My L3901 seems WAY under-powered to me

[SDT]

Drive ratio is shaft rpm in vs shaft rpm out. What is in the middle makes no difference. Until you understand how the pump displacement changes while the motor displacement remains constant your confusion will persist.
Your assumption that the hydrostatic pump operates at a fixed displacement is incorrect.

Agreed.

SDT

 

[SDT]

I see what you mean! Looked at a diagram online. I thought it was controlling fluid flow using a bypass circuit. Wrong. Thanks for the education on that point.

The swashplate mechanism is ingenious. However, I still maintain that decreasing pedal pressure, and thus fluid flow, reduces power to the wheels. That is definitely the case with both of my HST tractors.

I have one tractor that has Stall-Guard and it will compensate by revving the engine up when needed. I usually don't operate with it turned on. But at a constant RPM, I definitely have less power at the wheels with less pedal pressure.

Stall guard does not compensate for increased load by increasing engine RPM. Rather Stall compensates for increased load by reducing speed (increasing effective gear ratio), same as backing off on the HST pedal.

Having both stall guard and auto throttle advance on at the same time can produce some interesting results when the going gets tough, confusing the issue even more.

SDT

 

[CH4Ohio]

Stall guard does not compensate for increased load by increasing engine RPM. Rather Stall compensates for increased load by reducing speed (increasing effective gear ratio), same as backing off on the HST pedal.

Having both stall guard and auto throttle advance on at the same time can produce some interesting results when the going gets tough, confusing the issue even more.

SDT

I stand corrected. Again. You're right. It's the ATA that I don't use. Went out and looked and I have Stall Guard turned on -- I think I actually use it most of the time. There's also a feature called Auto H-DS that I don't have activated.

Agreed that having several of these on can create some confusion about what your control inputs are causing vs. the auto-compensation systems of the tractor.

Thanks.

 

[s219]

This is not how a HST works at all. The HST pump & motor are completely separate from the rest of the hydraulics. There is a fixed displacement gear pump(s) that run power steering, 3pt, loader & any other auxiliary hydraulics. The HST has its own variable displacement pump directly coupled to a fixed displacement motor. On Kubotas at least, probably most machines, the HST also has its own filters for the HST separate from the aux hydraulic filters & circuit.

Any slowing of the machine is due to the engine slowing down due to the extra load from the aux hydraulic pump going from 0 load to some load.

That is a much more thorough and correct way to explain it, thank you for correcting me. I think the point I wanted to make is that HST and hydraulics will compete for the total resources on the tractor, whether it's engine HP, available flow rate from the common reservoir, thermal control/cooling of the common hydraulic fluid, etc. There have been so many cases reported on TBN where a problem with hydraulics saps HST performance, that it's one of the first things I think people should check when the HST seems to have a loss of pep.

 

[Steppenwolfe]

Not sure what to say... Maybe you bought a bit on the small side? I have an L3301 gear drive that is fantastic to use; but does not hold a candle to the MX5400, also gear drive ...

 

[Smokeydog]

Hope the OP resolves his issue. Either by repair or understanding how best to operate.

Surprised the misconceptions how HST operates. The more gears the better when pulling, pushing and FEL work. Hard to beat almost infinite variable ratios the HST provides.

So many operator actions that can contribute to low power. Then there are mechanical problems too. I once was getting slightly more black smoke from the exhaust and low power from the B26. Mice or ground squirrels had packed the air filter box full of acorns! No nesting material just little small acorns. Hasn’t happen before or since. Check the easiest stuff first.

 

[Kubota user]

I have several hydrostatic machines and I'm fully familiar with how hydrostatic machines work. I bought an L3901 a couple year ago and haven't used it too much (because, as noted, I have other machines). However, this year, I got the tractor out and it was time to see what it would do. Which, to my surprise, isn't much. To put it in perspective, I must run the tractor in low range for ANYTHING other than basic travel across mostly level ground. Even in mid-range, if I stop on a small hill, the tractor will barely take off. I'm aware the tram peddle is not a gas pedal...it operates more like a gear box...applying a little tram will give the most torque, etc. But the main thing convincing me the tractor may have a problem is its behavior. Specifically, I can't make the engine "bog down" under tram load. For example, if I try to start off on a hill in high range, the engine does not complain even though the hydrostatic transmission is whining away and the tractor isn't moving. Ether the engine has so much power it can engage the transmission pressure reliefs with ease, or the pressure reliefs are set too low (or, the transmission has a problem).

I'm posting to get the experience of some other 3901 owners. Does this sound normal to you? When in mid or high range, can you bog down the engine when attempting to take off or transverse on a hill that's too steep? Before I take the tractor in for service, or trade it for another model, I’d like to get an opinion as to whether this experience is normal for this tractor.

If I remember correctly, there was a minor recall from a collapsing hydraulic hose that could be causing your issue. A clogged hydraulic filter can do this as well.

 

[Blackgreyhounds]

Stalling/bogging in H range on hills is totally normal on my L3901. I use M range for almost everything and rare stall the engine on hills or working a pile with the FEL.

 

[gilraine]

How many hours on the machine? Is the hydraulic filter been changed? It sounds like you may have a flow restriction

 

[MHarryE]

I have a L6060 always in stall guard and a few weeks back actually had an engine stall when accelerating a heavy load, a 20,000 lb grain trailer. When pulling a load like that up a steep hill near my farm, I have actually had the tractor come to a near complete stop with the engine still above 2,000 rpm as the hydro pump automatically destrokes before. This was a surprise to me - I finished my trip delivering the load to the mill by manually using the pedal to keep engine speed up. When I got home, I replace the water separator filter - problem resolved.