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Discussion Starter #1
I decided that I should purge the hydro on my YT3K today so I pulled the disengagement pin and put it in reverse. As expected, it just sat there so I slowly gave it more throttle and then returned it to neutral. I then moved the drive selector forward slowly to the highest "mowing" speed and as expected, it just sat there. Then came the "OH ****" part.:fing20:

As I pushed the selector further to the highest "travel" gear, suddenly the tractor spun its wheels in the gravel and lurched forward, taking me quite by surprise. It was a good thing I had a good ten feet of free space in front as I burned it up right quick. I checked the disengagement pin and it was still in the disengage position so I'm not sure what happened. I'm guessing the purge procedure shouldn't be done at full throttle.:duh:
 

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Purge procedure can be done at full throttle, But I would get the back tries off the ground.
 

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Is this purge procedure something that "needs" to be done every so often? I've never heard of this before.
 

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LLigetfa,

Do you have a manual that spells out the purging process?

My manual specifically says to disengage the transmission, start the engine, then REDUCE THE ENGING SPEED TO SLOW before slowly moving the motion lever full forward, hold for 5 seconds, and back to reverse, hold for 5 seconds, repeating three times. Afterward, shut the engine off, reengage the transmission, start the engine and mover the throttle TO 1/2 SPEED, then move the motion lever slowly to forward allowing the tractor to move approximately 5 feet forward, then slowly to fully reverse for 5 feet, repeating 3 times.

Running the engine at full throttle when trying to purge the transmission is self-defeating because the speed of the transmission pump would tend to make the oil foam. Instead of purging the air, it would pump air into the transmission fluid.

Runningbare
 

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Purge Instructions: Jack up rear wheels, disengage drive-train (set to free-wheel using clip lever),
start engine and run at 1/2 throttle, slowly move shift lever from forward to reverse several times,
stop engine, engage drive-train (set to lock using freelwheel clip lever), start engine and run at 1/2
throttle, slowly move shift lever from forward to reverse several times until wheels turn without indication
of air in hydraulic system.
 

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How often do you have to purge the tranny's or is that just for certain types or all hydros
 

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Discussion Starter #7
Do you have a manual that spells out the purging process?
Actually, I do and I did read and follow the instructions four years ago when the tractor was new. I should have reviewed it obviously since my recollection was flawed.

The manual makes no mention of having to repeat the purge procedure. I think if the purge is done with the wheels on the ground, no air would get purged from the hydraulic motor that is driven by the hydraulic pump except during the second stage of the procedure when the disengagement pin is back in the drive position (bypass valve closed). Then it is once again a closed loop so if there was any air in the second half, it would stay in the loop.

I think that pushing the tractor with the hydro disengaged (bypass valve open) works to purge the second half while the prescribed procedure purges the first half.

As for whether the purge should be performed on a schedule, the manual makes no such reference but there is anecdotal evidence that old tractors do benefit from the procedure.

There is also anecdotal evidence that the simple purge procedure outlined in the manual is insufficient to remove larger quantities of air as might be the case when the oil is changed and that the wheels do in fact need to turn (with the bypass valve open) to evacuate air from the second half. I think also (as was mentioned by someone) that air may get entrained in the oil (foam), and that time (and a slow throttle) are factors in a successful purge.
 

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Another problem that can make it difficult to purge air is the viscosity of the oil. The oil recommended for the transaxle is 20W-50, which is very thick and difficult for the pumps in the transaxle to pick up, especially when the temperature is cool or cold. This combined with purging the transmission at high engine speed can make successful purging difficult.

I switched to synthetic 5W-50 in the dead of winter when it was down below zero because the tractor would barely move going forward and reverse was nearly non-existant. The original oil was so thick it drained like molasses, taking about 12 hours to ooze out. After I installed the synthetic oil and performed one purge cycle, the transaxle was fully functional, performing better than before the oil change.

Runningbare
 
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