Smoke Unit RAW

Well, first off, the silkscreen on the actual part and the nomenclature on that graphic is incorrect.

• 1 - Smoke Heater Raw Power (unregulated +V)
• 2 - Smoke Heater Control (PWM control to DC common)
• 3 - Common (+5v)
• 4 - Smoke Fan Negative (PWM control to DC common)
• 5 - Sensor

The smoke fan negative pin-4 and the sensor pin-5 both return to the Common pin-3.  Pins #1 and #2 have no connection to the other pins on the smoke unit.  Common is used somewhat interchangeably with some of these parts.

In case you're wondering how I know this, I sorted all this out doing this project.

Building a Lionel RCMC Board Test Fixture

Awsome! That does now make sense. Thank you John!

Any insight as to what temperature the smoke unit circuitry begins backing off the heater resistor?

@BobbyDing posted:

Any insight as to what temperature the smoke unit circuitry begins backing off the heater resistor?

As it turns out, the thermistor is only used in specific circumstances.  One time is on initial energizing the smoke resistor, they boost it to speed up the heating, then the thermistor is used to avoid overheating things.  I think the problem is, if during the initial startup they detect excessive heat, you'll have a problem until you fix the issue.  Typically the issue is spacing of the thermistor.  I've honestly never seen a bad thermistor, other than one that was physically broken.

That's interesting. I had assumed the thermisters mostly came into use for whistle steam, where the loco could run for hours while the smoke fluid for that function just sits cooking without ever venting. So your saying they also use it on startup to verify the smoke unit is working as expected?