On Jun 17, 4:46*pm, wrote:
On Sunday, June 17, 2012 4:04:05 PM UTC-7, Robert Macy wrote:
Typical!

In AZ we have the Air Handler in the attic and the Heat Pump sitting
on some square beside the building.

Shut off the AC last winter, and when went to turn it on this one
time, nothing happened. Well, except the fan came on, but no big power
hum, no fan blade inside the heat pump running.

Just as a check I measured all the voltages coming from the the
thermostat out through to the unit, and they all seem correct. OFF no
voltage, ON 26 Vac, yet unit doesn't come on. *There is a distinct
power hum, but nothing happens.

I turned OFF the 220Vac going to it, and measured the voltages around
the huge relay right at the access and was disappointed to find almost
nothing there. I expected the relay[whatever it's for] to be powered.
Perhaps that's the key.

Inside the access panel lid is atype of schematic. One interesting
item caught my attention 'low pressure switch' so I thought perhaps
the unit has lost its charge and the swicth won't come on to prevent
damage when it's not properly charged. *But I don't know much about
HVAC units. *There are three access spigots for measuring/filling I
think all right under the access panel.

Anybody out there with a bit of tutorial to help? Like what pressure
range should be there, what to do if the pressure is too low, etc etc.

I've got two other identical, operating units to do comparisons to,
but it still helps to know WHERE to look.

I’m confused: You say that you “turned OFF the 220Vac going to it, and measured the voltages around the huge relay right at the access and was disappointed to find almost nothing there”
Why would you expect to find any voltage if you say you turned it off?

My thinking: the 24 Vac coil should have been activated, reading the
voltage at the coil should have seen it try to turn on. The 24 Vac is
supplied from the attic unit comes in under different breaker. The
220Vac is to supply the 'husky' stuff.