S Helper Service 2-8-0 TMCC Conversion
Super detailed engine gets TMCC

Great looking and running engine
Click on images for larger view.

The S Helper Service 2-8-0 was the most challenging TMCC conversion to date. Very limited space was available and the factory wiring used a +5 volt common, switching the device loads to ground. TMCC uses an opposite scenario using a common ground to the devices, supplying voltages to the loads. The engine could be rewired to accomplish the conversion but that would require a complete disassembly of the loco chassis, not a desirable option. 
This is the bare tender chassis. The rear posts have to be removed for the sound board clearance. 

The Sound Commander has a full complement of sounds including synchronized 4 cadence
chuff, whistle, bell, air pump, coupler clanks and blow downs.
The original loco wiring diagram is here  
The  loco disassembly instructions are here 
Be sure to remove the smoke tube first.
Re-assembly is not an easy task. I found removing the piston drive rods and piston guide assys. from the steam chest made it much easier to reassemble.
I wanted to do the conversion with as little modification to the loco as possible. Fortunately the wiring in the loco was all isolated from the chassis and track voltages in the loco. This allowed turning the +5 volt common into a ground common and then making the loco's original common return in the loco a -5 volt supply. Using the -5v supply made the polarity sensitive loads like the LEDs, Opto Isolator and Smoke motor happy. Basically it turned the wiring up side down. 
No extra room left in the tender... at all.
The smoke unit's 4.4 ohm element did require it to be replaced with a hand made one to be compatible with the TMCC driver triacs current limits. I made a replacement element using a piece of core from an old wire wound power resistor and wound it with nicrome wire to a resistance of 25 ohms. The smoke unit it a itty bitty thing but puts out adequate smoke. 
You can see the original smoke element. It is  very ingenious. It looks like an axial grain of wheat bulb in a brass sleeve inserted in a braided wick. Unfortunately it is too low a resistance for the TMCC driver triac.
The smoke works best with 17-18 volts on the track. I use A-Normal-1 smoke fluid due to its lower operating temp. It only takes 5-8 drops to charge due to the small smoke chamber. If you over fill and it will not start smoking after filling blow down the stack to clear excess. It will run 10-15 minutes between fillings.
I added a transistor smoke motor speed modulator synchronized with the loco's chuff sensor. At low speeds it has a very realistic effect with the stack velocity changing with the chuffs. I used a relay to sense the TMCC "smoke on"  voltage to supply the motor driver supply voltage. The 25 ohm smoke element is driven directly by the TMCC smoke driver triac. The smoke motor has it's own circuit board.
      The smoke board is on the left.
The electronics are mounted to a heatsink made from .032" aluminum that holds the Mini, voltage regulator, component board and smoke board.

The header connector is a Mouser part number 575-641001.

The electronics mount to the tender posts. You can see the header plug mated to the  tender's original circuit board. 
A second chuff opto isolator is used to translate the -5 volt pulses from the loco to a ground closure for the sound board chuffs.

The tender headlight connector wiring was reversed to make the polarity correct for the TMCC's voltage. 

The program/run switch is located under the tender's water hatch but should only be needed if you forgot the locos ID number since the Mini Commander uses SoftSet to program it.
The Mini Commander from the
Electric RR Co.  used custom firmware for engine functions that I load.

This is the schematic
The 2-8-0 is getting workout on the treadmill for some quality assurance.