JCB JS70 Hydraulic problem.

[Ferguson man]

Hi. I have a JCB JS70. It could even be a SH60 import, but they are both the same. It has worked fine untill last week when I was doing some heavy digging. When you ask for full power and hit some resistance, the hydraulics cut out. As soon as you release the control and try again, full power untill you hit resistance. If you ask for half speed, you can blow the relive valve off. You can grade all day with no problem. It is rod control, so no problems with servo's.
I'm no expert, but to me it feels like the pump control is shutting down on full flow.
I have access to pressure gauges and a reasonable tool kit, but not sure what to try first.
Any help on what to do next would be great. Many thanks, Pete.

[si25]

have you checked any of the filters on the hyd system?
not sure if your hyd system would work the same but if there is any resistance in supply they will automatically shut down capacity so not to effectivly suffocate the pump and run it dry.
you should find strainer and main filters in the tank.

[Ferguson man]

Hi Simon. The filter is on the return line, so I don't think it should alter the flow much unless it is in a very bad way. I will take a look at it all the same as all things should be checked. I did forget to mention that all the time I have had a problen it has been below freezing. I don't know if this has added to the problem or not.
Thanks for the idea. Peter.

[si25]

i'd be very surprised if the cold is affecting it in such a way, we are talking uk cold not alaska cold?! assuming the oil is not the right grade or mixed with another oil? ...shall have a ponder and a coffee.

[Stu in Wales]

Hi did anyone get to the botton of this as I have one doing the same thing and it's driving me round the bend!
Feels like a relief valve that sticks of full power and doesn't close until load is released.

Cheers,

Stu

[wxmman]

Check the line relief pressures first

[geoff_n]

Pressure Sensors are often a source of trouble and confusion, the classic one is the engine oil warning light problem. a faulty sensor or a lose wire could be the source of your problem, assuming there is one on the machine.
regards Geoff