A, B, E & F Trane

Capacity Control on the Trane models A, B, E & F is accomplished by mechanically lifting the suction valve disks off the suction seats. When the valve is lifted all refrigerant in the cylinder is pumped back into the suction side of the compressor on the piston up stroke. Trane offers three types of unloading on these compressors, suction unloading, electric unloading and electric unloading with remote mounted solenoids.


Electric unloading or remote electric unloading does not use the bellows and relay assembly but directs oil pump pressure directly to the power elements thru electric solenoids mounted either directly on the compressor hand hole cover or remotely on the unit.

The suction unloader control is basically the same on all models. There are two main components in this system, the hydraulic relay and bellows assembly and the power elements.

The bellows and range adjustment spring control the position of the hydraulic relay piston by varying the oil pressure on top of this piston. This oil pressure is referred to as the “control oil pressure” and is the oil that comes thru a small orifice in the top of the relay piston. Pressure is varied by a controlled dumping of this oil back into the crankcase thru an orifice drain. Three forces control the orifice drain, suction pressure in one direction and the range adjustment spring plus external air pressure or atmospheric pressure in the other direction. A balance to maintain the desired suction pressure is achieved by changing the spring pressure using the external adjusting stem.

The hydraulic relay controls the oil supply from the oil pump to the power elements by opening or closing the supply oil lines to the power elements. Detent balls and springs in the relay control the steps of the relay piston. All the compressors have three steps of capacity control, each step may control one or two cylinders depending on the model and number of cylinders the compressor has.