And here's a good description of how these suckers work...
http://www.aeroelectric.com/articles/strtctr.pdf
If you aren't well-schooled on electrickery, this might be over the top but hey, it may help.
The key takeaway is that typical engagement currents on starter solenoids is about 35amps - well short of the current required to blow F105. Something is sucking a LOT more current down that line...