While most of the general ideas above are correct, some are a little off. Older EMDs did in fact draw current for the grid fans from dynamic braking effort, the more braking effort exerted the faster the fans went. I can't speak for older GEs however. ALCOs grid blowers functioned similarly. My experience with Dash 8 GEs all the way to the newest GEs is the grid blowers are powered separately, where that power comes from I don't know, but the blower motors are AC powered even on DC traction units. If you put the unit into dynamics with the unit standing still, the blowers turn on, but at a low speed. As the current increases the blowers are sped up to match. New EMDs (SD70ACe) function similarly with the grid fan, even at a stand still the grid fan is running, albeit at lower rpms. I don't think the 70Ms ran the fan with the unit standing still. Any modern unit emitting any kind of glow from the grids is very bad, and a sign of impending failure, there should always be sufficient air flow to prevent enough heat to build for the grids to glow. That said, some older units, e.g. GEs U25B or C, had its dynamic grids at the rear of the unit, inside of the radiator air intake grilles in plain sight, and could be seen glowing under heavy dynamic braking.