MTH PS-2 Steam wire colors?

Depends on whether this is a basic engine, or one with mux boards.  You have to modify the engine for smoke and lights anyway so you can choose which to use.  G

@GGG How would I know whether mine is basic or mux? What would the color be in each case?

I don't want smoke or class lights, just the directional headlight.

Based on a wiring diagram I have, a wire went from the PS2 board to the headlight. Can't tell whether it was positive or ground.

Steam headlights are always on prototypically.

@gunrunnerjohn in that case, I just want it to be on.

Since you're really starting from scratch with the wiring, I'm confused about the wiring questions in any case.  The color of wire to use is the same color you connected to the headlight connection of the Cruise Commander!  I actually have a adhoc color convention I use for wiring ERR installations and using the MTH 10-pin steam tether.

• Red: Track Center Rail
• Black: Track Outside Rail
• Yellow: Motor
• White: Motor
• Green: Chuff
• Orange: Headlight
• Blue, Purple: doubled up for Smoke

If you're confused on the other end in the locomotive, your ohmmeter is your friend to find the mating wire.

It's not just one color. This is a huge piece of the problem. You need to REWIRE the loco because natively, headlight and smoke unit are tied to the motor via diodes. Going a step further, TMCC boards like the R2LC and thus ERR upgrades outputs 3rd rail "hot" outputs for lights.

As stated, the native system in a PS2 steam engine, the lights would be connected to PV- positive unregulated track voltage via diodes connected to the motor AND the PS2 control board PWM grounded the return line from the bulb to PWM approx 6V equivalent pulsed DC across the 6V bulb.

So no part of that is just you figuring out a single wire.

Again, you have to disconnect ALL of that. You have to remove the diodes or remove the PV wiring from BOTH the smoke unit heater resistor feeds, AND the headlight. You then have to change the bulb, and if doing that, have to change the wiring scheme to then frame ground the bulb socket to match typical ERR TMCC wiring, and use an 18V bulb.

The original colors aren't really 100% suitable for wiring TMCC.  The chuff, cab light, and smoke all have a common frame ground.  There's no reason to follow any color codes from MTH.  The exception is that the four 22ga wires should be used for power and motor, the other six wires are 28ga and can be used any way you like.