JD 790 loader and hydraulic issues

[ARansier]

Hello everyone, I recently acquired a JD 790 with a 300 loader on it. It runs great, tires aren't bad, but there is one glaring issue:
The loader only moves when the 3PH is being raised. I've checked both relief valves on the rockshaft cover and now I'm out of ideas...

Any help would be appreciated...

Adrian

 

[zzvyb6]

Is it mis-wired, hydrauliclly ? So, return oil has to pass thru the 3pt flow path. Check the connector order.

 

[Hoss710]

sounds like either a bad check valve. or hoses may not be connected correctly?

 

[ARansier]

From what I could tell from the parts diagram the lines look right, they're all steel until the quick-connects...

-Adrian

 

[zzvyb6]

Pull one of the lift hoses to see if there is no difference, implying that it's a supply problem. If the lift is now inoperative, it's in the valve or cylinder hose routing.

Presuming you have enough oil and you have cycled it plenty to bleed out all the air.

 

[oldnslo]

Did this failure occur suddenly or has it not worked since you purchased the tractor?

Since the 790 has fixed displacement pump all of the valves must be plumbed in series. Most common system flow goes to the loader valve and then the 3 point. A way to verify this is to check How many hoses or lines are connected to the loader valve? 6 or 7?

Symptoms you are describing would point towards inlet flow to loader valve is flowing in parallel to the 3 point. We need to identify where this leak point is allowing this parallel flow path.

 

[ARansier]

Did this failure occur suddenly or has it not worked since you purchased the tractor?

Since the 790 has fixed displacement pump all of the valves must be plumbed in series. Most common system flow goes to the loader valve and then the 3 point. A way to verify this is to check How many hoses or lines are connected to the loader valve? 6 or 7?

Symptoms you are describing would point towards inlet flow to loader valve is flowing in parallel to the 3 point. We need to identify where this leak point is allowing this parallel flow path.

I got it at a reduced price because of the problem, so it's been like this.

There are 7 hoses. The line from the pump goes to the rockshaft housing, and there are two going from the rockshaft housing to the loader valve

 

[LD1]

Take some pictures and detailed descriptions of how the hoses are routed, from pump, to rockshaft, to loader valve......

 

[ARansier]

Pump to rockshaft

Rockshaft to loader front

Rockshaft to loader rear

To loader quick connects

And there's another one coming off the rear of the loader valve under the inlet going to the trans housing

 

[LD1]

Definitely dont look like it has the ability to be anything other than original.

Now you say the loader only works when moving the 3PH and doesnt work if the 3PH isnt moving.....

What is the position of the 3PH when the loader does NOT work? Is it anywhere in the range that the loader doesnt work.......

First reaction of most people when getting on the machine is to raise the 3ph ALL the way up. If the feedback linkage is slightly out of adjustment and still trying to raise the 3PH hydraulically even though its at its physical limits, it will cause the symptoms you describe.

Try lowering the 3ph a little bit and see if the loader works then?

 

[ARansier]

The loader will only function if the 3ph is trying to raise. If I hold back on the loader, nothing happens unril I move the 3ph handle, the the loader will rise until I let go of the loader control, and then the 3ph comes up to wherever I left it's handle. It's like the system relief isn't working and the loader only gets pressured oil when the 3ph relief is involved.

 

[LD1]

Puzzling. Im not familiar enough with the 790 to be much more help so I'll defer to others....

However....if you can.....see if the loader valve is labeled. Not concerned with the 4 going to the loader quick connects.

The other three however......need to understand their relationship a little better and how its connected to the pump, rockshaft housing, and sump.

One should be labeled P which would be where the loader valve is receiving its hydraulic fluid from.

On should be power beyond. Sometimes labeled (in the casting) as Y, PB, or BYD.....

And the final one should be the return....labeled T or R.

The PB and T should be close in proximity and the P on the opposite side of the valve.

Under normal operation fluid comes into the P port. IF you are using the loader......the fluid expelled from moving the loader will exit via the T port. IF you are NOT using the valve it will enter at the same P port but exit the PB port to feed other functions down stream.

See how its labeled. You mentioned that the pump goes straight to the rockshaft. Then two connections from rockshaft to loader valve and one going to sump.

On the surface that doesnt sound right because having a PB on the loader is only a requirement IF it is feeding something down stream. So at this time I am unsure whether your loader or 3PH is fed first.


Also...being unfamiliar and given that the 790 is an older machine......some older machines had a lever or diverter that you had to switch between what you wanted to use. IE.....turn knob or lever to use FEL.....but that disables 3PH. Turn other way and enables 3PH but disables loader. Not sure if the 790 has something similar

 

[ARansier]

The manual I have (almost more confusing than helpful) shows the pressure line connecting on the rearward side above the T port, it calls the line going from the front side of the valve and back into the front of the rockshaft as a return...

 

[LD1]

The manual I have (almost more confusing than helpful) shows the pressure line connecting on the rearward side above the T port, it calls the line going from the front side of the valve and back into the front of the rockshaft as a return...

Not sure I can help much more without being more familiar with the machine.

But manuals are extremely hard to find online. IF you can....snap some pics of the relevant sections in the manual and post them here. MAy help someone else help you figure this out.

 

[ARansier]

Thank you for your help.

If anyone wants the manual for this tractor I'll gladly share it.

I'm currently pulling the rockshaft cylinder head off. The manual mentions something about either the relief bypassing, or the "SCV plug" bypassing if there's no pressure at the loader connections. Bad part is, the "SCV plug" is only mentioned in the diag and test procedures, nowhere else.

 

[ARansier]

Quick update, pulled the rockshaft cylinder head off, and out fell this plug . I'd bet dollars to donuts this is the "SCV plug" that sits in the bottom of that hole to prevent oil from skipping the loader. The threads were even staked, but it still backed out. So back in with red loctite and we'll see if it's good!

 

[ARansier]

Fixed...

Offer still stands if anyone needs the JD790 service manual.

Thanks for the help!

-Adrian

 

[oldnslo]

Adrian,
thanks for posting the pictures of the ultimate problem. My guess is that plug isolates pressure from 3 point when installed and may not be used on units with out auxiliary valves installed. Strange that you have to remove cover for access though.

 

[LD1]

Dang, nice find. And sorry we couldnt help you figure it out