JD 650 3PH Slowly Lowers: Tractor Engine Repair Rebuild  -- Tractor Maintenance Discussion Forum and Review JD 650 3PH Slowly Lowers: Tractor Engine Repair Rebuild -- Tractor Maintenance Discussion Forum

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Discussion Boards > Active Subjects > Messages as Posted > Tractor Engine Repair Rebuild Forum

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 08-20-2002, 21:50 Post: 41443
Lallen0810



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 JD 650 3PH Slowly Lowers

I have a JD 650 tractor w 800 hrs. Recently, after adding a mower to the back, the 3ph will not keep the set mowing height. It slowly leaks down until the mower is dragging the ground. Have replaced all the seals on the Rockshaft valve. What's next, replacing the rockshaft valve? Many $$.
Open for suggestions.






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 08-21-2002, 08:47 Post: 41461
TomG

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 JD 650 3PH Slowly Lowers

If it's a position control 3ph, the easiest fix is that the tension adjust on the quadrant lever is loose and the lever is vibrating down. Draft control is another story.

If the 3ph lifts normally, most type leaks in the control valve will cause the 3ph to cycle up and down when the tractor is running. I guess there are a few exceptions and some adjustment possibilities, but I hope the problem is the easy fix.






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 08-21-2002, 21:14 Post: 41492
Lallen0810



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 JD 650 3PH Slowly Lowers

Thanks for the information and suggestion. But, nope, don't think that is it. Have tightened everything up. Have closed the stop valve and the mower will NOT go down. Proves that the piston is not leaking. I believe that I know what the problem is, I would just like someone to confirm what is going on, before I shell out the dollars.






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 08-22-2002, 08:21 Post: 41508
TomG

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 JD 650 3PH Slowly Lowers

I wonder what you think it is? I fancy myself as more learned (or at least studied) than experienced, so I might learn something here.

Anyway, my take on it might be helpful. If it lifts normally, then the lift spool valve is actuated and nothing is grossly wrong with hydraulics such as the unload, poppet and relief valves. If these valves, and the check valve as well, don't seal then the 3ph will leak down when the 3ph is in neutral. Normally, the lift spool will again be actuated when the lift goes below its position setting and the 3ph will be pumped back to its position setting. I'd probably start by wondering why that isn't happening.

I think that on my Ford, I can see the ends of the poppet and lift spools with some minor disassembly. Starting from neutral mode, the heads of the lift spool and poppet valve both should have clearance. If the 3ph leaked down, the lift spool clearance should disappear and the poppet valve clearance should remain.






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 08-22-2002, 22:06 Post: 41533
Lallen0810



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 JD 650 3PH Slowly Lowers

I suspect that the spool valve is not moving like it should. It is my understanding that when you move the rockshaft control lever (to lower the lift arms), that causes the spool valve to move and open the the sump side of the control valve. This directs the oil from the piston to the sump, until the feedback lever says the lift arms have moved to the position in relation to the rockshaft control lever.
My theory is that the spool valve does not move enough to cover the sump side of the control valve, so the oil is allowed to seep past the spool valve to the sump.
Not sure how I can prove this. I just know that when I replaced the seals, the spool valve had to be pushed out of the control valve. More that a "snug" fit.






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 08-23-2002, 08:05 Post: 41545
TomG

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 JD 650 3PH Slowly Lowers

It's a possible theory. I'm not sure if the lift (or control) spool sees cylinder pressure when the hitch is in neutral or lower mode. I think the check valve is between then lift-cylinder and lift-spool, and the check valve is closed in neutral and lower modes.

A couple of comments about general 3ph position control workings may help the theorizing. As far as I know, the lift spool sees the sump when in neutral and lower mode. The unload valve is open and the lift spool is closed. Oil circulates from the pump through the unload valve and back to the sump. The poppet and check valves are closed, which isolates the cylinder from the sump.

On lift, the lift spool opens, the unload valve closes, which blocks return oil to the sump and allows pressure to develop. The check valve opens and the cylinder extends. On lower, the lift spool and check valve are closed while the while the poppet valve and unload valves are open. The path of return oil from the cylinder goes through the poppet valve to the sump rather than through the lift spool. During lowering, flow from the pump follows the same path as in neutral mode.

In a normally working 3ph, the lift spool has to actuate for the lift to raise and the poppet must actuate for it to lower. One or the other has to be open, or they are both closed in neutral mode. Which spools are open or closed depends on a combination of quadrant lever position and lower-link arm height. It's easier for a draft control hitch to flop on the ground than a postion control hitch, but raft control usually flops fast if it's going to.

Lord, I hope this is at least a little helpful. It might be easier to look at oil flow diagrams in a manual.






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 08-24-2002, 07:40 Post: 41575
Lallen0810



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 JD 650 3PH Slowly Lowers

Anything is appreciated at this point. I visited w the local JD dealer service rep. And his take was that the spool valve should move to either raise or lower the arms. And then once the lifting arms are positioned, the valve goes into a "Neutral" position where the return passage from the lift piston is covered and the oil cannot return to the sump. This keeps the piston in place.
So, I am back to the theory that the valve is not moving to the proper position to cover the hole that lets the oil return to the sump.
Going to take the control valve apart again today and see if there is a burr or something on the spool valve that would keep it from moving properly.

Thanks for the information.






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 08-25-2002, 06:14 Post: 41602
TomG

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 JD 650 3PH Slowly Lowers

I'm not sure it's been said one way or the other if the 3ph has draft, position or other type control. Some of the comments sound more like draft control.

For draft control on my Ford, the control valve is centered in neutral and does move one way for lift and the other for lower. In neutral, I believe that the control valve spool rather than the check valve does block the return oil from the cylinder. I suppose I took position control as an illustration because I don't have to look in the manual much to write about it.

Irrespective of the type of hitch, I hope all this helped the thought process and the problem gets solved. If the valve is scored, I have heard that some types can be repaired by electroplating and re-lapping. I'm not sure how the results compare in either cost or durability to new parts.






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 08-25-2002, 21:00 Post: 41630
Lallen0810



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 JD 650 3PH Slowly Lowers

Well, embarrasingly, it seems the problem was solved very easily.
On a whim, while contemplating how to tackle the problem of taking the control valve apart and taking the spool valve out, I checked the oil level in the hydralic system. It was half way between the full mark and the end of the dipstick. Fully within the normal range. But, I went ahead and filled it to the full mark.
Lo and behold, everything begin to work. I could park it with the implement raised and it would hold it up even when I turned the tractor off.
So, before I did anything, I hooked the mower up and took it for a spin. Worked like a charm. Just like it was supposed to. So, I have backed off on fixing anything else until I have a littl confab w the JD Dealer and walk through the repair manual trouble shooting section.
Sometimes the obvious is what you need to check first.

Thanks for the info and support.






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Discussion Boards > Active Subjects > Messages as Posted > Tractor Engine Repair Rebuild Forum

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