Problem still remains. Put a new tstat and fired heat. It runs great. As soon as tstat goes to AC, fuse blows. Checked all low voltage wiring in and out....all fine and no sign of chewed or damage.
Diagnostic LED flushes at AC setting as:
24, meaning as the panel reads: Secondary Voltage Fuse is Open check for short circuit secondary voltage (24 VAC) wiring.
2 rapid flushes followed by 4 delayed flushes.
I removed the tstat face, loosen all wirings and connected are to why & G at the same time, turned on unit, fuse blew.

Carrier unit Model #: 58STA090---1114
Serial #: 4404A40864
Any suggestions?
Thank you,