Sounds like you've got a good diagnosis of your problem. Based on what you've said, that small excitor wire to the back of the alternator is likely damaged and shorting out somewhere. I can't remember on the 140's whether it goes under the crank pulley like the 240's or not -if so that's a common problem area. If necessary trace it back to one of the junction blocks and splice in a whole new piece of wire, re-routed along the fender and firewall. I find high-temp oven wire from the local hardware store is good for that job.