I am trying to see if I can fix my girlfriends AC unit (if it is an easy fix). The air comes on when you turn on the thermostat, air starts blowing, but it is not cool. Checked the outside unit (compressor) and it was NOT running. No fan, neither pipe was cold or hot, nothing from the outside unit. Last time this happened, I went to take off the side cover and it started working again, just like that. So this time I thought I could do the same thing, like there was a safety switch on it that was not getting triggered. No such luck, took off the cover, there was no switch and I didn't see any loose wires. Checked the voltage, there seems to be voltage 24vAC coming in from the thermostat wires, and there is AC power (110vAC I assume) coming in from the breaker box. Beyond that, don't know what to check. Thoughts? Help?

There should be 240 coming to the unit for the compressor and fan. Sounds to me like either the contactor is not closing or something else.
When the unit calls for cooling is the contactor closing? It is the main control for the fan and compressor.

I checked the voltage (with just a voltage sensing probe, not a volt meter) on either side of what I think the contactor is (wires coming in from the breaker and then going out the other side to the compressor)... there seemed to be voltage on both sides of that device.

If there is voltage any where near the contactor it will show up... need to get a meter.

Is the contactor closed or open. If open push it in with a screwdriver and BE CAREFUL not to touch any metal. See if compressor turns on or not than.

Might want to call in a service tech. 240 volts can and will hurt you!!

OK, that makes sense. I do have a volt meter and I did see where the "plate" could be pushed in to make contact. So a) I will check the voltage coming out the other side and see if it is 240 vAC and check to see if the contactor can be activated manually.(I will be careful, thanks for concern). If the contactor works manually, does that mean it just needs to be replaced? Or is there something that activates it that might be broken? Also, speaking of safety, what would the small white barrel looking thing in that same area be? A capacitor? Am I going to run the risk of discharging that somehow if I replace the contactor and if so, how do I discharge it before I do that replacement so that I lesson my risk?

By the way,, thank you for helping !

That would be a start cap... no need to bleed it just be careful when removing wires from contactor if that is what needs replaced. Make sure to label them or make a very very precise wire diagram so you know what goes where.

You rock... won't work on this until the weekend, but you have been very helpful, thanks for taking the time.

Good luck and let me know what you find...