Question in regards to the HST auto transmission?

[buck1974]

I have a M59 with HST auto transmission, what I'm starting to notice is that the forward and reverse seems to be getting sluggish. It hesitates before take off and slowing down, where before it was responsive. What I was wondering does these transmission get air trapped and does it need to be purged. I understand what I'm about to ask has nothing to do with my tractor and is totally different, but I have a riding mower with an automatic transmission, in the manual it explains how to purge the air from the hydraulic motor ( trans ) from time to time, if the systems feels sluggish. My M59 feels if is low on fluid and I have checked that over and over again. I also have the HST knob that would increase the response, but it still has a hesitation on the max, but a stronger take off if that makes sense.

 

[Egon]

Have you lubed up the pedal controls lately?

Any air should be vented out of the top of the transmision case.

 

[Sysop]

In your typical daily usage, which of the following do you find yourself doing?

Do you use a high range and very little go-pedal to produce your torque?
Or do you switch to a low range and use the full go-pedal.

 

[Hersheyfarm]

There is zero need or way to purge 'trapped air'. Your m59 is cycling 26gpm, every minute all your fluid has went through twice. Your 12 gallons of fluid is not blocked by air. Clogged filter? You have adjustments on your go pedal for hst engagement as well as throttle engagement, they are very sensitive and more than likely are your issue. when was last time you changed hst filter? How many hours on machine?

Also, look at weep hole under transmission. It will show if fluid is in mid case. It could be possible for rear main to leak motor oil on clutch, but you would likely smell that. Could cutch need adjustment?

 

[rbargeron]

An HST doesn't have much in common with an automatic transmission.

The HST's go pedal connects to a servo regulator that controls how the drive engages. The regulator operates on pressure from a small charge pump inside the HST, that turns whenever the main clutch is engaged (whether the HST drive is engaged or not.)

The symptoms you describe sound like charge pressure is rising slower than it used to. It could be something simple like a worn main clutch. The discussion should start there. Does the tractor have normal pushing power once the HST engages?

 

[Fallon]

My L3200 had no adjustment at all. My new L4060 like all newer grand machines has adjustable HST sensitivity. Have to scroll through the menus to find it. A dozen odd stages bet between instant jarring stop the second you let off the pedal & nice slow gentle deceleration over a second or 2 after you let off the pedal. Not sure if the TBLs have that adjustment option. If not, lube the linkages.

 

[MHarryE]

Other than wear, problem sounds like restricted flow to the servo pistons. Filter? Time since oil change? Outside temp and type of oil? In cold weather UDT will create sluggish propel response compared to Super UDT2. If you have capability, check charge oil pressure. Charge oil provides control pressure plus fills the high pressure loop. A bad component can Rob charge flow and create what you describe.

 

[rScotty]

Buck, my M59 had a similar lag time or hesitation for a few hours....and then the throttle cable from the foot pedal broke. That cable is a stranded wire type working in tension to control the engine RPM so that the RPM matches the foot pedal's opening of the servo valve to the HST. My suspicion is that as the cable began to fail it was stretching just enough to lose the registration between RPM and HST valving. I wonder if that is what is happening with yours? It's hard to tell, though. If so, it's no biggie.

It could be hard to tell if a failure is happening because mine failed inside the rubber bellows that protects the wire cable from dirt. You might try laying under the tractor's right side, double-jointing your wrist, and then try to feel of the wire inside the protective bellows - see if it feels intact...., but I'm not sure you can tell anything that way. As I recall it's not possible to retract the rubber and actually see the wire. It's not the kind of thing anyone would normally check and not made for easy access.

From an engineering standpoint, Kubota did do a good job routing that wire, protecting it from dirt with a rubber bellows, pre-lubing, and also they passed it over a proper diameter drum so that the strands wouldn't fatigue in bending. The only problem is that the wire itself turns out to be slightly undersized for the load. That became obvious when I took mine off and looked at the break under magnification. It is a pure tension separation. However, ours is an early M59 and Kubota may have redesigned that part by now.

Old style M59 owners might want to keep a new throttle cable in their spare parts kit. Not expensive, and it's about a 30 minute job with hand tools and fingers to swap it out. The good news is that if yours does break you aren't absolutely stranded. It's awkward, but you can use the hand throttle to finish up and get home.
luck, rScotty