Here's another decoder install I did recently on an MTH 4-8-4 Greenbrier. This is a 2007 catalog engine, so it's PS-2 with the wireless tether (but not 3-2 convertible, despite the catalog description). I got this beast from a fellow forum member, and it's in really nice condition cosmetically and mechanically. Ready for some new electronics!
I like to start with a complete disassembly, both for purposes of planning the decoder install as well as for performing a complete mechanical overhaul. Here it is with the old electronics removed, shortly after arriving in the shop:
With the compact size of the Loksound L decoders, it's easy to install them in the boiler, rather than in the tender, which really simplifies the wiring between the boiler, chassis, and tender. Here's the block diagram I worked up for this guy while planning things out.
As you can see, with the decoder and related electronics in the boiler, the only wiring that has to go to the tender is for the speaker and the reverse light. Similarly just the pickups and the motor need to be wired to and from the chassis.
The light gray box represents a polystyrene sheet that I cut to mount the decoder and other electronics on. It screws into a couple existing holes in the boiler, so it makes for a nice subassembly. The power board, regulator, and power switch are part of a standard approach I use for powering smoke units, and are described in detail in this thread. Here's a pic of the assembled electronics board, prior to snapping the decoder in:
Here are the power board, regulator, and power switch, with the two smoke unit connectors to the right. The red/black connector is for the track pickups from the chassis. The large harness to the bottom is one of the original MTH harnesses that I re-used for the speaker and reverse light connections.
And here are the four connectors for the headlight, cab light, firebox light, and number boards.
And here it is with the decoder and electronics board installed in the boiler:
The tender was fairly straightforward, just the reverse light and new speaker. Lots of room for a nice big speaker in there now... I reused some of the existing 10 conductor harness again here. I didn't wire the electrocoupler, as I'll be replacing that with a kadee, however that could also be accommodated as described in the referenced thread above.
The chassis was pretty easy also, just the motor leads and track pickups, as well as the harness to the tether.
Next, I 'buzzed out' the wiring with an ohmmeter. To do this I hook up everything electrically between the boiler, chassis, and tender, and do continuity and isolation checks on the adapter board. This is a really good step to perform to ensure everything is wired properly and to avoid smoking a new decoder. Things to check include:
- Isolation of ALL other connections from the track power and ground. This is vital, especially on older engines (including this one) where the chassis is also the electrical return to the track.
- Continuity of the red lead to the track pickups.
- Continuity of the black lead to chassis ground.
- Speaker continuity (5-10 ohms depending on the speaker)
- Motor continuity (anywhere from 25 to about 200 ohms)
- Lighting continuity. If using incandescent, look for a few ohms. If using LED, you can test using diode mode.
- Smoke heater (roughly 8 ohms)
- Smoke fan (varies, 100 to 200 ohms)
In this particular case, I found I had wired one of the leads for the reverse light to an incorrect solder pad on a connector. Nothing would have blown up as a result, but it's nice to be able to fix any issues like that before powering up.
With all that checked out, I applied DCC power to the track pickups, still without the decoder installed, and set the output voltage on the regulator to 5 volts. This powers the smoke heater and fan.
Finally I popped the decoder in, and things are ready to close up at this point!
After programming the decoder, as described in this thread, here's the end result. Amazing sounds (recording doesn't remotely do it justice, sadly), and enough smoke to fill the room, if you want.
How's the low speed performance you ask? Check it out on speed step 1...
Anyway, just bench tested so far, but this thing is going to be an awesome smooth puller on the layout. Just needs a kadee for the tender now... 
