What fuse?
120 V power to the air handler?

It's a STO 3 Amp fuse on the circuit board.

Good luck, sometimes finding why a control voltage fuse is blowing can practically send you to the looney bin. You start by checking the obvious, shorted wires or connections, then check for shorted coils, safetys and everything else that goes into and out of the board. It helps to know at just what point in the system operation the fuse pops. Is it when the fan starts, or the compressor, or on and on and on! Good luck and I hope you have some control experience,,, that and a handfull of 3 amp fuses.

Thanks for your response mr.havac.

It does not pop when only the fan is turned on. It pops when I tried to turn on the compressor.

Yeah...most likely I won't be able to fix it since I have pratically no experience in the area. The guy put the system in will come on Saturday, hopefully! service is very poor in South Carolina. I do want to learn more about the system thought.

There does not appear to be any damage on the part, I will examine it in a little more detail.

I measured the electrical resistance on low voltage side of the contactor and it is only 2.3 ohms. Can anyone tell me what the specification is? Unless I am missing something, at 2.3 ohms it would draw more power than a 3 amp fuse can provide.

Sounds pretty low.
Some equipment allows the coil to be replaced, but not all.

Highly unlikely a weak coil would cause a fuse to pop, especially immediately on a call for cooling. Pull the wires from the contactor coil and try to start the unit with a fresh fuse to see if it blows then. If it does you probably have a wire short. If not then check for 24 volts between the two control wires. Be careful, you'll be working around a live circuit that can knock you right on your keester.

I disconnected the coil from the circuit and manually pushed in the contacts. The system ran okay and so I replaced the contactor; that fixed the problem.

Thanks for your suggestions.

"Highly unlikely a weak coil would cause a fuse to pop, especially immediately on a call for cooling. "

If he measured the coil at 2.3 ohms it will pull over 10 amps @ 24 V.

Sounds like a SHORT in the coil, not an open.
While opens seem to be more common, the resistance would have been infinite, not just a few ohms.

We just paid $192 to find that the wire in the condensing unit was vibrating and caused a short by wearing the wire insulation cover and as it hit the copper tubing our unit would blow the inside air handler fuse when we turned on the unit. First the thermostat needed batteries, then we were unable to get the unit to blow heat because of fuse blowing. I find it really unbelievable that install did not wrap wires with at least electrical tape ( now shrink tape is used) and zip tie wires together so this issue did not occur. The unit will be 10 years old in 6 months. The wire in question had a small dot of a wear hole. Repair of this issue was to cut the wire and use a wire connector. We used electrical tape on all wires that have possibility of hitting any of the components and zip locked wires together also to keep secure off components, i.e. copper tubing. Hope this helps DYI.