New transmission, and now a different problem???

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by rolls canardly, Feb 23, 2024.

  1. rolls canardly

    rolls canardly Road Train Member

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    Took my 2007 Freightliner 26,000 GVW Rollback to an Allison automatic distributor/service center and had a new transmission installed. My PTO works, but now just tried to load a heavier vehicle, and the winch pulls the car on, but there is not enough power to pull the bed back up onto the truck frame. seems to be slipping or bypassing somehow. Wasn't that way before. I once got a chain caught and didn't notice it soon enough and broke the cable with the 8000 lb winch. plenty of power - now wimpy? PTO has a clutch pack and air operated engagement. I was thinking pressure was bypassing from an incorrect over pressure setting.
    I believe you screw in a 4000 lb. gauge on a tee to set this, but I don't have one over 2000 lbs.
    I have an appointment to take it back Tuesday 8 AM. I'm afraid they will say they see nothing wrong.
    They wouldn't put a new transmission in with my old torque converter, would they?
    What could be wrong here?
     
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  3. Goodysnap

    Goodysnap Road Train Member

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    May sound silly but is the hydraulic system full and no air. All shutoff valves on and no hoses kinked?

    Something changed, but maybe not related to the transmission. Surely the pump was removed and put out of the way for access?
     
    Magoo1968 Thanks this.
  4. rolls canardly

    rolls canardly Road Train Member

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    Thanks!
    I checked all that, and AOK, New hyd. filter, too.
    Check the simple stuff first.
    They took the PTO off totally, and had to remove hyd. lines from old tranny,
    so I hope they removed the correct plugs on the new one to put those hoses back on?
    Seems funny it works empty or light load, but not real heavy load.

    I even greased the ways on the bed rails, Poor truck said "What's that stuff?"
     
  5. AModelCat

    AModelCat Road Train Member

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    If fluid level good and no air in system your first check is going to be to plumb in a gauge and see what the relief is set to.
     
    JB7 and beastr123 Thank this.
  6. rolls canardly

    rolls canardly Road Train Member

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    Copy that, I have a 2000 lb. gauge, but don't know the range of pressure coming out of tranny.
    Buddy of mine that is more hydraulic savvy than I am said you need a 4000 lb. gauge first to verify the range.
    Don't know. I would think 2000 lb.range would be sufficient for a transmission?
    If i knew where the relief setting was, I would just bump it up.
    Keep watching the gauge and screw it in.
    I believe its only a spring and plunger you would screw in to increase system pressure.
     
  7. AModelCat

    AModelCat Road Train Member

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    You're not dealing with transmission pressure, you're dealing with a seperate hydraulic system. I would even go as far as starting with a 5k PSI gauge.
     
  8. rolls canardly

    rolls canardly Road Train Member

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    I agree, I have an appointment with service Tuesday 7 AM.
    It's a warranty job now!!!
    You engage the PTO switch and increase idle to 1200 RPM.
    Transmission port puts out power to push 3 rods into the PTO clutch pack, to engage the main pump for that separate main hydraulic system.
    You hear a rumble as pressure increases, but not enough to move the rollback bed forward 6' off the ground.
    I think the small hose is transmission fluid that exits out the inside of the pump casting, draining back into the transmission body, after compressing the clutch pack.
    I think there is a way to increase this pressure; hope they can do it with programming from a scanner, IDK?
     
  9. brian991219

    brian991219 Road Train Member

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    Have you checked your hydraulic tank to make sure they topped your system off to replace the fluid you would have lost when they took the pump off, presuming they did disconnect the hydraulic lines and not just set the PTO and pump aside ad one unit?

    They should also check the shim/gasket (gear lash) spacing to make sure the new transmission and PTO are performing ad designed.

    Also check for a kink or change in hose routing that would cause a restriction to fluid movement.
     
  10. rolls canardly

    rolls canardly Road Train Member

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    Thanks Brian, for the reply.
    I filled the hydraulic tank, it's full. (ask me how I know?)
    I pulled the bed back up to cab, and the added volume
    from the 6 foot cyl. overflowed the tank a little.
    On billing the mechanic made a point to add "the backlash was checked and within spec."
    I can see the hoses from the pump and no kinks are visible.

    I notice that when engaged, it makes no sound, until I increase the idle.

    That leaves torque converter, (I sure hope not,that means it has to come out again,)
    relief or bypass valve stuck or broken - not probable; as it works marginally,
    or low pressure from trans. to operate the wet clutches inside hyd. pump to engage,
    that I hope can be turned up with their laptop or scanner.
     
  11. Ridgeline

    Ridgeline Road Train Member

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    Let's drop the idea that it is a trans problem, the trans has little to do with this, so let's start with two things, change the filter in the return line for the hydraulic tank, this being plugged will surely cause a problem. There should be a gauge there to see what the condition of the filter is.

    the other thing is this one is the possibility that the pump is bad. this is a very common thing for older trucks with this set up. Pumps wear out and they leak between the inlet and outlet so they won't produce enough pressure to move a heavy load. I would just change it myself, they are cheap <$500 and are rebuildable at that.
     
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