Air handler blown 3a fuse

Hi, just to let you know, never jump out a blown fuse as if blown, you have a problem in the control voltage side of this unit, anywhere from a shorted stat to any part that is controlled by 24 volts. When you by passed the 3 amp fuse and took out the transformer, lucky for you it didn't take out the control board as well. After you replaced the transformer, you say you now have correct voltage again. You will need to locate what part or find what is shorted on this furnace by doing some trouble shooting, process of elimination. If the fuse blows as soon as you turn on the power, its a dead direct short. Check the thermostat wiring coming into the unit for a spot on any of the wires that are visabal to see if they are touching metal. I find this a lot. To rule out the stat, low voltage controls on the condensing unit, remove all of the wiring from the terminals, but make sure you draw a picture diagram on how they were or you will forget.Now turn on the power. If fuse does not blow, your problem will be in the control stat wiring, could be a shorted contactor 24v. Coil, you will need to connect the condenser up and hit power to see if it blows the fuse. If it does you will need to take it to the outside unit checking the wires from furnace to condenser first and check any 24 volt control on the outdoor unit, contactor coil, low and high pressure switches, time delay boards, look at the schematic to find them, but disconnect one at a time until you find what part is shorted. If all is OK there, back to the indoor unit. Your circuit control board may be your short. How I check one which takes some time is to look. You will need to remove the 4 stand off screws at the corners, stat wires and any other wires that you need to move around to take a look at the back side of the main board. It is a little difficult, but you will see right away if it is as it will be covered with black smoke and burned circuit paths. You will know then the board is the problem. You will need to take all of the numbers, letters from the board, model and serial numbers from the furnace and go to a retail appliance parts house to get or order one. If you know any one who knows a contractor, you can save and go to a a/c, refrigeration wholesale parts supply to get it. Once the new board is in and make sure the new fuse is in, turn on the power to see if the fuse holds. If it does, connect the outdoor unit and hit power. If it holds its OK. Connect the rest of the wiring and energize it. If it works, your off and running. If at any point it blew the fuse, you will have had it isolated and will no where to go to find the short. Such as a shorted safety pressure switch, the gas valve coil, roll out switch, etc. It takes some time, but its really very simple if you follow my instructions as best you can. If you replaced the transformer, you will be able to find this short by disconnecting the parts that are in the 24 volt control circuit. Just remember not to jump out the fuse as it protects everything, board and all 24 volt controls. Don't forget the thermostat and the wiring from the stat to the indoor and outdoor unit.
Sincerely, and let me know the out come,
Shastalaker7
A/C, Heating, & Refrigeration Contractor