JD 650G blade rides over obstacles

Hello all

I have a small problem with my little dozer that I just can't seem to figure out.

What is going on is that when I lower the blade and begin to push a small about of dirt/brush, the blade will lift and ride over any rock or root until I'm passed the obstacle then drop back down. It acts similar to what would happen in float mode. If I have a fair amount of dirt in front of the blade it seems to hold well. The blade will also stay down just fine during a big push if I apply any down pressure at all. It really only happens if I set the blade at a desired height such as grading, and a rock or root happens to be in the way.

It almost acts like there is a small amount of air in the lift cylinders that act like a shock absorber, but only in one direction. The blade will never suck down further than where I put it, only push up.

The machine itself is very tight, no leaks. I have completely rebuilt the control valve with new factory seals, orings, and backup rings. This didn't seem to help. I don't see anything else in the circuit that might need looking at. My next guess is to look at the lift cylinders themselves? Thanks.


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Even if the lift cylinders were shot they can't bypass in that direction b/c the total internal volume shrinks as the cylinder retracts due to the cylinder rod taking up more volume. Loading the blade up creates down force that would keep the blade from jumping up under those conditions. It's kinda long shot, but maybe the hose is ballooning and letting the lift cylinders come up some. I would think any air would get worked out cycling the blade, but maybe there is a quirk to these machines.

Thanks for the reply. The ballooning is an interesting idea, but it would have to get pretty big in order to let the blade rise as much as it does. Sometimes it'll rise enough to jump over a stump.

I should have also mentioned in my earlier post that if I put the blade down and raise the front of the dozer, it'll hold there for a long time before settling...



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It's probably a different issue then. You are getting an accumulator effect from something. I can't come up with anything better than air in the cylinders. Try slowly cycling the blade all the way up 10-15 times and see if that improves things.

Yes, exactly like how an accumulator acts or how our old cable blade dozers rode up and over rocks and the dropped back down.


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IMO there is some issue with the JD G only cause I have one (450 model)....if I push the blade control down the blade goes down and stops at the grade but if I continue to hold the control in the same position, then the blade will go down further and the tractor front lifts in the air if so held that long...it is as though there is a difference in gravity down and then power down...
Aside from that is there possibly a 'load check' in the control that allows the blade to hop up due to "bypassing"?

This was bugging me, so i went digging in the parts catalog. Anybody know what the "Lift Section Positioner" does?



http://puu.sh/qkxRm/4fee6c1459.png

Good morning hvy.

I really appreciate you continuing to help me figure out what is going on.

The assembly you have shown is the mechanism that is used to hold the blade in the float position.

I am beginning to think that the valve body section on the lift circuit could be worn or maybe have a small internal crack. I think my next move would be to swap this section with the ripper section which has seen little use and should be in good shape.
If that doesn't work, I might just have to live with this feature.


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I have a 1988 450G and my blade does the same thing.

I know when I drop my blade it takes about a second or two to be able to raise the tractor front end up. On mine this is at least partially due to a worn trunion ball.

Pack the cylinders

Pay attention to the bores with a bore Gage

http://www.starrett.com/metrology/product-detail/452B

Could also be a loose nut

It looked like it was for float, but i thought it was weird it had it's own section.

Price said:

I have a 1988 450G and my blade does the same thing.

I know when I drop my blade it takes about a second or two to be able to raise the tractor front end up. On mine this is at least partially due to a worn trunion ball.

Click to expand...

That is what mine does to.


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Heavey Metal said:

Pack the cylinders

Pay attention to the bores with a bore Gage

http://www.starrett.com/metrology/product-detail/452B

Could also be a loose nut

Click to expand...

Repacking the cylinders is on my list for sure.
The loose nut is something I hadn't thought about, thanks for the tip.



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I am having the same problem with my 450G. My trunnion was also worn so I upgraded it to the new style bushing hoping it would cure my problem and my blade will still ride over obstacles. Very aggravating.

If I hold my blade lever forward to lower the blade, once it makes contact with the ground it will take 3-5 seconds before it begins to lift the front of the dozer off the ground. I also have not seen any external leaks.

Hmm, so all of us with a G model that has posted to this thread have the same issue. Surely when these machines were new they didn't act this way? I never had the chance to run one back in the day.


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Here is a video of an H model that does the same thing i am refering to when you place your blade on the ground it takes a few seconds to pickup the front end. I suspect it has the same problem we have.

https://youtu.be/JsvALrVgNNM

Mine does that too, but it got a lot better after I rebuilt the control valve. The delay on mine is barely noticeable now, only about 1-2 seconds at full throttle. Once I raise the front, it'll stay there for a long time.


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I have a little more info to add in for our problem.

If I lift the blade an inch or two off the ground and go back down with it, the front end will pick up almost immediately.

If I pick it up all the way and then go to the ground, it takes 7 seconds before pressure is applied to lift the front end.

With mine, if I drop the blade, let go of the handle and push down again, the front will lift instantly. If I maintain steady pressure, I get the 1-2 second delay. That doesn't bother me so much, it's the "riding over obstacles" that's driving me crazy!



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moriboy said:

With mine, if I drop the blade, let go of the handle and push down again, the front will lift instantly. If I maintain steady pressure, I get the 1-2 second delay. That doesn't bother me so much, it's the "riding over obstacles" that's driving me crazy!

Click to expand...

Been following this for fun. Does this machine have a closed or open center hydraulic system? Reason I ask ... Could the blade be dropping by gravity drawing a negative pressure in the system ? Then after releasing the control, when you ask for another response the pump has built control pressure and reacts instantly.