I have a 14 YO Lennox builder grade 2 ton system. The orig. transformer blew about 1 year ago, I replaced it (with help from this board) and 6 months later it blew again. This time, I installed a 3amp car-type fuse inline on the 24V side of the transformer before putting in the new transformer. I energized the system and within a few minutes, the fuse blew but it did its job and protected the transformer. I put another fuse back but this time had a helper turn the thermostat to cool while I observed what happens at the outside compressor. The compressor fan started and then quickly quit but no starting of the compressor motor. I ran back inside and noticed the air handler fan was running. I let it run a few minutes, went over to the thermostat and set the AC set point about 5 degrees higher than the inside air temp - basically not allowing the AC cycle to activate. I should also mention that I looked at the contacts on the contactor and they appear pitted - I believe the contactor is original. I read that they should be replaced every 5 years. The capacitor was not bulging. I do not know how to test either of these devices but am willing to learn. Any advice on where to go next is appreciated. Please give me advice on where I should go next!

Possibly a shorted out contactor

The coil of the contactor could be bad. Also low voltage wire to the contactor could have a short or the insulation is nicked. The pitted points and capacitor are all part of the high voltage side and will not cause the low voltage 3 amp fuse to blow.

All, it was a short in the 24v wire between the airhandler and the outside compressor unit.