overcharging alternator

We have a 9170 Deutz Allis that we have been fighting charging problems for a few years. All of this started after the cab was pulled off to fix the rear end. The problem is that it charges around 14 and jumps to 16 and maxes the gauge out and jumps around. It cooks out batteries. When running at night the lights go bright and dim. The dealer is at a lost and doesn't know what to do. The tractor was is there this winter when it quit charging. A wire under the dash was burnt off. I was driving it there when the cab filled with smoke and it started charging. They have replaced the alternator about 4 times, key switch, ac blower, ac switch, batteries, cables, wire from key to alternator, pulleys, belts. They where able to get the alternator to start charging after the tractor is running and not rev to 1500 the last time but nothing else is right. We ran 80 acres since then and the batteries were bone dry and water ran out the bottoms when we filled them. We replaced them last fall. The only thing powered by batteries is a fuel pump, ac, gps, tool bar monitor, radio. We dont run much at night. The alternator has a 3 wire pigtail and a post for the battery cable. Can you explain the pigtail wires. I think one goes to the tach and one gets power from the key. The delaer was out today and they didnt find anything. We installed 2 new batteries and keep going!  Im open for any help. Thanks Brandon

I've found in cases like this that it is in the grounds. Make sure of the ground from the batt. to frame, or however it is grounded. Run a ground from the ground post on the alt. to engine. Also run another ground from the cab to the frame. Look for any loose connections. Check for any corroded or rusted connections, even the light switch. This may give you a place to start. Let us know how it turns out.

I have no idea of how that alternator is wired, but if it uses external sensing the sense wire could be intermittently shorting to the frame somewhere.

Sounds like it broke hub out of clutch.      MACK