Weird FEL behavior

[83Ttype]

I have been using a friends JD4300 tractor for the summer and the FEL has some weird behavior. I did a full fluid / filter change and it made no difference. It has a single control lever with front/back to raise/lower and side/side control the bucket.

When lifting the loader:

Full pull back on control lever, everything seems normal until you quickly stop lifting. The bucket then drops about 6 inches before stopping.

If you only pull back slightly on the control lever (0 - 12%), the bucket will slowly drop.
A little more pull (20-40%) and the bucket will stabilize, but not lower or lift.
More than 40% back on the lever and things seem to lift normally until you go back to the neutral position where it drops a few inches.

I switched the quick disconnects on the control valve with the bucket controls (bucket seems totally normal) and the loader behavior was exactly the same even though it was using a different control valve.

So I am totally stumped as to what to do to fix this. I do know that the tractor sat for over a year without being used and I don't know if it was doing this before I borrowed it. I want to have it fixed before returning to him but not sure what to look for.

 

[RNeumann]

That sounds like pretty classic cylinder seal failure. One or both of the lift cylinders will need some love.

 

[RNeumann]

Bypass testing a hydraulic cylinder - YouTube

 

[wdchyd]

Yep, RNeumann got it right, piston seal failure

 

[Leejohn]

Could maybe be the load check valve spring or seat bad.

 

[ovrszd]

I'll take the other side. I suggest valve failure.

 

[ovrszd]

The cylinders aren't very smart. They hold or they don't. If cylinder failure was causing the drift it might continue all the way to the ground. Only controlled by valve sensitivity. If valve is failing, nothing will control drift.

 

[83Ttype]

Could maybe be the load check valve spring or seat bad.

Would that still happen when I switched the hoses from the bucket valve? The behavior was exactly the same and would have been using the second spool. Cylinder seals are a lot cheaper / easier fix.

 

[83Ttype]

The loader did seem to hold the load fairly well once it had settled that 6 inches after lifting. I was moving dirt so it really only had to hold for a minute or so, but it wasn't constantly lowering itself down like I assumed it would if it was leaking past seals. Since the 2 cylinders are "tied" together in the hydraulic system, I figured it would slowly drop even if only 1 of the cylinder was leaking. It is a JD 430 loader and rebuild kits run about $50 per cylinder (parts AH157965 Seal Kit & H169904 Ring).

 

[83Ttype]

I'll do a test tomorrow by lifting up a load and timing how long it takes to drop it down, or a least get a rate at the speed it is dropping.

 

[ovrszd]

Lets think about this. If there are no external leaks and the valve is not leaking, it is impossible for the cylinders to compress regardless of the condition of their seals. Where would the displaced fluid go?

 

[oldnslo]

Lets think about this. If there are no external leaks and the valve is not leaking, it is impossible for the cylinders to compress regardless of the condition of their seals. Where would the displaced fluid go?

You are correct that a cylinder will not retract with bad piston seals but on a loader a 1/4 inch of cylinder movement could be 6 inches of bucket movement. 1/4 inch of cylinder movement would just be the pressure equalizing in the cylinder

Like others I suspect lift cylinder leaking internally

 

[Threepoint]

Lets think about this. If there are no external leaks and the valve is not leaking, it is impossible for the cylinders to compress regardless of the condition of their seals. Where would the displaced fluid go?

I agree with Richard. For the cylinder rods to move further into the boom lift cylinders during compression they must displace an equivalent volume of oil. Assuming no leaks at the gland seals, fittings, hoses and valve, the oil has no place to go. So even if the piston seals are badly worn, or even missing altogether, the cylinders are locked and can't compress further.

The cylinders can extend in those conditions, but extension doesn't seem consistent with the FEL behavior 83Ttype describes. :scratchchin:

 

[Threepoint]

You are correct that a cylinder will not retract with bad piston seals but on a loader a 1/4 inch of cylinder movement could be 6 inches of bucket movement. 1/4 inch of cylinder movement would just be the pressure equalizing in the cylinder

Like others I suspect lift cylinder leaking internally

Interesting point about pressure equalization! But I'm not visualizing how that could account for even 1/4" of the rod entering the cylinder if the cylinder is completely full of oil. Now, if there's even a small amount of air in the cylinder, I could see it. But the air would have to have entered somehow, so that would mean an external leak. Perhaps, e.g., past the gland seals?

 

[Leejohn]

Look at the picture and you can see the load check valves. Some control valves have them on the bottom. Easy to check and fix, just make sure all pressure is off of system. Read how they work in that picture.

 

[ovrszd]

Leejohn, very good file. I saved that one. Makes sense even to a monkey like me!!

 

[83Ttype]

You are correct that a cylinder will not retract with bad piston seals but on a loader a 1/4 inch of cylinder movement could be 6 inches of bucket movement. 1/4 inch of cylinder movement would just be the pressure equalizing in the cylinder

Like others I suspect lift cylinder leaking internally

This makes the most sense to me. I couldn't mentally picture how it would be leaking, but still hold a load. The 1 good cylinder is doing all of the work (and holding) and the pressure bypasses in the bad cylinder and equalizes in both. I can at least kind of picture it now. There are no external leaks at all on the tractor, just FYI.

I did a quick test to see how long the loader holds up with no load on it. It drops 4 inches every 10 minutes, 2 ft per hour.

I'll pull one of the cylinders and do some testing. Rebuild kits are only about $50 / side so even just rebuilding both of them wouldn't cost a fortune.

What about disconnecting one of the cylinders at a time and capping the lines so the other cylinder does all the work. The "bad" cylinder should show the symptoms and the "good" one should be completely normal.

Thoughts?

 

[oldnslo]

Look at the picture and you can see the load check valves. Some control valves have them on the bottom. Easy to check and fix, just make sure all pressure is off of system. Read how they work in that picture.

Lee,
Valid concern when the spool is shifted but not valid when the spool is in the center or home position since the spool now blocks flow to load checks.

 

[oldnslo]

This makes the most sense to me. I couldn't mentally picture how it would be leaking, but still hold a load. The 1 good cylinder is doing all of the work (and holding) and the pressure bypasses in the bad cylinder and equalizes in both. I can at least kind of picture it now. There are no external leaks at all on the tractor, just FYI.

I did a quick test to see how long the loader holds up with no load on it. It drops 4 inches every 10 minutes, 2 ft per hour.

I'll pull one of the cylinders and do some testing. Rebuild kits are only about $50 / side so even just rebuilding both of them wouldn't cost a fortune.

What about disconnecting one of the cylinders at a time and capping the lines so the other cylinder does all the work. The "bad" cylinder should show the symptoms and the "good" one should be completely normal.

Thoughts?

since the cylinders are mechanically tied together you would have to either remove one of the clevis pins & cap the lines or figure out some way of catching the oil being expelled from the disconnected cylinder since you can not just plug those ports.

 

[ovrszd]

since the cylinders are mechanically tied together you would have to either remove one of the clevis pins & cap the lines or figure out some way of catching the oil being expelled from the disconnected cylinder since you can not just plug those ports.

The rate of drop he described cannot be pressure equalizing in the cylinders. The valve has to be leaking internally if there are no external leaks, which he said there are not.