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@Lou1985: See photo attached. The tender harness is a typical 10 pin, and I'm still really stumped on that secondary tender connector inside the tender. To be fair, this board was not original with the S1, it is stolen from a RK diesel.  So maybe I'm supposed to have a separate 10pin-10pin Female-female tender harness that goes from the loco connector, through the tender to the vertical connector, then somehow that feeds the board? The tender cable / harness you see here WAS already pre-wired to that connector, so It gets used somehow.


I've played around quite a bit with the boards and wiring and I've got some more things figured out, but that odd 10 pin connector mounted vertically in the tender is still confusing me.

I have found the connector that attaches the PS2 board to the MUX board in the tender, and I can sort of trace what wire do what based on the pinouts of the PS2 board.  There's a purple wire that runs through the tender harness to the loco and I'm thinking it has something to do with MUX board power or something, since it doesn't seem to be right for the "Normal" PS2 outputs.

On the engine side, I figure out that there are too may lights to be connected just to the RX MUX connector, and I realized I needed to install a secondary light distribution board (I had one sitting around).  So I just need to figure out which wires I still need to add to the empty MUX pinouts and run them to the aux board, then I should have enough connectors for all my lights.

I also think that since this particular PS2 board was stolen from a RK diesel, there are several pinouts that I could / should connect from the PS2 to the tender MUX in order to make sure I can take advantage of all lights in the engine. (photo is my own aux board I had from another project, so colors and IDs don't really apply.  I've got 4 wires I can use already (just need to plug them into the correct MUX pinouts) and one more location I can solder another wire to for 5 total jacks available.


For the lights, I can see (pretty sure these are right)


  1. Yellow connector with B&W wires : Marker lights in front of number boards (Grain of Wheat red LEDs)
  2. Blue connector with 2 black wires: Headlight
  3. Black connector with 4 black wires: Number board lights (mounted to the same bracket as the headlight, this was easy)
  4. Red connector with Purple & Brown wires: Smoke motor
  5. White / Olive Green Connector with Green & Gray wires : Smoke Heat element
  6. Black connector with Orange and Purple wires: Connects to Firebox and cab lights
  7. (Not shown in photos) Olive green connecter with black wires is another clear white LED with about a 10cm harness that I can't figure out where it goes. It's the same LED as the headlight.  Can't find any holes in the loco shell that make sense for it.  Is there some sort of beacon light or flasher?  Or does it somehow go in the tender as a backup light or something?

I figure any of the connectors that don't have a mate mounted directly to the MUX RX board, then I can use the auxiliary light board.  I just need to figure out where to add the pinouts to the empty MUX slots and then plug the correct connectors to the aux board.

There's a couple additional Overall photos attached, as well as this photo of the engine tether connector:


If anyone on this board has any photos of the complete S1 guts fully installed, it would be a huge help.  But I know that's asking a lot.

Once I figure out all of the electrical connections, I'm going to set it all aside and paint the shells.

Thanks for everyone's help so far!


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Last edited by Jeff_the_Coaster_Guy

The tender to locomotive tether doesn't plug into that 10 pin connector in the tender? Next time I have one of my factory PS2 3V locomotives on the bench I'll pull the tender shell off and snap a picture. One of the techs might have a wiring diagram they can send you.

PS2 3V boards are the same between diesel and steam. Same connectors and everything. So that board being out of a diesel has no effect.

Ok, after @Lou1985 and @gunrunnerjohn's replies, now I'm even more confused.

The tether that I have has a single 10 pin female connector that we're all used to seeing, and it appears to have all the same wires coming out on the tender side as the tether connector at the loco: 3 encoder wires blue gray orange; 2 power wires red black; 2 motor leads yellow and white; 2 smoke leads blue and brown; 1 mystery purple wire that comes from the MUX. The tether appears to match the loco pinouts exactly.

The wires come out of the tether and are configured with the standard Molex inline connectors and they plug right into the PS2 board. So I'm pretty convinced that the tether you see in my photos plugs to the loco and to the PS2 board, not to the odd port in the tender.

There would be no sense in plugging that tether connector into the 10 pin connector on the tender, it would just run in circles (and leave me without connection to the loco). But even if I got a different tether that had female 10 pin connector on both ends, I would then be missing the Molex connections at the PS2 board.

It really appears that the 10 pin connector mounted in the tender is some sort of way to connect an auxiliary tender?

A photo would really be helpful if anyone has a way to share one.


Last edited by Jeff_the_Coaster_Guy

Sorry, I didn't look at the pictures all that closely, I was reacting to the text and misinterpreted what you were describing.  The connector circled below goes to the switches and volume control on a small PCB in the tender shell.  There's a harness that comes down and connects to this connector so you can separate the shell from the tender frame.

If you're missing that PCB and harness, you'll have to trace it out and wire something up for the switches and pot.


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Last edited by gunrunnerjohn

Ok that makes more sense. I'm probably going to see if MTH had any of those boards in stock or I'll see if anyone here has one for sale before I try to cobble something together.

Does that PCB only act as a distribution board, or is there any voltage regulator or logic in it? If it's just a bunch of connectors and traces, then I should be able to rig something up way easier.

Still need to figure out how/where to add wires to the loco MUX board to control my additional lights.

By the way... What the heck does MUX stand for?

It's just a connection board, all the logic is down below.  I've never seen so much as a resistor on any of them.  Look under the hatch on the tender shell and see what switches (or holes for switches) are there.  The only S1 I find with PS/2 on the MTH site is the 2003 model which apparently wouldn't have 3/2 rail capability, so I'd only expect to find a smoke switch and a volume control.  Not sure why they have ten pins, so maybe this was a prototype that never made it to market.

@gunrunnerjohn: the tender shell  appears to have locations for two switches and a volume pot.

I'm in the process of trying to map everything out, but thought I'd share the attached photo.

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I'm thinking red is volume pot. Green is smoke switch (says so on top of the shell with cast-in lettering saying "smoke unit on/offc)


Orange has cast-in standoffs so the switch that fits there would be deeper / larger.

The tender appears to have two holes in the back for backup lights. Maybe something to do with that?


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John, this was one of the engineering samples from the MTH auction.  It came incomplete, but with most of the parts needed minus the board and motor / drivetrain (which I've since procured).  So that's why there's no cover over the switch ports yet (although I do have them).

Yes the charge port makes sense to me, should have thought of that.  I've got a BCR for it, so I will just leave that hole alone.  I think I've figured out most of the pins on the internal tender connection that we've been talking about.  I'll share a photo / diagram of what I think is going on later.


@Lou1985 posted:

Do you have the harness with the switches and charge port that mounts in the tender shell?

No, I don't.  But I do have a few random switches and vol pots laying around that I can use.  I'm not going to need the charge port since I've got a BCR installed.

Although just having that harness would be helpful to understand what the other pins went to.  I'm 79.2% sure it's for rear backup or marker lights based on what I'm seeing here.  But not 100% just yet


I think I've mapped everything out best as I can tell.  There's a few lingering mysteries, mostly because I don't understand how a MUX works.

I've attached a jpg of my wiring schematic as I have it now.  I named the pinouts as they are connected, unless I wasn't sure then I used pinout names that @gunrunnerjohn used in his diesel PS2 pinout diagram he posted a while ago.  I also included a couple of notes.

Here's what I am thinking / questions I have now:

  • The version of the S1 that I have (was an engineering sample) does NOT have a backup lamp.  The only location for any sort of lights in the tender that I can find is a pair of red grain-of-wheat LEDs that go in as markers in the rear corners of the tender.  With that being the case, I'm wondering if I need to connect those lights to either pin 6 or 7 on the mystery auxiliary tender connector:
  • The 8-pin connector on the PS2 board has pins 1 and 3 jumpered, with pin 1 going to the AUX connector in the tender. But pin 5 (a gray wire) also goes to the AUX.  I am assuming that one of these two should drive the rear markers instead of the green wire that would have gone to the backup light?
  • Why are the orange pins 1 and 3 jumpered like that?
  • I don't understand the purple wire going from the MUX in the tender to the tether connector.  It doesn't seem like it's Positive 6 volts like the other purple wires.
  • There's also the doubled up gray wires on pin 4 of the tender MUX that should be +5volts but I have no idea where that power comes from. It doesn't appear to connect to any source that can generate 5 volts, unless its done up in the front MUX board and that's what passes through the tender?  Then I don't understand the smoke motor circuit and how it is used on the main PS2 board.
  • After looking over the entire loco shell, I don't think I actually need the auxiliary power distribution board that I thought I did.  The only output that I can't find a home for is the front marker lights.  There's a pair of them that are wired to a connector in parallel, but no place to plug them in.  Is it possible that these can be one of the empty pinouts on the MUX boiler board? And if so, what pin of the transmit MUX corresponds to the empty pins of the receive MUX? It's not like I can just test continuity...
  • There's a bunch of internal traces that connect the two 10-pin tether connectors on a PCB at the tender tether mount in the loco cab.  Although I can't be 100% certain that these are the correct interconnects between pins, this seems like a logical arrangement based on the connections forward to the boiler MUX and smoke units. If anyone has a better diagram of this PCB with the traces, would like to see it:

I *think* I'm pretty close, but would be really nice if @Lou1985, @GGG, @gunrunnerjohn or anyone else who is smarter than me would check it over before I apply any power and risk blowing up my fancy board.

Also, this was my first crack at using AutoCAD's electrical schematic tools.  It's not the Mona Lisa, but I think it's clear enough if you zoom in on the attached overall jpg to see the details. Open or save the overall image from the attachments bar.   I tried to keep the colors of the wires correct per what I actually have on hand.

Edit: Seems like the image uploader compresses the JPG to the point that you can't read any text.  I also attached a PDF if anyone wants to download and look at a clearer version.

Thanks again guys (and gals?).



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Last edited by Jeff_the_Coaster_Guy

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