On Fri, 16 May 2008 18:23:24 -0700 (PDT), randallbrink
wrote:

I have a puzzling problem. When I select "Cool" on my thermostat, the
central fan does not come on, and so, while the air conditioner
outside runs, it quickly freezes, as there is no air movement. On the
other hand, if I select the heat-cool switch to "Off" and turn the fan
"On" the fan works fine. But if while the fan is running, I move the
Heat-Cool switch to "Cool" the fan shuts off. I have replace the
thermostat, and have the same problem.
Did this just suddenly happen to an existing installation, or was
anything changed in the installation before the problem began, e.g.
wiring, equipment, supporting electrical wiring, etc.