What could be causing my A/C not to come on?
If the system run normally when the thermostat is calling for heat, and when the fan control is switched to the ON position. Then the first thing I'd look at, is the contactor in the condensing unit.

WARNING: If you're not comfortable opening the condensing unit, and/or working with electrical devices. PLEASE contact a licensed and insured HVAC technician.


Start by turning off the breakers for both the furnace, and condensing units.
Next pull the serviceman disconnect at the condensing unit, and place the pull out in your pocket, or place a lock (that only you have a key to) on the disconnect switch.
Open up the condensing unit to expose the electrical components.
Locate and discharge any capacitors.
Locate the contactor.  


Label and remove the two low voltage wires connected to the contactor coil.
Using a multimeter set to measure resistance, measure the resistance across the coil.




Typically the coil should measure between 9-12 ohms, though this may vary by manufacturer. You may want to contact the condensing unit dealer, or the contactor manufacturer for model specific values.

High resistance

If the value is higher than 12 ohms, it's likely that the coil will not be able to pull the contactor closed. Which means the condensing unit will never turn on.

Infinite resistance

If there's infinite resistance across the coil, the coil is completely broken. Again, this will never pull the contactor closed.

Low resistance

If a low resistance is measured, it's likely there's a short in the coil. This will typically result in burnt out transformers, and/or blown fuses.