JD 750 compact loader problem

[JoeJChin]

HI all .

I have done some searching about troubleshooting the loader hyd system , to find a specific problem I am having with my JD model 67 loader, on my 750 tractor .

I thought if I described it here , maybe someone could narrow it down for me.

I recently bought the tractor , and it had this problem when I bought it .

It seems as if the loader cylinders are leaking by , or the valve for the loader cylinders is leaking by .



The loader will not lift the front of the tractor off the ground , nor will it lift the back of the tractor off the ground (no counterweight, or loaded tires) .

IF I lower the loader , and lay the bucket flat on the ground , then try to "dump" the bucket , it will just start to lift the tractor slightly, then just push the loader arms up.

With the bucket in the "dump" position , resting on the ground , sometimes I can get the tractor to start raising , if I find just the right combo of valve position and high engine rpms.

The valve on this tractor is a two lever type .

If I run the bucket cylinders to the end of their stroke , either all the way extended or closed , I can hear , what I believe is the relief valve operating ( sounds like turning your steering wheel on your car too far and holding it there)

If I run the boom lift cylinders to their extremes you do NOT hear the relief valve much , if at all , making any noise .

I don't know if this is enough info to indicate either the valve . or cylinder leaking by , but I thought I would ask anyway .

Any info for further checks will be welcome

thanks

Joe

 

[kennyd]

Joe, the cylinders are easy to rebuild and it's quite common for a machine that age for them to be leaking by internally. Most valves can't be rebuilt since there are very precise tolerances involves.

 

[J_J]

Check out the relief pressure with a 3000 psi hyd gage and see how close to specs are the hyd pressure readings.

 

[Leejohn]

I would rebuild the cyls as kennyd said about the age of you machine.

 

[kennyd]

Here is the breakdown of the cylinders:



HYDRAULIC CYLINDER
(650 AND 750 TRACTORS)
Cart KEY PART NO. PART NAME QTY SERIAL NO. OR YEAR REMARKS
1 JD7826 FITTING 2
2 PT5249 HYDRAULIC CYLINDER BARREL 2 BOOM
PT5251 HYDRAULIC CYLINDER BARREL 2 BUCKET
3 14H863 NUT 1 3/4"
4 PT5246 PISTON 1
5 PT5247 SEAL 1
6 PT5248 ROD 2 BOOM
PT5250 ROD 2 BUCKET
7 AM118390 SEAL KIT 1 (SUB FOR PT5252)
8 PT5220 HYDRAULIC CYLINDER 2 BOOM
Cart KEY PART NO. PART NAME QTY SERIAL NO. OR YEAR REMARKS
PT5221 HYDRAULIC CYLINDER 2 BUCKET

 

[JoeJChin]

kennyd

To rebuild the cylinders I would need parts 4 , 5, and 7 ?????

Joe

 

[Leejohn]

I was waiting for kennyd to come back and help, but he must be offline. All you need is #7. You might want to wait until he gets back to you. That # 5 you might need.

 

[kennyd]

Generally the seal kit, #7 will take care of everything.

Here is a quick and easy test: use the loader for a while, then feel the cylinders. If they are extremely hot, then you can pretty much bet they are bypassing-or at least one is.

Cylinders, like windshield wipers, brakes, headlights and socks should be changed/rebuilt in pairs IMHO.

 

[JoeJChin]

I picked up a pressure gauge , just wondering where in the circuit would be the best place to check the relief pressure ?

thanks

Joe

 

[J_J]

The best place is in a tee in the IN port of the first valve in the series path.

The gage will read what any of the attachments hyd is doing.

If you only want to check each valve, just plug it into any work for that valve and work the lever to show the relief pressure.

 

[Egon]

Before you rush out and tear things apart do a little test. Lift up the bucket, shut the tractor off and check how fast the bucket drops.

Also dig up some spec's on the loader. Not all loaders will lift the front end with the bucket flat. Sometimes you need the curl function to lift the tractor.

PSV's usually have a set pressure for a reason.

Does the loader lift a full bucket?

Chances are all is good but maybe not quite in a new State.

 

[JoeJChin]

Thanks for the replies .

I finally got a chance to do some other tests on my tractor, and I also picked up some gauges, and fittings to check out some things on my hyd system .

One of the first things I did was try lifting a known weight with the loader
I had some large steel plates , that I was able to calculate the weight of , plus some smaller plates that I was actually able to weigh

Note . that after I did this I had someone send me some owners manual info that listed the capacity of the loader as 700 lbs.

With 945 lbs in the bucket , it will lift it but it is working hard , you can tell it is over loaded . (I did NOT try to lift it to full height , just a foot or so off the ground )

800 lbs lifted quite a bit easier , and with the one large 650 lb weight , the loaded lifted pretty easy.
I left the 650 lb weight raised about 2 foot off the ground , it drifted down about 1 1/2 -2 inchs in 10 minutes or so .

There is a (double) relief valve mounted on the loader , that is set at 1075 psi (stamped in the valve assembly) one each for the lines that raise , and lower the boom .

I hooked up a 3000 psi gauge to the circuit that raised the loader arms and it read around 1100 psi
When I hooked it up to the circuit that lowers the boom it read only 900 psi

IN my original post , part of my concern was why the loader would not lift the front of the tractor off the ground .

I switched the hoses around (hooked up the "lowering" line to the 1100 psi relief ) and it lifted the front with no problem at all .
So I am guessing that the relief is the problem .

It is not important enough for me to worry a lot about it , but can the relief valves be "adjusted " ?
I thought I read somewhere about adding shims to some types to change the relief settings.

I am still not sure why , when I lay the bucket flat on the ground , and roll it forward to the dump position , that it pushes up on the loader arms .
Is it maybe overpowering the relief that is only working at 900 psi instead of 1100 ?

Or could it be as cylinder or valve leaking by .

One other test I did was to try to check (not sure if it did it correctly ) for the lift cylinders leaking by .

I lowered the boom all the way , to empty as much oil as I could out of the end of the cylinder that raises the boom , I disconnected the line on that side , capped it , and hooked up my pressure gauge to the cylinder .

I then tried lowering the cylinder further. (it was already all the way down ) , my thought was that if the seals were leaking by it would show up as pressure on the lift side of the cylinder .
I checked both cylinders this way and neither showed any pressure , so unless my method is wrong (it might be ) I think the cylinder are ok .

Don't know if any of the above makes sense at all LOL but if it does , looking for comments .

thanks again for all the help and suggestions .

Joe s

 

[J_J]

I am not sure why you have relief valves on the cyl hose. Are they perhaps check valves.

If you apply fluid to one side of the cyl, and it reads 1100 psi on the extend mode, and 900 psi on the retract mode, then you appear to have a one way leak.

Yes, you can adjust some relief valves by turning and adjustment screw or inserting shims in the relief valve.

If you have a stamped relief valve, at a specific, then it is not adjustable.

Can you show pictures of the relief valves on the cyl hose?

This is a double relief valve.

1/2 NPT 30 GPM 500-1500 PSI HYD CUSHION VALVE

 

[Egon]

Seems your lifting stuff. Not all loaders have the geometry to lift the front of the tractor unless the curl is used.

Don't muck around with the PSV's. Leave that to the proper shops?

 

[JoeJChin]

J_J

I don't have a picture of the relief valve on my loader , but it looks very similar to the valve you had a link to .
The oem parts list calls it a relief valve . It is in line with lift , and lowering cylinder lines.

I did notice that further down in the description of the valve you showed, it is also called a relief valve , so the valve on my loader could also be there as a cushion valve .

In the description of the cushion valve , it states that it relieves pressure to the other side of the cylinder if overloaded , to prevent cavitation.
That might explain why the loader arms start to lift (one side of the relief/cushion valve is opening at only 900 psi , instead of the rated 1075 psi.) when trying to lift the front of the machine by curling the bucket down while it is setting flat on the ground .

It might be bypassing to the other side of the cylinder.

Going to play around some more , have a couple tee's and other fittings , to put the pressure gauge in other places .

Since the loader "almost" lifts the front end I am not going to be real concerned with it . MIght just pick one of those valves up sometime and try it .
At $70 , vs $450 for the Deere one it is a lot more interesting .

At this time , I am a lot more interested in changing out my two lever loader control valve for a single joystick valve .

How do I tell if I have an open center or closed center valve system .

It is a JD 750 tractor ,with a model 67 loader .

thanks

Joe

 

[Tx Jim]

How do I tell if I have an open center or closed center valve system .

It is a JD 750 tractor ,with a model 67 loader .

thanks

Joe

750 came from factory as open-center hyd's. Short of you having the valve ID # you can disconnect the return line and point this line in a bucket or reservoir opening. Crank tractor with control levers in neutral and if oil flows out return hose it's an open-center if no oil flows out it's closed center or hyd pump isn't pumping oil.