ASE Master (auto) Tech here, newish to tractors but it's kind of all the same.
The PTO is driven by a shaft that goes
through the HST but doesn't interact with the HST's function, i.e. you could start hitting the HST with a sledgehammer and the pto would be unaffected until you smashed all the way down to where the PTO shaft goes through the middle of it. The 24' mower test suggests (unscientifically) that the engine is somewhat ok. I know someone said a mower not cutting anything isn't a test, but i don't think i could get a 24' mower spinning without smoking my clutch on my little tractors. I think it would take 20+hp to spin that mower at 540rpm not even cutting anything.
The HST is able to transfer enough torque to spin 4 tires pushing against a tree in low range. As already said, the HST moves the same amount of fluid regardless of whether you are in low, medium, or high range so the HST is able to move fluid, at least at a low enough pressure. Pressure will only rise enough to meet the load. It's possible that going up any kind of hill in high range requires more pressure than spinning all 4 tires in low.
If that's the case, we're talking about a problem that either starts, or finally becomes noticeable, at higher pressures
inside the HST. The only pressure
leaving the HST is essentially to push fluid through a remote mounted filter, if it has a remote mounted filter instead of one directly screwed onto the HST. Even if there was a restriction in the filter, it should have internal bypass valve which would open and allow flow to continue. If it didn't, or if something after the filter became restricted, it still shouldn't cause this because the HST can 100% definitely make enough pressure to simply blow out the filter o-ring or split the filter case if that path was THAT restricted, since spin-on filters can only take ~100-150psi before popping, from what i understand. So, we are talking about something that happens at high pressure INSIDE the HST.
The HST has a charge pump, a variable displacement pump, and a fixed displacement motor. If there were any problem getting fluid INTO those components, they would do something closer to freewheeling which would NOT bog the engine. If anything it would make a whining noise while the engine revs stayed mostly the same. But if there is a problem getting fluid OUT of any one of those components they become harder to spin and bog down the engine. All leaks leak more when there is a larger pressure differential so we probably have an internal leak making it harder to get fluid OUT of a component, which is not causing a noticeable symptom at low pressures because the flow rate of the leak is lower as well.
My only theory right now has to do with case drains. If you have a piston pump/motor, everything under/behind the piston rings is the 'crankcase' where any leakage past the rings goes. The pumps will have some kind of case drain releasing that leakage back into the sump to prevent the crankcase from filling up with fluid. My theory right now is that one of these pumps has a case drain which is just some kind of smallish hole. If this pump has a leaky cylinder, at low pressure the case drain hole can keep up with the leak, and the area underneath the pistons does not fill up with fluid. But, at high pressure, the leak is larger than the case drain in terms of flow, and the area underneath the pistons in the pump fills up with fluid, making it extremely difficult to turn that pump.
So i
think we're looking at a leaky cylinder in one of the pumps or motor that is outflowing the case drain at high pressure, causing that pump/motor to almost 'lock up'.
If that's the case, the only solution will be to take the HST out and apart for repair or possible replacement.
I hope i'm wrong..$$$
