Alternator

88 XJ 4.0, CS 121 Delco Remey. In the last three months I`ve swapped in two of my, shelf alternators (spares)(pretty old but funtional). I´ve just picked up a new CS 130 (100 AMP). What I don´t want to do, is have it fail also, so I thought asking a few questions would be prudent.
I checked out two of the failed alternators, one had a bad diode, the other a very worn brush, the third I haven´t opened yet (maybe just bad luck, maybe something more sinister). I´m thinking the alternators were all pretty old, the best, by judging the brush wear, had maybe 60,000 miles on it.
Maintainance free battery, that holds a good charge (for days). But doesn´t seem to hold the 14-15 volts the charging system puts out. Usually tests at about 12.5 volts.
The field wire to the alternator, seems to have a line loss of about 1 volt, tests a volt lower than battery voltage. Could this cause the alternator to overcharge and maybe damage (overwork) it? Could the battery have a weak cell, causing the alternator to work a little too hard? All of the above?
Where might I check for the line loss in the field wire? I learned that troubleshooting, without a plan or destination under the dash, can cause more problems than it cures.
Grounds are clean and tight, no noticeable electrical gremlins. System seemed to charge, a little high (15 volts at startup), with all three of the alternators. But they all seemed to settle in at 13-14 volts after running awhile.
Any imput would be appreciated, I´m gonna be seriously upset if I smoke my new alternator.

Thanx MudderChuck

Sounds more like you've just had bad luck with used alternators. Neither failed for the same reason. Since you're in there playing with everything, why not upgrade the wiring?

http://www.olypen.com/craigh/charge.htm

That link should help. As for the 12.5 volts across the battery terminals, not really sure what it should be though it sounds like it's right. Only way to get more would be to put one of the batteries that have three terminals, neg-12-14, used in racing, and I don't think they'd fit in the battery tray. You could reposition it in the rear if it was in a battery box.

A fully charged battery should read about 12.5-12.7v at rest, so 12.5v sounds OK. It's best to also test it with a hydrometer. Sometimes a worn-out battery will read normal voltage, but will still be weak.

Your charging system is working normally, with higher voltage shown at start-up, and quickly dropping down to 13-14v.

A line loss of 1v in the field wire sounds like a lot to me. There should be no appreciable loss at all. I would replace the field wire.