(Sent here from alt.hvac)

Does a heat pump's defrost control board change the voltage level
seen at the thermostat on the reversing valve lead?

When in heat mode but blowing cold air, the reversing valve
voltage at the thermostat is 0V as opposed to 26.6V when
working properly.

My heat pump puts out very good heat but it also seems to love to
periodically blow cold *far* more often and for *far* longer than I'd
expect to be due only to the defrost cycle. (The unit does not
have resistive heater strips).
Sometimes it seems to get "stuck" blowing cold for hours and sometimes it
blows cold at least 50% of the time.

I'm trying to determine if the thermostat or the defrost control board
has gone insane.

A tech came out recently and could find nothing wrong with the heat pump.
The refigerant level was perfect and its diagnostic board reported no
problems. It is a trane.
It was of course not misbehaving when he was here.

This is Arizona and therefore only about 50F with low humidity.

Also, the heat pump, during summer months, sometimes does the
opposite. It will blow hot air when it should be blowing cold.

Thanks in advance for any help.