you might try the following 'divide and conquer' approach.
- install a 3 amp fuse in transformer secondary.
- measure and record total secondary current in heat pump mode only (call for heat 1 degree above room temp).
- measure and record total secondary current in emergency heat mode only.
- measure and record total secondary current in heat pump plus aux heat mode ( run heat 5 degrees above room temp). the transformer should be able to handle all this current and the current should not exceed 1.6 amps (40va / 24v)
you can then run heat in each mode separately. if there is a current spike, the fuse may blow and you can focus on outside or inside.
if it is determined the problem is occurring outside, install a fuse in the common wire outside and jiggle the wire bundle. if the fuse blows, look for a break in insulation outside.
the current limit for the 40 va transformer is approx 1.7 amps (40va / 24 v). a direct short will blow a 3a fuse. an increase to 2 amps, however may take out another transformer. ultimately, you may need to look for an overheating relay or sequencer.