Yanmar Hydraulic Dead-Heading

Yanmar tractors, like almost all compact tractors, use what is called an 'open center' hydraulic system. This just means that, when no hydraulic work is needed,  the hydraulic oil is pumped by the hydraulic pump and makes an 'open' loop right back to the hydraulic reservoir at near zero pressure. That oil is always flowing as long as the engine is running.

When you move a valve to raise your loader, for example, you re-direct that oil to flow through the loader cylinders which causes a restriction, raises the pressure, and your loader moves. Eventually your cylinders will reach the end of their stroke or you try to pick up something too heavy and the oil pressure will hit the relief valve's limit. The oil will then bypass through the valve and return, safely, to the reservoir. The oil must always have that open path or you will 'dead head' your pump. 

This is all a massive oversimplification but gives you the general idea. 

The hydraulic pump is not a smart device. It does not have any way to regulate its own pressure. If there is no relief valve in the rest of the system, the pump will produce enough pressure to destroy itself. It can almost instantly hit 5,000 PSI or more before it fails. Each of the valves in the system must have a bult in relieve valve to prevent over-pressuring the pump. If you uncouple a quick disconnect fitting then you eliminate the pressure relief function of the system and create a 'dead-head'.   

The most common cause of dead heading is quick disconnect fittings. I, personally, recommend not using quick disconnect fittings unless you actually remove your loader frequently- which almost no one does. If your hydraulic system uses them then you must make sure that you always maintain a path for the oil to go from the pump back to the reservoir. If that path ever gets blocked then the oil will instantly build up pressure until the pump case breaks, the pump shaft twists off, or, if you are lucky, a pump shaft keyway might shear off. This can happen if a quick disconnect is not fully seated, a disconnect that is worn and does not open completely, or disconnecting the hoses when the tractor is running. Another common cause is a branch getting caught on a hose and uncoupling the quick disconnect while the tractor is running. 

So how do you know if your pump has deadheaded? The most common symptom is a hydraulic pump that leaks profusely through a crack in the side, a loader that just does not move at all, or one that moves but is very weak as the gear starts to slip on the pump shaft with a sheared key as the pressure increases.

Beware! If you simply re-place the pump without fixing the cause then you will likely have the exact same problem again.  

So what is the alternative to an "open center" system? Some bigger equipment use a "closed center" system which, basically, just means that the loop is designed to maintain a high pressure state even when nothing is in use.