in when it’s not as much a discrete topic or not anything I care if it is easy to find via search.

    2) Let us know how it pans out, do a followup post on the success of this experiment 🙂

    3) Text overlays & call-outs are great for instructional stuff, or where you want to be very explicit. Good job on that. let me know if you need any help photo editing!

    4) As you put it once, it feels so much better to be working on something to IMPROVE it rather than just fix it. I feel the same even in “cubeland” honestly, but getting out there and working on a project is a nice way to get away from the daily grind.

    Great post Mark

  3. ken permalink
    March 28, 2010 6:30 pm

    Should be able to pin point if pump or control valve is source of noise. May need a stethoscope if you have a hard time determining. If you are starving your pump of oil this is called cavitation and very damaging to a pump. After an oil change a pump needs to be primed with oil. Usually near the top of the pump is a plug that needs to be removed to let trapped air out. Oil should start to flow from hole due to gravity. Let oil flow for a few moments to asure that all air is out.
    A weak relief valve will cause a weak machine. If it is stuck open it will also cause slow functions. Cycle times can tell you if this could be the case.

  4. October 29, 2010 5:18 pm

    oil change should be done depending on the characteristic of the machine”:

  5. April 17, 2011 6:53 pm

    Mark, I’m in awe of anyone who can drive one of these things, much less work on one. And hydraulics, no less..

    I have a little Kubota BX-24 and it’s all machine I can handle 🙂

    I’ve been fighting the hydraulics on my boat’s Volvo Penta Duo-prop outdrive trim system. Who ever thought that screwing a small-diameter stainless steel hydraulic fitting into aluminum threads and then soaking the assembly in salt water should have thought twice.

    BTW, I don’t find too many gratuitous apostrophes around her. Not too many…


  1. CAT 955L | Materials Find

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