Thanks, cb&q bob. Our locos do not have a/c in them (the a/c was removed to save on maintenance as it doesn't get that hot here, or so management thinks as they sit in their air-conditioned offices...), and so the climate control tab on the FIRE screen only has "vent" and "heat"--which is why I'm so interested in this fresh air make-up blower. If the climate control system (HVAC) is not turned on, does the fresh air blower still operate? I doubt it, as I don't feel any air coming out of the cab vents when the switch is on but the HVAC is off. Just checking, though. Last post here, I promise!
I believe that blower runs whenever the breaker is on but it only draws air in to the compartment under the cab floor where the main blower in the heater or HVAC unit can pick it up and circulate it.
I tried looking in several operator's and maintainance manuals at work and I can find very little information about the blower other than that the breaker supplies power to it. Maybe the locomotive manufacturers dont even know what it's there for! Other than adding cost to the locomotive.