Usually the high current contacts get damaged by arcing or the moving contact piece jams. Pretty easy to service, half an hour and a flat file is all you need. Take the contact end off, remove the nuts on the studs and push the contact studs out. File the flat. File the moving piece flat. Clean out any crud, reassemble. I can't exactly describe the process but it once you open it up it will be easy to see what you need to do.
If the coil bit is jammed, it's toast. This is not really repairable.
