I purchased a brand new central ac unit and it worked last year fine. This year I tried to start it and it went on for 30 sec and shut off. I tried everything from checking breakers to buying a new thermostat. The fan won't come on by itself either. I checked the 24v at the main contactor and it's not there. I pushed the center of the contactor in and the unit worked fine,just compressor and fan came on but not sure of the air handler, it is in the attic the switch is on. I think the problem is in the handler but I am not sure how to check that. Please help me its gitting real hot.

Did you check the breaker to your furnace? Check the transformer in your airhandler output/secondary should be 24-28 volts
Does the blower run in airhandler?
If yes and OK to the above start looking at the tstat wires running from airhandler to the condenser unit for chewed /.broken/shorted wires

I don't know why the breaker to the furnace could be a problem. The ac units are outside and separate from the furnace. Is there something I don't understand? And No the blower does not work to the handler. Iwill check the tranformer. Thank you so much.

The furnace/airhandler has to have power in order for there to be power to the transformer in order for the tstat to have power so the tstat can send signal to outside ac to come on

PS, unless you have a package unit that has heat and air all in one big unit outside,the furnace breaker has to be on for the blower to blow cool air into the home

Ok I have a hot water furnace 2 zone and a two zone AC unit outside.does this still apply?
I went and checked the power to the handler there is 110vac to the unit and I would imagine 110vac to the primary side of the trans. But when I go to the secondary to the trans. It has 0 v going to the contactor in compressor. The Tstat does function and clicks but no go on the unit. The secondary of trans goes to some sort of bridge and a filter looking thing. Never saw these things. Thanks again WM.

You have a low voltage problem. The only thing you can do is to track down the problem using a meter and replacing the defective part.

NOTE You have a nonstandard system. While there is nothing wrong with that the wiring of the low voltage operating system will be different than standard AND there could have been many different ways they adapted the low voltage control system to work with the added A/C.

I suggest you contact whoever installed the A/C system last year since they did the adaptation of the low voltage system to make it work with the addition of the new A/C system. It would be all guess work to try and tell you how to fix it without a personal ON SITE visit to your home.

OK one more question . I disconnected the secondary from the trans and left the 110v and got 0volts. I think it's the trans right.

Usually that would indicate the transformer BUT you need to find out what caused the transformer to go out or the next one you put in might go out and also cause other collateral damage.

If the unit is only one year old call the company that installed it. It might be under warranty and parts and/or labor might be covered.