John Deere 755 adding hydraulic circuits

[3jd318]

I am a new owner of a JD 755. I am trying to add a 3 spool valve. I believe a previous thread on this subject was posted some time ago before I had my tractor so I did not save it. Now I can not find it. Can someone direct me to the correct thread or give me help on searching?
gp

 

[tomrscott]

I don't know much about the 755, but I suspect it is similar to my 790. I can tell you about the 790 and if you recognize similar stuff on the 755 it will undoubtedly work the same. In any case maybe this will help a little. (acronym key at the end). /forums/images/graemlins/smirk.gif

There is a green painted steel tubing line with banjo fittings on each end, under the seat, on the right side, that runs between the PB port from the FEL SCV, and the P input to the rockshaft. It looks like a kind of square corkscrew for 270 degrees. It is just a couple inches forward of the shaft where the 3PT raise/lower control lever attaches.

Using compass headings for direction of tube travel, the banjo fitting into the FEL SCV points N (toward the frond end of the tractor), and the line clamped by this fitting runs first W, then turns S for a few inches, then turns E for another couple inches, and finally turns N for a couple inches until it reaches the other male banjo bolt into the which is pointed E and screws into the rockshaft housing.

You take that line out and plumb the P line of your new SCV into the PB port of the FEL SCV. Then you plumb the PB port of your new SCV (which has to be OC, and PB), and connect that to the P input port of the rockshaft. Finally, you have to provide a line from the T output port of your new SCV into one of several ports that will return the low pressure fluid to the crankcase. I haven't actually decide which port to use for that yet, but a mechanic showed me several options.

FEL = front end loader
SCV = selective control valve
P = Hydraulic pressure supply
PB = Hydraulic pressure "Power beyond" to next device
T = low pressure fluid "return to tank"
OC = Open center, allows fluid to pass through when not in use
Banjo Fitting = If you imagine the fitting on the end of the tube after you remove the bolt that secures it, it terminates in a ring that if you put skins over the opening, would resemble a banjo body (with a little imagination). Hard to find tubing with these fittings, but I think it might be replaceable with an SAE fitting for 3/4". Still working on that myself.

 

[3jd318]

Thanks Tomrscott.
I will take your response tomorrow and look at tractor. Idon;t think I can't tell which is power beyond.
gp

 

[3jd318]

Apparently there is very little similarity between 755 and 790. I guess if I could get a copy of instructions for installing 7 backhoe it would help.
gp

 

[tomrscott]

3jd318,

My JD technical service and parts manuals finally came in today. Ouch those things are expensive! The parts manual wasn't too bad, I guess because it is just for the 790 - $50, but the service manual covers the 670, 770, 790, 870, 970, and 1070 and they get $180 for it. I think when I asked the guy the pricing when I ordered them I thought he said "Ones is fifty and one is thirty, for a total of eighty." but it was $50 and $180, for a total of $230!

Anyway. I will get plenty of good out of them, so I am not going to quibble over the cost. The drawings are pretty good and there is enough stuff on the hydraulic systems to be a big help.

Unfortunately, some of the info I had been told by a mechanic, and repeated in the previous posting above turns out to disagree with the service manual. Further complicating matters, is that I think I agree with the mechanic and not the book!

If you have read that posting above, the squared-off 270 degree corkscrew line I had been told was hydraulic pressure from the SCV to the Rockshaft, well the manual says the flow goes the other direction, that it is the PB from the rockshaft into the P of the SCV. It also shows that the T return to sump line from the SCV is connected right below that, and the PB outlet is on the opposite side (front side) of the SCV.

But the more I study those drawings, the more I am becoming convinced that the mechanic may be right and the manual may be in error.

Every monoblock or stackable multi-spool SCV I've seen has had a P port on one side of the casting, and the fluid flows from that side, through both spools and then on the other side of the casting is both the PB pressure outlet, and the T low pressure return to tank. It just makes sense that both outlets, the PB and the T would be on the opposite side from the P port and grouped near each other. But the manual would have me believe the P and T lines are on the side of the SCV toward the rear of the tractor, and the PB outlet is on side of the SCV facing the front of the tractor!

It's late now and I'm not going to get this sorted out staring at it, but stay tuned as I work to sort this out. /forums/images/graemlins/confused.gif /forums/images/graemlins/wink.gif