Why does my engine short when I try to on A/C

A/C take a lot out of your engine it requires a lot of generating power,i would have your mechanic to check your components under the hood. Generator, battery, electrical system, wiring system or harness, fuse e.t.c.

There is also the possibility that your vehicle is underengined to run an A/C compressor, the worst thing to have in a vehicle is an engine that I'd not powerful enough to run the vehicle and it's option, and components, in some vehicle the electrical system are not strong or powerful enough to run your components such as radio, A/C, light e.t.c

Now when the A/C is not on there is no resistance been put onto the belt by the compressor, when you turn on the AC, it creates friction to run the compressor, this make alternator and belt pulley work harder. If you are able to check what your alternator is putting out for voltage using a voltmeter 12.5- 13+ when over 2,500 rpm,if it isn't this check if anyone you know has a midtronic battery tester  and test your battery, your AC compressor could be frozen causing your engine to work harder and put a lot of pressure against the drive and the accessory belt causing your vehicle to off.

It may also cause from the poor power to the computer a faulty fuel pump, faulty ignition coil, spark plugs and the nurzles