What did my alternator in was the main contacts on the glow plug relay.
Even with the coil disconnected, they still erratically connected the 200,+ amp glow plug load.
This was discovered after premature replacement of the battery, twice, as the malfunction was erratic and wasn't showing up until I suspected this scenario and was watching it with a meter.
The 140 amp alternator, trying to hold 14.2 volts or so on a battery loaded with over 200 amps took the alternator out within a few days. Fried the terminal right off the back. The windings had been pretty hot just to look at them.
Incidentally, a brand new glow plug controller did the same thing.
That's when I bit the bullet and rewired to manual control with a White-Rodgers industrial contactor
That was about two years ago. Not a hint of my old woe resurfacing, as by now I know exactly what to look for...unexpected wags of the voltmeter, and my Carter electric fuel pump having erratic pitch. 200 amps on the battery rail is quite a load, as I can hear the pitch change that even the turn blinkers cause. Yes, it's a loud fuel pump.
If you suspect this, you have to disconnect the big 6 gauge wire from the glow plug relay to see if the unexpected voltmeter wagging goes away. Disconnecting the relay coil was inconclusive, as mine was failing regardless of coil current
Should you try this, I will suggest disconnecting both battery negatives first. That glow plug feed comes directly from the battery rail. Insulate it well and guarantee it won't see ground. I wrapped mine in bubble wrap and stuffed it in a toilet paper tube to make sure it wouldn't touch anything.
If you are still getting unexpected wags, this wasn't it. All I can figure out is that one company is making these relays, others are reselling then under their brand, and someone changed the manufacturing method. As I had experienced the same identical problem on two different brands