Bruce,
If the wires are clean and tight, I am with Keith, the alternator either has to be replaced or rebuilt. It should be putting out 14V. If it is the original it is 14 years old, by now the bearing grease is dry or depleted and that is an issue. If rebuilt, it would best be done with a Leece Neville parts kit that includes new bearings, new brushes, new regulator, and some smaller miscellaneous parts. If the shop has one in stock or can get one then you know the parts are right.
By now the heat sinks will be filled with dirt and diesel soot, so it is likely not getting the cooling that at times it could use. If there is a reputable shop around that has the parts you can probably have it back in a day or two.
Those that went with an alternate brand may want to chime in on their experience with another brand. I had mine rebuilt and would plan on a 10 year interval roughly, as a preventative rebuild point. Better if it can be done at a more convenient time then when failing on the road.
Later Ed