Lionel smoke unit not working

The smoke unit is a "dumb" smoke unit, and it gets both the fan and heater drive from the RCMC board in the center of your picture.  Did you check the resistance of the smoke unit resistor?  Note that the wiring is just a 4-pin plug to the RCMC board. If you can power the fan and measure the heater from the RCMC end of the cable, the smoke unit is probably OK.

In that case, try a board reset, if that doesn't do it, you may have a bad RCMC board.

Click on the diagram to expand to full size.

This is one diagram from Bruk Bannister's excellent Legacy documentation series, available from this link.

Bruk Bannister's Legacy Documentation

Thanks John for the info.  I tried reseating and that seems to have fixed the problem.  When I had the engine apart I took DC voltage readings at the smoke unit.  RAW = 19v, HTR=18v, FAN=2.8v.  As I varied the input AC voltage, the fan voltage stayed at 2.8V, but the HTR & RAW changed.  I am assuming these reading all seem normal?  What is the RAW designation for.

Thanks,

Bob  D

The heater gets a PWM voltage from the RCMC, and so does the fan.  I'm not sure what polarity they run, my guess is that you're getting bogus readings because of the nature of the drive.  You get the same kind of odd readings if you look at an MTH board smoke fan and heater.

Is there a reason why they are using PWM.  Are they varying the duty cycle of to change the effective voltage applied to the heater and fan?  

PWM is the easiest and cheapest way to digitally manage power to a load.  That's why most microprocessors have built in PWM capability.  That's also why it's so widely used in industry for motor control, lighting, etc.  PWM also dissipates MUCH less power in the switching components than trying to do a linear voltage or current control of the same load.