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.
JD 790 loader and hydraulic issues
- Thread starter ARansier
- Start date
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LD1
Epic Contributor
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
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
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
Epic Contributor
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.
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.
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.
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!
. 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! oldnslo
Super Member
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.
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
Epic Contributor
Dang, nice find. And sorry we couldnt help you figure it out
