low hydraulic pressure in excavator

[Tiny Tim]

Hi All
Hope somebody can help me out. I have a Nissan N350 excavator which will not meet it's upper pressure set point.
It has three pumps which feed both sides of the spool valve via a Tow valve. The highest pressure I can achieve is approx 1500 psi which I have measured at all three pumps and on both sides of the spool valve.
Each side of the spool valve has a relief valve. I have removed and cleaned these and they will increase the pressure to approx 1500 psi. At this point in the system the set point is 2600 psi but the pressure seems to be cutting off before this point.
The only common point where all three pumps come together is in the Tow valve. My question is are there any relief valves at this point in the system which is restricting the pressure to 1500 psi which I am seeing throughout.
Any insights or info on the machine in general or the valve will be gratefully received.
Hope I have explained this issue sufficiently but please ask for clarification if required.
Many thanks
Tim

 

[akkamaan]

Please post a high-resolution hydraulic schematic with a complete position list

Does it have hydraulic pilot controls? The check the pilot pressure that should be 30-40 bar...

 

[Tiny Tim]

Hi Akkaman and thanks for the reply.
I have attached the only hydraulic schematic I have. Hope this helps.
As you can see the only common point is the 'Tow'valve. I'm thinking there is a relief valve in here which is restricting pressure 591500 psi. I'm thinking this as all hydraulic circuits are affected.
The system is mechanically operated direct to main spool valve so all hydraulic lines are shown in the diagram.
Cheers. Tim

 

[akkamaan]

OK, this schematic does not show anything else than how components are connected with hydraulic lines.
I am talking about a schematic like this that shows the complete function on every valve.
This is a schematic example from a Wacker Neuson 75Z3 mini excavator...

Have the machine had full hydraulic pressure earlier while you were the owner? Or did you buy the machine like this?

 

[Tiny Tim]

Sorry don't have anything like that schematic. I assume that would be in a service manual?
I have just bought the machine like this. Previous owner said it was usable for about 20 mins or so until it got hot. As the machine heats it will gradually lose all hydraulic power until it becomes unusable.
As I am seeing a uniform pressure across all three pumps when I dead head an appropriate circuit I am assuming the relief valve effecting the three pumps will be in a common component back from the spool valve. The only such component is the tow valve. The three high pressure Supplies enter this block and then two Supplies leave to go to each side of the spool valve. I can adjust both relief valve on the spool valve (marked 2 on drawing) upto 1500 psi but no higher. There seems to be a relief valve further back kicking in.
Was hoping somebody would know what the internal of the tow valve consisted of and whether my theory made sense before I pulled the tow valve out.
Thanks . Tim

 

[akkamaan]

This "tow pressure valve" must have some lever for on/off?
Have you checked for that and is it in the right position?
Seems like it has the #1 position on the pressure list on the left, ie approx 1500 psi...

 

[Tiny Tim]

There is no lever in evidence. I have read elsewhere that some tow valves are switched by screwing an allen key in or out. Unfortunately there is nothing obvious on this valve and there is no reference on how to operate the valve in the user manual.
I'm thinking there is a relief valve in there that is faulty or stuck which is not letting pressure build further into the system.
Does this make sense?
I'll have another look around the valve for some kind of actuator but failing that I will remove it and see how it works.
Many thanks. Tim