(The content of this post is intended for consideration by trained service personnel)
The reversing valve is the component of a heat pump system that determines whether the system runs in heat or cool. They are in fact, an assembly of two valves: the main valve which actually directs the refrigerant flow in the system, and a pilot valve which controls the main valve. The pilot valve applies system pressures to the ends of the main valve, suction pressure to one end, discharge pressure to the other, creating a pressure differential which will force the main valve slide piece to shift in one direction or the other. This design allows the heat pump system pressures to actually switch the reversing valve position. A solenoid capable of switching the main valve directly would, no doubt, be very large. I’m oversimplifying the design and operation a little, but my intent is simply to summarize the function of the valve, because if it fails mechanically, your only option is replacement…an intimate knowledge of the inner workings turns out to be academic.
Valve failures will generally be 1) solenoid coil failure, 2) “stuck in heat or cool” position or 3) stuck somewhere between heat and cool positions. Coil failure is usually fixable. You only need to verify the absence or presence of coil voltage in the appropriate cycle, to eliminate wiring problems. Coils can short out or go open and in most cases, a new coil can be substituted.
Stuck valves could be the result of a pilot or main valve problem. In either case, I’ve had no luck in making a “repair”…I’ve “unstuck” a few, but the fix was only temporary. They most often stick again.
The last failure situation is the valve stuck somewhere between cycles, which is usually a difficult situation to diagnose. The valve slide position is such that the net result is a significant amount of leakage between low and high side pressures, producing symptoms of a faulty compressor…high suction and low head pressures.
There are several methods used by service people to confirm or eliminate the valve, involving tubing temperature measurements. More involved procedures amount to isolating the compressor, then eliminating it as the problem.
You can see a more in depth explanation of reversing valve operation and troubleshooting techniques in the “Diagnosing Reversing Valves” rental videos:
(The content of this post is intended for consideration by trained service personnel only)
WayneShirley.com 
Good morning, I recently had a brand new Goodman 3.5 ton 15 seer heat pump installed, I am very satisfied so far with the unit, however periodically the unit will make a LOUD moaning sound when going from normal running mode to defrost mode. The unit was installed the beginning of this year; 2013. The tech that installed the unit is unsure of what would cause this noise. It does not happen all the time, but when it does it is rather loud. Any ideas?
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The unit brand name is airease packaged heat pump.
When in heat mode the valve will not move to let the 410a feron work the heat.
I replaced the board and get the same problem.
If i connect the orange wire to the red wire on the board the valve works fine but will not let the unit deforst.
Any help will be great
woodrow boyd @outlookmail.com
Thanks
thank you