The central AC is not working. There is no air coming out of the ducts nor does the compressor come on. I have already replaced the transformer/relay combination. Without the thermostat being turned to Cool, I can hear the transformer/relay humming when I switch on the 110V. I figure the compressor would come on even if the blower did not or vice versa. I also replaced the thermostat, one was handy. I would appreciate some pointers. If the blower motor is bad, would that prevent the compressor from coming on because of a biult-in safety feature ? I do not have a schematic diagram to see the flow.
Thanks.

First make sure the blower door is installed correctly activating the door safety switch to allow current to pass to unit and all switches and breakers are turned on. Next check to be sure you have 24 volts at the transformer. Then check to see if you have a blown fuse on the control board in the inside unit.
Next when the unit is calling for cooling check to make sure 24 volts is at the contactor in the A/C unit outside and that there is 240 volts on one side of the contactor. This check should be done after the A/C is turned on for at least 5 minutes just in case the thermostat or system has a time delay in the circuit.

If not try to activate the outdoor unit by pushing down the contactor contacts with a insulated device. (240 volts present) be careful. If unit starts the problem is in the 24 volt low voltage control circuit and you will have to track it down.