[LazyPup]

All 24volt control voltage originates at your 24volt step down transformer which is usually located inside the indoor HVAC unit or wall mounted close to it.

The power goes to the thermostat then to the AC condensing unit contactor.

The contactor has an internal 24v electromagnet. When the thermostat calls for AC the 24v energizes the magnet and pulls the contactor armature down to complete the 240 volt circuit to the unit.

To test the control voltage first turn the thermostat to AC and set the calling temp well below the room temp to insure the thermostat will be calling for AC.

Locate the signal voltage connections on the contactor. They will be on the sides and will have the small diameter thermostat wires connected to them.

Using a volt meter measure the voltage from one of those low voltage signal connections to the other. Do not measure from signal voltage to the heavy line voltage connects as this could damage your meter, the control components or your thermostat.

While the control voltage is often stated as 24v it is not uncommon to get readings ranging from 18 to 24V.

In your post you stated that when the thermostat is turned on you could hear a click but you got no responce until you actually pressed down on the armature with a screwdriver. Generally the clicking sound would be indicating that the magnet is energized and trying to pull the armature down, but for reasons unknown the armature is not moving far enough to effectively complete the circuit. This is very typical of a defective contactor. The armature may be fouled by dirt and debris in the slide mechanism or their may be a burned contact that is not closing properly. The best solution is to replace the contactor.