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Do you have air or cable hoist control? If cable check adjustment. If air check the air plunger that is mounted on the PTO pump. Sometimes they get dirt in them and won't travel the full stroke. Sometimes a piece of dirt will score the inside of the bore thus allowing air to bypass. Sometimes the o-rings fail.

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I'm still going to say check the air plunger for travel first. I've had to replace new ones because they were scorded inside. But yes, if new, will be warranty.

 

I'll get pics later. Going out in a bit to get truck ready for tonight. Minus 25*C here, so gunna go clean the snow off the truck while the sun is out, LOL.

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check the valve for proper adjustment. 

my 2020 T880 would just barely go up at 1500 RPM, and drop like the box had 25 ton in it when empty blowing the fluid out of the hydraulic tank.

after properly centering the control piston it now raises at idle, and comes down without blowing out the hydraulic fluid.  

Edited by tjc transport
fix typo

when you are up to your armpits in alligators,

it is hard to remember you only came in to drain the swamp..

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See the cylinder with the 2 air lines with the Drive Products decal on the bottom of the PTO. That is the air shift plunger. The back side is where that little keyway slide is and you can see if it is travelling. Get someone to go in the cab and get someone to shift it up and down and see how that is moving. In the old days it was open on the back so you could hit it with a hammer but now they all have that cover on them.

You can also take the air lines off (and check for air supply) and remove those 2 bolts and slide cover off. There is a piston with o-rings inside. Make sure it is clean and lube it with some Krown or Fluid Film orsomething similar. 

That is assuming you can remove it. Sometimes the pumps are rotated to a different orientation on the transmission. I had one Freightliner Classic that I had to  drop the PTO from the trans and drop the oil to get that plunger off. Not a fun job in the bush when you are working.

 

Hopefully this helps. I suck at trying to explain stuff.

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so the second  last picture can you manually move that  shaft to the front any further that cap with the hole is meant for a cable to attach to  pull the valve to the neutral position  when the box is raised WHEN  operated by cable  system on an air set up it becoms redundant Can you push the rod with the pin thru it forward any more! Maybe contact Drive products and ask  some questions!

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