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

 

[oldnslo]

My first thought is system pressure is to high. Do you have a gauge to check the pressure?

 

[California]

If no answers here, repeat the question over in in the Yanmar (classic) sub-forum. The 750 was built by Yanmar and is probably identical to Yanmar of the same era in the hydraulic system.

I've seen similar questions, and excellent answers, over in that forum. We have to be pretty much self-sufficient because Yanmar left the US market and shut down their dealers, leaving the 1980's US Yanmars as orphans. That sub-forum is where all the expertise resides today, also covering the grey-market Yanmars which are identical.

Dealers of New Yanmar, from when they returned to the US market, aren't much help for the old classics.

 

[California]

As for breaking the pump drive key, that has to be a case of excessive pressure, maybe continual rather than momentary. Is the knob under the front of the seat wide open? Could the lift lever beside the seat be stuck beyond Hold and into Lift position? Any squeal from the overload pressure bypass?

The usual suspect for breaking the key is a hydraulic line to an implement disconnected, leaving nowhere for the pressure to go. Possible here?

 

[750JDLA421]

My first thought is system pressure is to high. Do you have a gauge to check the pressure?

No sir what type of gauge would I need and where would I test it at ? Also thanks for your response.

 

[750JDLA421]

As for breaking the pump drive key, that has to be a case of excessive pressure, maybe continual rather than momentary. Is the knob under the front of the seat wide open? Could the lift lever beside the seat be stuck beyond Hold and into Lift position? Any squeal from the overload pressure bypass?

The usual suspect for breaking the key is a hydraulic line to an implement disconnected, leaving nowhere for the pressure to go. Possible here?

I don’t recall any squealing and all lines are connected I do think initially the knob under the seat was either all the way open or closed I ran it out about half way and the hitch shot up. When I started the tractor after changing key I had the handle in the down position once it raised it was left about 3/4 of the way up. Now that o think of I did hear the tractor idle down and after that the lift didn’t work anymore. Could that be when the shaft key broke? I do appreciate your help and response.

 

[California]

I haven't experienced this but I have seen it described, and solved, over in the Yanmar (classic) sub-forum. It seems to occur after some change to the hydraulic plumbing with unexpected consequences.

The most obvious case is removing an implement or something then leaving a line going to nowhere, plugged, that previously had a return through the implement's return hose.

The general Hydraulic sub-forum here is another collection of helpful people experienced with issues like this. But my experience was I had to sort out what was helpful from the many replies, when I asked about replacing a loader control. 'That could never work' was a common reply describing the way my Yanmar Operator Manual said to plumb the dealer-installed loader control that had worked fine for near 40 years before I replaced it, replaced simply because it dribbled. YMMV.

I hope you find the solution.