I just finished upgrading a PS-1 GP38-2 to PS-3 using MTH new PS-3 Diesel Kit 50-1912. Here are some items that will help others. The current instructions are not worthy of use. It is a modified PS-2 book, and if you have not done one before, or are familiar with MTH wiring protocol you can get yourself into trouble. For the tech and experienced folks this should help.
PS-3 Supplement instructions
PS-3 board has Flash Code 3239. When I tried using this with PS-2 Sound file or a PS-3 sound file there was no control of LED function other then Head Light on and off via DCS. I believe this is a test all code so you definitely need to load the appropriate diesel flash code for the engine you are upgrading.
This was a GP38 so I used a 2014 Sound file which worked fine with flash and sound file loaded.
Some things to consider:
In general Blue wire is Positive Voltage and used as common for couplers and Heater element.
Purple is common PCB ground and used for LEDs, Volume/Smoke Pot and tach reader common
LEDs are parallel wired when double
Marker LEDs are still series wired
The tach wires were soldered to the lower 3 holes, this prevents the tach reader from fully inserting into the bracket. I had to resolder them to the upper 3 holes.
No MARs 2 included.
No Beacon is included.
Smoke Heat PV is a blue wire not purple if you are following the instructions.
The Heater has 2 brown wires available for return, and the fan motor has 2 green available.
The number boards are all driven off one return wire. So 4 total LED controlled as one.
The Markers are also controlled by one wire. So you could leave an older PS-1 Marker harness alone, just rewire each LED set to series vice parallel if not already series.
Spend 15 minutes or so using the provided chart to identify the wires on the harness. You will need to untangle and I recommend removing wires not needed.
The harness has a switch for DCC/DCS. This is not explained in the instructions, but when connected you are in DCS mode. Open is DCC. It is a large switch and fits nicely in the Coupler or Smoke On/OFF slot.
Both Volume pots have the same color wire, so you need to identify via terminal number using the provided chart.
These harness are extra-long, and this is fine wire so I did not shorten, I just bundled neatly and used wraps to keep in order.
I removed harness wires not needed to make it neater.
LED fit nicely in the bulb insulators and they do light up nicely.
You do get Rule 17 lighting and interior light goes off with motion either direction.