Title pretty much says it. When thermostat turned to cold, the compressor and fan on the outside unit don't turn on. The indoor fan will turn on. Additional info:

Worked fine yesterday during the day
Didn't work last night or today (was turned off in the evening and didn't work when turned back on)
Fan turns freely with a slick
Capacitor (visibly) looks OK
We have 2 units and the other one works fine
When I take out and put the circuit breaker back on the working unit it sparks a little as a put it in
When I do the same with the unit that isn't working, no sparks
I swapped the thermostats between zones and same one doesn't work
Reset circuit breakers at the panel (they weren't tripped but cycled them anyway)
When the AC is turned off, at the thermostats I can hear a click coming from the circuit board/contractor area

Please let me know if you have ideas on what else should be checked. I'm suspicious of no sparks when putting the outdoor circuit breaker back in, but don't know how to try to trace that back anywhere.

Of course this is the unit for the bedrooms and it's the hottest weekend of the summer,

Thanks,
Steve

 

Steve, there are a couple of things you can try. First, do you have a multimeter (volt/ohm meter)? Things will be a lot easier to troubleshoot if you do. If you do, check to see if you have 240VAC coming into the outside unit. The easiest way to check for that is at the contactor. The contactor is a relay and is usually rectangular in shape. There will be two terminals at each end of the contactor, and one terminal on each side of the contactor. Set your voltmeter to AC volts and measure across both sets of end terminals. One end should measure 240VAC, and you'll probably measure 0V across the other end pair of terminals. If you measure 240VAC at one end of the contactor, you have power at the outdoor unit. If not, check for fuses inside the outside electrical disconnect box located near the outdoor unit.

If you have 240VAC at one end of the contactor, the next thing to check for is the control voltage. With the thermostat calling for cooling, carefully measure the voltage across the contactor side terminals. You should measure 24-28VAC. If no voltage, find where the control voltage cable coming from the house enters the outdoor unit. The cable will usually be a small diameter cable with 2 or more conductors in it. Where it comes into the outdoor unit, it will connect to the outdoor unit wiring using wire nuts. Check for voltage across the wires at the wire nuts. If you have 24-28VAC there, but not at the contactor, you have an open pressure switch (if present), or a problem with the Time Delay Circuit (TDC), and you'll need to do further troubleshooting to identify where the voltage is being lost.

If you don't have a voltmeter, about the only thing you can try is to take an insulated screwdriver or small stick and push in on the center of the contactor (there may be a button to press). This will manually close the relay. If the outdoor unit comes on, you know you have power to the unit, however you'll still need a voltmeter to troubleshoot further.