Our upstairs AC stopped working earlier this week. Probably on Tuesday or Wednesday after Harvey passed through town.

I noticed the thermostat display was dead.
Replaced the batteries in the t-stat and the display came back to life.
No AC, or blower operation though.
Even putting the fan switch to on, and no blower.

This morning i checked the telltale lights on the air handler circuit board in the attic and both were off.
The 3 amp fuse on the circuit board was blown. Went ahead and replaced it.
The telltales lit up and the fuse popped after about a minute or so.

Looking over the wiring schematic, it appears the fuse provides 24 volts to the thermostat.
and the circuit goes through the left and right flame rollout switches and then to the thermostat R terminal.
When the AC is on, the Y terminal gets current and this goes to the ignition/blower control board to run the air handler fan and then goes on the the outside unit.

Going from memory here, the Y goes to a high pressure cutout switch and then to a contactor which powers the compressor and outside unit fan.

The wiring I checked in the attic appears to be in good shape. The circuit board is not burned from what I can see.

Am i most likely dealing with a faulty contactor or wiring to it?
I was looking for a relay that controls the air handler blower motor but it appears to be part of the ignition blower control board. Am I reading this correctly?

One more thing, I replaced the air handler blower with an OEM unit in 2013 due to a noisy motor bearing.

Sorry for the wall of text but i am trying to cover all bases.

Thanks for any help on this.

Did the furnace control board get wet?

You could always wire the system to operate without the board, for testing purposes. Make sure to wire in a fuse/circuit breaker to protect the transformer.

No evidence of water intrusion on the control board.

I was going to measure the current draw of the contactor to see how much current is going across the windings when energized. I could probably test this on the bench, pretty sure i have a low voltage power supply floating around.

That seems like the easiest next step with the tools I have now, Fluke 88 DVOM that can measure up to 10 amps.

I hadn't thought about bypassing the board.
One thing for sure, it is blazing hot in the attic. 5 minutes up there and i am drenched.

I don't if it should matter but could be issue at outdoor condenser. You could try disconnecting control wire going to condenser and see if it still blows fuse.

I had driving rain and ants get into one of my condensing units because of Harvey. The contactor was not engaging. I banged on it a few times and back to work. Ordered a spare and spare cap but have not needed them.

Since no water is evident, I would look for a bare wire that's touching the metal cabinet.

As Mikito stated, unhooking the C and Y wire feeding the condenser would point to where the problem exist (indoor/outdoor).

I've been working 2 jobs for a while so I won't be checking this until Saturday.

First, I'll check for chafed wires inside and out.
Then I'll try to isolate the circuit and take a look at the contactor by the condenser.

Thanks very much. I appreciate the advice.