stock alternator burns brushes

FWI
I recently had the pleasure of working on a 77 Dodge Sportsman van.
The vehicle ran perfect except when I pulled the lights on.Charge voltage was good. I felt the voltage to the coil was a little low. I cleaned everything in sight, charged the battery. Took her for a spin. Everything was good. Pulled the light switch and all the world was smiling. Turned the heater blower on and as soon as I came to a stop, the engine stalled. I full-fielded the alt(which was new) and it was putting out plenty. Charge voltage was right on.

Well I got to thinking. And at my age that sometimes runs into days. You might not believe this, but when I get a problem like this where everything checks out, but there is an obvious problem,and I get stumped, well I just walk away. I present the problem to my brain and let it work on it like it was a separate entity. Sure enough a couple of days later, while I'm doing something completely unrelated, the answer comes to me. Man, I don't know how that works but I love it!
The thought comes to me that the ammeter is defective. And it was. The shunt was not passing enough current on the run side thru what was left of it. So I fabricated a new shunt way bigger than it needed to be, and strapped it across the posts . And voila all is right in the world again.
Except of course the ammeter doesn't work,anymore.
No problem says the customer, I'll find you a replacement some day.
So moral of the story; just cuz the ammeter looks like it works, might not in fact mean; that it is working correctly.