John Deere 750 hydraulic issues

[750JDLA421]

I was gifted a John Deere 750 tractor by a good friend of mine. The 3pt hitch will not go up or down so we drained all fluids replaced with new fluids cleaned all filters still would not work. Next step we took pump off and took apart found that woodruff key was sheared so we replaced with new one before putting back together we inspected all gears and pistons nothing appeared to be damaged, so we put her back together salpped it on tractor after bleeding system the 3pt started working. YAY. Only for a short time it worked for about 2 hours and back to square one we went. We figured because it wasn't a (John Deere ) part that might make a difference so i ordered a couple of keys from JD they came in we installed them, and it worked again only this time for 15 minutes. So, question is what could be causing the woodruff key to shear? I know it's designed to be the weakest link and its working all of the gears appear to be good on the back side if the impeller as well as the blades on the impeller, So I'm lost! Any and all help would be greatly appreciated.

Thanks

 

[winston1]

Sounds to me like your system might be over pressuring. With the new key installed does it sound like your engine is loading until the time the 3 point quits working? Do you have a loader? Is your stop valve open?

 

[750JDLA421]

Sounds to me like your system might be over pressuring. With the new key installed does it sound like your engine is loading until the time the 3 point quits working? Do you have a loader? Is your stop valve open?

Yes the engine did seem like it was under load when the 3pt quit it sounded like the Rpm picked up. No loader. The stop valve I’m assuming is the knob is under the seat it’s opened about midway.

 

[winston1]

If it is open midway that should be fine. Are there any quick connections on your hydraulic lines? If not, then I would next start looking at the controlling mechanism on the 3 point. Sounds like something is causing over pressure and gear pumps will keep pumping till something gives, in your case, the key.

 

[rScotty]

If it is open midway that should be fine. Are there any quick connections on your hydraulic lines? If not, then I would next start looking at the controlling mechanism on the 3 point. Sounds like something is causing over pressure and gear pumps will keep pumping till something gives, in your case, the key.

It does sound like a blockage is building up too much pressure. There should be a relief valve that opens before damage is done...but that might have only lasted for the two hours. Often they are noisy in bypass mode. And they heat up the hydraulic fluid remarkably hot. Next thing to break will be a pressure line or fitting. Then high pressure oil will cut you like a buzz saw blade. Losing a limb is possible.

Right now the first thing I would do is put a hydraulic "T" fitting in the output line from the hydraulic pump so you can mount a hardware store 5000 psi pressure gauge. Those two items will cost under $50, and will tell you that the pressure is less than 3000 psi - which is max for your tractor fittings. If more than that, get help before you get hurt. When that key shears, you are closer to disaster than you know.

If 3000 psi or less, make sure that the pressure doesn't go past that when you raise the 3pt and continue tracing the system until you come to a blockage. Probably ought to repair that relief valve before you do anything else. A shop manual will help.

Has it ever had a front end loader or power steering fitted? If so, those valves are probably where the problem is. More likely to be the loader valve than the power steering. Both also have their own relief valves, but those may not work if the plumbing has been changed.

Good luck,
rScotty

 

[bmaverick]

If it is open midway that should be fine. Are there any quick connections on your hydraulic lines? If not, then I would next start looking at the controlling mechanism on the 3 point. Sounds like something is causing over pressure and gear pumps will keep pumping till something gives, in your case, the key.

Winston, recently you posting something in another forum about the vent-hook. If it's clogged by mud-dobbers or an insect cocoon, would this aid in having the pressure raise? I'm just guessing some on this.

 

[rScotty]

A clogged vent would raise pressure by tens of psi, not thousands. Not enough to shear a woodruff key or cause an engine to labor. If you are worried about it you can open the fill port or loosen the dipstick. Both of those can also vent the air above the hydraulic sump.

 

[bmaverick]

I was gifted a John Deere 750 tractor by a good friend of mine. The 3pt hitch will not go up or down so we drained all fluids replaced with new fluids cleaned all filters still would not work.

When you say new fluids, what fluid did you use?

 

[750JDLA421]

NAPA Hydraulic Fluid, Agricultural, SAE 20W, 5 Gal. I changed it about 3 months ago but it has not been used ​

 

[winston1]

Woops, may not be the right fluid, however, that fluid should not cause over pressure. I'm sure Maverick will be along to comment on fluid.