New package unit, Start up procedure, Used old

Good morning. Ohm out the transformer secondary coil. If the secondary coil is "open" then the problem is on the low voltage side. If the side that burnt up was the primary side, then look to the line voltage side for your problem.
If the low voltage wiring is correct and your problem was on the low voltage side, disconnect all low voltage at thermostat and at unit....then ohm out all the wires to see if there is a "short" at any of the wires.
When you replace the transformer, add an inline fuse to the secondary side. A 5 amp ATC fuse will work.....use female spade connectors to hook up to the fuse. Then wrap with electrical tape. This will save you from the expense and trouble of having to replace the transformer in the event of a future short. If you have any further questions, comments, concerns, please don't hesitate to contact me.

I hope you find this information to be very helpful to your moving forward.

Please leave a rating for fixya.

You had a direct short. One of the wires shouldn't have been where it was. Tell me the color of the wires and the hook up.

rule of thumb from thermostat
red to R
White to W
Green to G
Yellow or Blue to Y
no comm hook up from t-stat
now the control for the ac is the Y terminal
and the only wire left to Common. Generally speaking. Let me know the wire colors? ken