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Discussion Starter #1
Hello,

I have a F935 front deck mower that Ive been having trouble with. The implements quit working - the power steering and front deck hydraulics stop operating, It has the Sundstrand 15 series U type hydrostatic pump, and I have been troubleshooting it and did a lot of work to it and still can't get the implements to work. What I have done is,,,

Changed filter - no improvement

Checked relief valves both charge and implement relief, also check the charge check valves, all looked good and were clean/clear

I removed and checked the charge pump - bearing was failing so I replaced the unit with a nice used one. After that I tested good charge pressure on the test port and it gets almost 300 psi at full speed. I bleed the hydraulics extensively, and still no improvements to the implements.

My f935 manual says to test the implement pressure on the pump, but is not clear where to do that. The pump has 4 pipe plugs, one is the charge pressure, the others, I assumed the one next to it was the implement pressure - I only get 80 psi testing that port at idle and full speed, and while operating implements, I assumed the pump motors inside has failed. So I tore the pump apart and replaced the pump motors valve plates, blocks and pistons with a used set. I reassembled everything only to have the exact same problem.

I'd like to confirm that I'm testing implement pressure at the correct port, I found the sundstrand manual, and it says the port to test is on the side of the pump, but I'd have to take a case bolt out to be able to access it. Is that what I need to do ?

I also am wondering how I can verify that the Relief valves are working and not leaking ? Also how to make sure the charge check valves are good and don't need to be rebuilt ?

I would like ideas how to troubleshoot my mower.. because I'm just about out of ideas.



Thanks
 

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Discussion Starter #2
Here’s a pic of the hydrostatic transmission torn apart.
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here’s the new pump motors and valve plates cleaned up and ready to go in.
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I know zip about your mower. What I do know is that the Type U hydro is a transmission for propelling the tractor. It has no provision for powering an implement at the same time beyond lifting and lowering into position.

There is another, and separate, pump involved if the implement is hydraulically driven.
 

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Tudor, on this mower the sundstrand u type 15 series hydrostatic pump moves the mower and the same pump drives the power steering and deck lift arms. I have attached a picture of the hydraulic system on my mower.. it has one hydrostatic pump for everything..

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Jeep, You DO HAVE a seperate pump for your lift & steering. The drive shaft from the engine connects near a flange held on by 2 bolts. Inside this flange is a gerotor pump which produces/supplies oil pressure to your implements and also a flow of oil for lubricating your internal pump & motor that move your mower.

I can't help with too many of your questions, but the 80 psi you found when testing is not good! I know off no area in that system that makes/uses 80 psi. After looking through the Sundstrand manual, I find pics showing a test gage installed on the left side, when facing the input shaft, and reading it's a 1/8" NPT port. You should also have a hex nut in that area, that's your system relief valve and no need to mess with that. On the top, front, center of the hydro is another hex nut. THIS is your implement relief valve and may be an issue. You can remove that inspect for damaged seat and/or broken spring. If everything appears OK, put it back together. I'm suspecting your charge pump is worn out.

You mentioned check valves. If you're referring to the 2 valves on top of your hydro with pins, maybe 1/8" dia., sticking up, they're fine. If your tractor moves, valves are fine. If it doesn't move (which is not the case) your valves could be an issue.

One last comment. I'm pretty sharp on these units and there're others too, but I know of NO ONE more knowledgeable on the hydros than Tudor! If he should reply and it's not in agreement with someone else, go with Tudor's reply.

And Tudor, if you're following, this a a plain ol' Sundstrand 15 U. Bob
 

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Don't know about this specific system, but many hydraulic systems are "open center". This means the pump is always pumping fluid at the same flow rate. That is what pumps do. They generate flow. Pressure comes from resistance to flow, like when you close a valve. In an open center system, when the valve is "off" it still let's fluid flow from the pressure port to the tank return. When in this state, the pressure in the line will be very low. Now when you open the valve, the fluid gets directed to the output port and the tank port is closed. The pump continues to try and move the same amount of fluid. Since fluids don't compress, it will cause the cylinder (or motor) to move. Once the cylinder gets to the end of its stroke, there is no more room for more fluid. But the pump keeps pumping more in. This is what builds pressure. The relief valve will then let go to protect the system and allow the excess fluid to return to the tank. The pump and engine driving it have to work much harder when the pressure is high.

There are also closed center systems, but those require a variable displacement pump. Pretty sure gearator pumps are not variable displacement. If you have one of those, you have an open center system. I would expect at least 800-1000 psi on a tractor that size. My CUT is spec'ed at around 2000, bigger ones go to 3000, really big ones 5000 or more.
 

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The separate pump to which Bob (rwmeyer) is referring is the charge pump for the hydro. The charging circuit is the ONLY circuit under constant pressure, and it can be anywhere from 50-170 psi. That is where the 80 psi at the test port is coming from.

The same pump also supplies the implement lift and power steering hydraulics via the outflow from the charge relief valve. That relief valve that doesn't dump directly to tank, but to the implement lift and P/S circuits. The pump has a displacement of 0.3 cu-in and is theoretically capable of flowing about 4.5 gpm less what is required to maintain the hydro (its primary task). The implement relief valve is normally set at about 800-950 psi (maximum rated 1000 psi). That pressure combined with the approximately 3 gpm flow available after maintaining the hydro calculates out to about 2 hp. That is not enough to feed the mower.

There is no provision to take hydraulic power off of the hydro pump itself since it would be worthless to operate a continuous duty attachment such as a mower deck while changing speeds and direction of travel for the tractor. Any attachment connected would change speed and direction in lock step with the tractor.

Since there is no other pump, you're missing a drive shaft that connects to the splined shaft on the front of the hydro via a clutching mechanism. In studying the pic that I found of your tractor, there appears to be a drive shaft coming from under the footrest to the mower.

I suspect that the auxiliary lines noted in the pictorial at the front of the tractor are for the chute control for a snow blower attachment since they originate at the same valve set as the implement lift hydraulics.
 

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Tudor, if the deck lift hydraulics and the power steering has stopped working, but forward/reverse is still working, were is the most likely of the hydraulic loss?
 

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Charge pump on very front of hydro or implement relief valve, also on front of hydro between free-wheeling valves.
 

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Discussion Starter #10
Thanks for the replies, I appreciate the inputs. I'm not a expert on hydraulics and this is the first time I've really worked on a hydraulic system. So, all your comments help me understand things more.

Tudor, there is another driveshaft coming from the engine, and that one goes to the PTO, and goes to the mower deck gearbox that turns the blades. That part is working fine, the mower blades will turn on and mow the grass. I just can't lift the mower deck, which is lifted by hydraulic rams, and those are powered by the sundstrand correct ?

rwmeyer, I replaced my charge pump, and I get 150 psi at idle and almost 300 psi at full engine speed - so that is good correct ? I am thinking maybe my implement relief valve has worn out and just isn't working - is that possible ? The spring is not broken, and the valve visually looks fine. Also, what does "also of hydro between free-wheeling valves" mean, can you explain more what that is ?

Heres a picture of me testing the sundstrand at the implement pressure, Is this the correct test port ? I have a 5,000 psi gauge, but the test port will not move it, so I used the 300 psi gauge. The Sundstrand manual shows to test on the left side port, but I would have to remove a bolt that bolts the caee halves together. I can snap another picture of this...

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Tudor, if the deck lift hydraulics and the power steering has stopped working, but forward/reverse is still working, were is the most likely of the hydraulic loss?
Implement relief valve.

If the charge pump is lost, the hydro goes down for lack of fluid.

The free wheeling valves are a defeat setting for the charge relief check valves and have nothing to do with the auxiliary hydraulic circuits unless the engine is running. If the engine is running, why would the freewheel valves be activated? The charge pressure should be enough to reset them if they're stuck so that the springs alone can't pop them up.

Jeepsarebuilt, implement lift pressures are normally tested with the cylinder dead headed, as in at the end of stroke while the control valve is activated.

With an open circuit hydraulic system, the fluid has a free flow path from the pump to the tank when the valves are centered. As previously noted, the charge circuit is a special case where the flow from the charge relief valve goes to the auxiliary (implement lift) circuits where it has a free flow path back to tank. You are checking the pressure on the pump side of the charge relief valve.

The same port can be used to check the implement relief pressure as described above. With the control valve actuated, there is no free flow back to tank path available until after the implement relief valve, but the indicated pressure goes all the way back to the pump, including the charge relief circuit.

Check your implement control valve body for an integral relief valve that may be a contributor to your problem. From the pictorial that you posted with the work circuit ports on the bottom of the valve body, any integral relief valve will be on the tractor left of the valve body beside the left hand spool. JD has a tendency to not use the implement relief valve provision available on the Sundstrand and instead uses one in the valve set. The same max pressure rating of 1000 psi applies since that is the rating for the pump.

You may find it beneficial to come back and reread some of my posts that seem indecipherable at first read as I tend to pack a lot of technical information in some of them that takes time to digest. I've gotten bogged down myself on a couple of them over the years and had to do extensive re-wrights to simplify things a bit, especially for those new to the topic. Don't be afraid to ask questions when you don't understand. I'm here to help, not confuse.
 

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Jeep, "also on front of hydro between free-wheeling valves." is the location of the implement relief valve. Bob
 

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Discussion Starter #13
I pulled the Relief valves out of the sundstrand today, and here's a couple pics of them, The implement relief valve has a shorter but stiffer spring - dose this look correct ? Also the implement relief valve has a wear ring that looks like it wore a shiny spot from where it seats in the bore.

I also decided to take apart my steering valve, I discovered broken springs inside - not sure if that's the problem all along or not... but I guess I will fix this part and then try again.

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