hydrostatic issue

mikehaugen · Jun 28, 2012

I know this is a tractor forum but I have a question about a bunton mower I recently purchased. It is a older (1998-ish) 3 wheel mower. It needed an engine rebuild which I just did. When I got it started it had a leaking hose, so I replaced it, then found another leaking hose. I searched forever and talked to multiple dealers and could not get a straight answer on what kind of hydraulic oil to use. Finally after getting hold of the manufacturer (who was actually quite helpful) he sent me an impossible to find owners manual and told me to use 10w40 motor oil. After all of this it would move but had no power. (I thought the hoses would have fixed this). I finally found the bypass lever that I had been looking for, the linkage and sticker was missing but found the lever on the pump and was able to get it out of bypass mode. Now it will spin the tires.

So my issue is this... although it has plenty of torque now as you are driving it surges. It will accelerate and them seem like it is coasting for a few feet, then accelerate then coast.... It makes a sound like the pump is cavitating, but there is plenty of oil in it. I don't want to drive it too much until I figure this out because I don't want to cause damage. I don't think there is any more air in the oil, in fact the bleeding instructions are just to let the engine run for about 5 mins at half throttle, doesn't even instruct to jack it up and let the wheels spin though I did that anyway. I did not change the filter but think I might, though I don't see that causing this issue. This mower only has 800 hours, and I know that they are made to run for 2000 to 2500 on the hydros. Should I maybe try a different oil? Any help would be appreciated.

mikehaugen · Jun 28, 2012

I just thought that I would add that this system is a single pump with 2 hydraulic motors plumbed in parallel to simulate a differential- not a true "0-turn"- it has a single rear wheel that does the steering.

oldnslo · Jun 28, 2012

Does it seem to surge when climbing hills or just when traveling on level ground? Depending on the style of HST you have some of them can take some time purge all of the air out of them.

Hydrogear recommends raising the drive wheels off the ground and running in forward and reverse under no load for at least five minutes forward and reverse. NOTE: May take longer on some models don't know why but I have had a couple that where real troublesome in getting primed once the fluid once changed and they where bled dry.

Eaton ball type HST's I am not sure of a purging procedure but would suspect similar to Hydrogear.

mikehaugen · Jun 28, 2012

no, it surges on flat land. Also maybe I should add that the fluid seems to get hot rather quickly. Granted it was fairly warm here yesterday when I tried it (mid 80's), the tank which is about 2-3 gallons was quite warm after about 30 minutes of driving around as well as the line going into the cooler. It wasn't too hot to touch but you wouldn't want to hold onto it for more than about 5 seconds. There is a small cooler about 4 inches above the engine fan intake that is about 4 inches by about 10 inches and about an inch thick. Not sure if that is normal, this is my first hydro anything though I have used plenty of hydro mowers I never really paid attention.

oldnslo · Jun 28, 2012

Getting hot usually indicates something is by-passing oil. You mentioned the by-pass on your first post. Are you 100% sure it is fully closed. These can rust or get gummed up and not function properly which would allow some oil to bypass and create heat.

J_J · Jun 28, 2012

My Dixie Choppers use 10W-40 motor oil , or 10W-40 Mobile-One as the hyd fluid.

Can you see bubbles in your hyd tank. Check the suction hoses and re-tighten.

Are the bypass levers fully closed, as on some of the hydro's, the bypass is keeping the relief valves open so the fluid can recirculate for towing, and you can't tow very far as the fluid will heat up.

mikehaugen · Jun 29, 2012

I can't say for 100% that the lever is fully closed, it is as far as I could get it to go. Maybe I will try to clean it up and see if it helps. The lever did feel a little sloppy, so one of my initial thoughts on this is that maybe it was bouncing, but it does it too consistently on fairly smooth ground that I don't think this is the case anymore. I will see if I can get it to close anymore and also try to purge it a little more.

Also that is an interesting thought on the suction lines. I will check those as well.

Thanks guys for the input, hopefully this will solve it.

CJONE · Jun 29, 2012

Like JJ said pay attention to your intake line, either it is pulling air or collapsing. If it is a hydrostat do NOT run it like that for very long. CJ

mikehaugen · Jun 30, 2012

Okay, so I checked all connections and they were tight. The hoses seem in pretty good shape. I verified that the lever was all of the way closed and jacked up the wheels and ran them at about half throttle 10 mins each way each wheel then jacked them both up and ran them both at the same time 10 mins each way. Absolutely no change. Although at half throttle there is a little surge in the engine that I noticed that I did not notice at full throttle (probably harder to tell). Sound like the governor is hunting, I will try some sea foam and see if maybe it is carb related and maybe there is no hydraulic issue at all. It makes sense since it has been sitting for a while and I didn't touch the carb when I did the other engine work. I was actually surprised it ran and idled and nicely as it did. If the sea foam doesn't work I will pull the carb apart and clean it manually. If that doesn't work then I will have to start looking at the hydraulics again. It is still possible that the pump is actually causing the motor to surge I think. I may pull the belt back off and see if the motor still surges. Thanks again for all of the replies and I will keep you posted on progress in case anyone is interested.

mikehaugen · Jul 3, 2012

now I have got the motor running smoother and after driving around a little more, I am convinced it is a hydro issue.

The pump pulls from the tank (and the filter is in this line) then the return from the pump goes through a cooler and returns to the tank. The 2 motors for the wheels are piped in parallel (each with two larger lines from the pump) and now I am trying to figure out what purpose the 2 smaller lines from the motors to the tank serve. I have a parts drawing for the hydraulic system, but not really a schematic. They are referring to these lines as "motor drain-L" and "motor drain-R". Anybody know what these do? I am learning quickly how different a hydrostatic system is than just standard hydraulic pump/ motor system. I have a new filter on the way, I will try that but am doubtful.