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AM General M939 6x6 military dump

Discussion in 'Off Road Haul Trucks' started by Fixer, Aug 14, 2022.

  1. Fixer

    Fixer Member

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    Aloha!

    The PTO for the dump bed engages, but there is no hydraulic pressure, and will not go up or down. The hoses were changed about a year ago. I’m wondering if it’s a cylinder or pump issue; does anyone know how I would I go about troubleshooting this issue?

    Thank you for your help!
     
  2. Delmer

    Delmer Senior Member

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    It's air until proven otherwise. I'd try to manually put oil into the pump and the suction hose. Unless the pump is submerged in the hydraulic tank? I couldn't find any pictures to show the tank or pump setup. The cylinder would be unlikely to fail ever, even less likely to fail suddenly, or while sitting. Cylinders start leaking down, or leaking fluid usually.
     
  3. mitch504

    mitch504 Senior Member

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    I just went through a 939. It was nothing like a civilian dump truck. A tank on the frame rail supplies oil to a pump which pushes it to a very expensive valve, then to 2 sets of double acting cylinders. The hoses are just tee'd to the cylinders, no sequencing valve or anything. There are 2 small cylinders which move hooks on the frame which keep the bed down, then 2 big cylinders which raise the bed and then pull it back down. The first question, is the oil tank full? Next, when somebody moves the handle to dump, does the valve spool move? Do the hoses jump? Is the pump driveshaft turning? The one I worked on had a half a dozen problems, but the one that finally stopped them from using it was that the valve spool locked up:eek:. I haven't pulled it apart yet to see what happened, I just replaced it with a common valve.
     
    digger doug likes this.
  4. Fixer

    Fixer Member

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    Hi Delmer, thank you for the advice. The hydraulic tank itself is full, so we’ll try what you suggested with the pump and hose. Thank you!
     
  5. Fixer

    Fixer Member

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    Hi Mitch! Im so glad you’re familiar. We’re able to see the hooks that hold the bed down, and they’re not engaged, but didn’t know about the two smaller cylinders, so I’ll watch those next time I’m under there. Where is the valve spool located? I may have gotten that confused with the pump drive shaft when we were checking it the other day? That is moving… and as far as the hoses jumping, it’s really hard to tell with everything vibrating under there, but I’m gonna lean towards probably not?
    Do you have a photo of the valve spool or could you direct me to where it is? It’s dusk here, so I hope the photos are clear enough for you to see.

    This spins when the PTO is engaged:


    It’s probably irrelevant, but we also changed the large hose with the new fittings:
    53C5C72C-A4CE-4BD1-B53D-A18C07E216B4.jpeg

    One more question, would crossed hoses damage the pump? Meaning lifting the handle up makes the bed go down, and vice versa… Like would it have more oomph for the upward pushing motion, or does the pump have one outlet making it not matter?

    Thank you again!
     

    Attached Files:

  6. Delmer

    Delmer Senior Member

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    Start from the beginning. Did the bed ever lift while you've seen the truck? did it stop after the hoses changed? or did it keep working for a while? What else has been done and when?

    With two double acting cylinders, it would be easy to hook one's hoses up opposite the other and there would be free flow of oil with no movement of the cylinders.
     
    Gary Layton and 398370 like this.
  7. mitch504

    mitch504 Senior Member

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    The valve is on the right (passenger) side, just in front of the transfer case. It has a control cable hooked to it that comes down from the handle behind and to the left of the driver's seat. I think you can just see the edge of the valve on the right side of your pic of the new hose.

    The pump is the piece with the shaft and u-joint sticking out of the front of it.
     
  8. Fixer

    Fixer Member

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    The bed did lift, and kept lifting after the hoses were changed. The first time I changed them personally, and they were hooked up correctly, but I had a small leak, so when we had a hand, my husband had him redo the lines, but they were crossed. The dump still worked, it was just opposite, up was down and down was up. We used it for a handful of loads and it stopped working. Nothing else was done to the dump bed or hydraulics since.
     
  9. Welder Dave

    Welder Dave Senior Member

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    Double check the hoses are going to right side of the cylinders. Normally the lift would work but not come down if a couple hoses were crossed but it might depend on how heavy the box is and the way the cylinders are configured.