My final hypothesis is: without tearing the model back open and is strictly based on the CV data shows voltage setting to be 255. That is full track voltage from the Decoder. I have come to the conclusion that on this Model 21985 and most likely on the 20752 is that the smoke unit voltage is controlled by the main circuit board of the loco and not by the Decoder. It appears they are only using the Decoder as an on / off switch. Stay tuned after Christmas ( Christmas Present and Wife won't let me play yet) as I will be testing another MFX Loco with poor smoke output. A totally different set up as it uses a pulsed smoke unit instead of a standard 5 volt unit. Richard