Hi everyone, our AC is down for a couple days. I've been trying to diagnose for a couple days, and here is what I find out.

At the beginning, the fuse on the control board get blown, so nothing was working. I replaced the fuse, control board and transformer, but the fuse still get blown when thermostats was on cold mode. The fan mode and heat mode work fine.
Then I disconnect the low voltage to the outside units, this time the fuse didn't get blown, and blower started working.
Next I connect the low voltage to outside, but disconnect 240v to the outside units, the fuse didn't get blown. And there is 24 volt coming to contactor.

So IMO it seems that one of 240v is short with 24v in outside unit. And I replaced the contactor, still didn't work. But when I press the contactor, the outside unit gets on.
Let me know what you think. Thanks!

 

If the 240V is shorting the breaker in main panel would trip. Is this what's happening?

 

No, nothing happened to the breaker. Only the fuse on control panel would blow.

 

It's rare and doubtful to have a 24vac to 240vac short.
I'm assuming this is a straight split A/C and not a heat pump system.

It sounds like one of the wires is shorted inside the condenser. Sometimes it's tied or routed by a sharp piece of metal.