Hello.
I have a three year old Luxaire Central Air Conditioner - ACCLIMATE - Model# AL3A036F1A with a YORK Air Handler Model# N4AHD14A06A. It is connected to a White-Rodgers thermostat Model# 1F90-71 (transformer = 24VAC secondary voltage)which is also connected to a hot water base board heatings system with a 2 TACO zone valves. The second zone valve is controlled by another White-Rodgers thermostat.
The problem I am having is intermittent. When the thermostat turns on "COOL" the air handler fan (inside the house)comes on and the outside air conditioner comes on and cools the house. Then, after it has been off and comes back on only the air handler fan comes on and the air conditioning unit doesn't come on. I went outside to look at the unit was off and the fan wasn't spinning. This happened a couple of days ago (and once last year near the end of the Summer)but it running fine now. I want to be ready when it happens again. What steps should I take to troubleshoot this? Can I rule out the thermostat since the air handler fan turns on?
Any help would be appreciated.
Bob

I found the problem:
The central air unit outside was working all day then later that day the thermostat called for it to Cool the house and the outside unit did not come on but the air handler did as usual.
1. I measured to make sure the 24VAC was at the outside unit and it was there (across coil)
2. I measured the 220VAC input to the contactor and read approx. 240VAC
3. I measured the output of the contactor (which should be the same as the input 240VAC) and it was not there.
4. During measuring the voltage,I noticed the contactor pick up and drop out (an the unit wnet on then off). I tapped on the contactor and it stayed on and worked until the next day when I could purchase a new contactor.
5> Purchased a new contactor and installed it and it has been running ever since (approx 10 days) without any problems.

Note: I took the contactor apart and found that the contact on the secondary side and pitted so badly that a piece of it fell off. I must have fell off in a spot where it couldn't fall out completely so it would make contact whenver it was in the right spot (therefore tapping the contactor must have moved it to make contact). Would have done me a favor if it would have just fallen clear of the contactor all together.