Something caused the transformer in my Trane XE900 heat pump system to burn out.
Info on components: Inside air handle/electric furnace - Trane model TWH7188140A0 Outside unit – Trane model TWD 718B100A2 Thermostat – Trane model 3AAT80B1A1
All components put into service 6/91.
A couple days ago my daughter called to tell me the unit was not working . . . she said the blue indicator light on the thermostat (for the auxiliary heat) was on but very dim. By the time I got home, no light on the thermostat . . . checked and found no current to the thermostat. I opened up the air handler and found no current on the secondary side of the transformer . . . looked & smelled burned. Installed a replacement transformer the next day & confirmed voltage on the secondary prior to connecting load. Connected load with a 5 amp auto fuse in line and checked out various functions. Units blows fuse immediately when switched to “heat.” Did not blow fuse on fan-only setting or on emergency heat setting. I replaced the 5 amp fuse wit a 10 amp & tried “heat” again . . . did not blow fuse . . . indicator light on thermostat glowing dimly, relay to energize electric heat buzzed loudly, and neither the blower motor or compressor started. Transformer started getting hot & I disconnected before it was ruined. I did confirm that the compressor will run by manually closing the contacts. In the outside unit I also observed a red LED on a circuit board was lit & blinking . . . not sure if this means anything but thought I’d mention it.
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Any thoughts on what I should try next?

Your transformer is shorting out somewhere. Youre going to have to start disconnecting various 24 volt users until the transformer operates normally. Three years ago, I had the same problem with xfmr burning up. Finally found the two wires to the outdoor condensing unit shorting together somewhere in the wall. Had to run new thermostat wire to correct it.
Bob
coastalappliances.com

First, Never use a 10 amp fuse on a board rated @ 5 amps, fuses are less $$$ than boards.

You have a short in the low voltage circuit, could be several different things.

The one thing that comes to mind is the contactor coil, disconnect the yellow wire @ the unit, switch it to heat, see if it pops the fuse, might have a shorted coil on the contactor.

There a many things that it could be, buy a box of 5 amps fuses issolate each componate, you'll find it.

Due to extended bad weather I chose not to look into the outside unit until this weekend . . . this system is one of three units in the house & I've just been running it as needed in emergeny heat mode.

I isolated the short to the yellow wire in the control system and yes it appears to be the coil in the outside unit's contactor. Will update again once I've replace it.

My thanks to those who responded to my post!!

yes, that should fix it. Make sure that you replace it with the same type of contactor. Most contactors you will find with be eather single contact and duel, go back with what it came with.