Was having transformer problems on residential unit as previously mentioned. Put 3 amp inline fuse on 24v circuit as suggested. Indoor fan comes on now with manual ON switch, but switching T-stat to cool blows the fuse. Getting 27 volts TO T-stat on all wires. Getting 239V on outside line side of contacter. Push in on contacter and comes on. Getting continuity on contacter coil. T-stat will not send low volts to outside unit, just blows fuse. Also replaced wire to outside contacter. At a loss... help!

Change the contactor!

Could it be that simple, even if the coil was checking out ohm wise? And working when pushing in on it? Sorry if dumb question, new to all this...

Most of the time if you are blowing the 3 amp fuse while trying to run the ac it ends up being the contactor.. but if yours is not stuck or burnt. Maybe just the coil is grounded. But that is where I would start. They only cost about $20 bucks

Replacing it crossed my mind, but threw me for a loop when I checked it and "seemed" to be OK. Thank you, will get one and post what I find.

Great, deffentally would like to know what you find out. Hope that is the problem.

Me too, a simple "freon charge" has turned into an electrical troubleshooting class. You can bet your bottom dollar I'll remember this awhile. Thanks again for your time... and knowledge!