The Quick Coupler Circuit
How it works
If the couplers are connected correctly, there is a path for the fluid coming out of the FQCA and it trips (closes). The LO_C opens all the way.
If the couplers are not connected correctly, because the FQCA is a normally open valve, the area between the CX_A and the unconnected quick-coupler becomes pressurized and prevents the LO_C from opening. The FQCA stays open because there is no flow through it. In other words, because there is no path for the oil to get out of the accessory circuit, no oil is allowed in.
How to apply it
The LO_C XDN and the CX_A XCN have to be sized to handle the flow of your accessory circuit. The setting of the FQCA XAN should not be critical. In most cases, 0.25 gpm (1 L/m) should suffice.
Circuit courtesy of Jim Kaas - Iowa Fluid Power