I have an 25 year old Trane Elite Super Efficiency Weathertron Heat Pump. The heat works fine. But when I switch to cool, the outdoor unit starts, sputters, then the transformer in the indoor unit gets blown. I added a 3A fuse after changing few transformers. Sometimes, just switching the tstat to "cool" blows the fuse. Othertimes, switching to "Cool" works and runs a minute then the outdoor unit sputters and fuse get blown again. Thank you for any help you can provide.

Maybe the contactor coil has shorted windings drawing more current.
Check low voltage control wiring, the compressor or fan should have little to no effect on the low voltage from transfomer/circuit.
I would check compressor contactor.