Electrical issue

[waynep]

I have a 1990 truck with later model carberated 454 engine with electric cooling fans. I was driving the truck and started smelling melting plastic. Then my volt Guage was all over the place. I took the alternator to a rebuild shop. He told me all the diodes were bad and something else was melted and that was what I was smelling. I reinstalled the alternator and no change. Then I disconnected the negative battery cable and truck shut off. If the alternator was working shouldn't the truck continued to run?????? I went back to the shop for a recheck on the alternator and it is making 14 volts. I have tried 3 different batteries and there has been no change.
Any advice????