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?