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I posted this previously in my thread about the S1 rebuild, but I think I've got a pretty good mapping of all the PS2 components that get connected for a 3V premier steam setup with the 10 pin tether.

This particular engine also uses what's called a MUX setup (I have no idea what it means) but my understanding is that the MUX converts several operating signals for lights and features into a single digital signal so so multiple lights can be controlled using one wire through the tether instead of one wire for each light.  At least that's what I understand.

There are a couple of lingering questions I have on this schematic and I've put them directly on the drawing, but I'll copy the main ones here:

  1. Why do the motor leads go to the MUX receive board in the boiler?  Does the MUX convert that power to DC for the lights?
  2. Is the purple wire that goes from the MUX transmit board to the tether to the MUX receive board the signal carrier?  Seems like it, but I also see other schematics marking it as "PV" which might also make sense because I can't find any way the boiler MUX is powered I think.  There's just all of a sudden PV available at Pin 1 of the 7-pin MUX connector.  So is it the purple wire that brings DC power, or is it rectified from the yellow / white?
  3. Is there anyone who can map pins 4 through 6 on the 7-pin boiler MUX board to a corresponding input on the tender MUX board?
  4. In the tender, why are pins 1 & 3 on the 8-pin PS2 connector jumpered? (I've seen them labeled "Mars" and "Ditch" respectively).  Are we assuming that the mars and ditch lights will always work together (note there is neither on this particular engine, so I don't know if it's important anyway.)

I've attached a JPG and a PDF if the JPG is not clear enough.

Thanks for your help!


PS2 S-1 Schematic


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  • PS2 S-1 Schematic
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Last edited by Jeff_the_Coaster_Guy
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John, I think I get it, but not sure. You're saying the motor leads feed the MUX board DC power, but how is the board powered when the engine is in neutral? In my tests here, the headlights and cab lights, which are controlled by MUX, are definitely turning on when when the loco is in neutral. So in theory there isn't any DC power coming through those leads? What is feeding the headlight?

It looks like there's a couple of diodes and I think a red zener diode on the boiler MUX. I'm guessing they make sure the lights are always fed the correct polarity regardless of the motor direction current. And there's a couple capacitors, not sure what they're doing.

Am I getting close?

Also, it looks like your digram (way cleaner than mine, but it looks like I got everything right) doesn't account for the tether connector board having two connectors on the PCB. It confused me because pin 1 on the tender side connector isn't necessarily pin 1 on the boiler side connector. I spent some time matching them up with my meter yesterday and the result is in my drawing.

Thanks for the tips!

I've always wondered a couple of things about this steam wiring too. The diesels are direct wiring. Some times I've used those boards in steam and don't have the MUX boards. I just steal some wires for things like headlight power, etc.

So the tach blue wire I have connected on the Williams engine right now. It appears that the tach grey wire is positive voltage and is shared with smoke fan positive pin 4. The orange wire must be connected thru so I'm guessing that's the tach's output for the board to read?

So where can I jump the blue tach wire to, to gain another wire on the tether? Did I see it correctly that it's the tach's LED output?

I've always connected it thinking it should be, yet I know on stock steam wiring, only the orange goes straight thru.

The motor leads are diode-or'ed to provide the PV voltage, that's the 20-22 volt unregulated DC voltage that is used for motors, lighting, couplers, etc.  I suppose you could consider it a "return", just a positive one, it's positive compared to DC common on the PS/2 boards.  The only reason they did that was to save a wire, one of the motor leads are always at PV through the drive relay, so it was a cheap way to get the PV to the locomotive without using an extra wire.  Even in neutral one side of the motor is sitting at PV, it's the other side that is controlled by the FET that determines the power actually supplied to the motor.  The FET provides the PWM pulses to DC ground to determine the average current fed to the motor, obviously in neutral that current is zero.

It looks like there's a couple of diodes and I think a red zener diode on the boiler MUX. I'm guessing they make sure the lights are always fed the correct polarity regardless of the motor direction current. And there's a couple capacitors, not sure what they're doing.

PV  is always positive, so there's no need to "insure" the lights get the right polarity, that happens anyway.  The two diodes are likely to create the PV voltage as I previously described.

Also, it looks like your digram (way cleaner than mine, but it looks like I got everything right) doesn't account for the tether connector board having two connectors on the PCB. It confused me because pin 1 on the tender side connector isn't necessarily pin 1 on the boiler side connector. I spent some time matching them up with my meter yesterday and the result is in my drawing.

Since My drawing is right from the MTH documentation, it's hopefully pretty close to correct.   Your comment confuses me, my diagram clearly shows the mux board connected to the tether, that's the second connector.

Not suppose to be posting those diagrams John.

The PS-2 board always has one of the motor leads hot at PV.  Yel in fwd, wht in reverse.  The board FET controls return to ground for motor control.

So with 2 diodes, one on each motor lead, the MUX now has PV available.  Yes the Head Light wire in 10 pin becomes the MUX engine signal.

Be careful what you do with tach.  The blue PCB return goes through a 181 ohm current limiting resistor on the PS-2 board when using 7 pin connector only.  If you use one of the other PCB grounds, you smoke the Tach reader immediately.

The MUX has the resistor for the blue tach wire.  This is why no speed control when you try to run an engine with the shell off and the ten pin from shell to chassis unplugged.  Orange is pass thru.  Gray 5V is common and ok.

This is also why if your swapping tenders of dissimilar wiring, you can smoke the tach reader, or worse for some engines.  G

@GGG: Thanks for the feedback.

To be clear, you're talking about the blue tach wire in my diagram that is connected to the tether pin #6 on the boiler side via a black wire?  Some of the pins on the PCB are connected through to the opposite side, but pin 6 on the boiler side tether connection is only being used as a convenient spot to tie the tach blue wire to pin 7 on the MUX pin board.  I assume then the resistor is between pin 7 on the MUX board and pin 5 on the 6-pin MUX board before going back to PCB ground in the PS2 board through the tether.  This clears that up for me nicely. (See yellow highlighted wire below).  The orange tach wire passes through the tether via pin 10 on the boiler side connector, through a trace to pin 6 on the tender side connector.  So I'm pretty sure I've got that right.

@gunrunnerjohn: I think maybe I'm still not describing it correctly, but I was talking about how the PCB that holds the tether connector actually has TWO 10-pin connectors, one pointing towards the boiler and one pointing to the tender.  Although the PCB has traces that connect some of these pins together, they are not all connected by traces (such as Pin 6 on the boiler side) but some are (such as pin 1 on the boiler side being connected via a PCB trace to pin 9 on the tender side, see red arrows below).  But I'm confident that I am showing the internal traces correctly now.  What confused me is that the pins are not 1:1 from one to the other, meaning traces are connecting dissimilar pin numbers. The PCB is outlined in Gray as "harness PCB" and everything between those pins is actually a PCB trace.  Only wires if they come outside the rectangle. (also, sorry for getting you into trouble with G for tricking you into posting those diagrams... )

I've ordered some 10-pin connectors to tie together the stuff in the tender shell (still need a backup light, anyone got one in their parts bag?) but I feel like I'm about done and am going to close this up soon and see her run.  I've got some practical questions I'm still hoping to get feedback for on my S1 thread.


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  • mceclip0

No I was answering Joe but describing in basic terms.  Remember all signals must come from the tender PS-2 board.  Mux needs 5V and PCB ground, plus activation signal (HL circuit wire) from the tender mux.  Engine needs PV so the mux makes it via motor wires. Tach needs current limit and they use the mux board to do it.

So what goes up and what comes down can vary by engine but is similar design wise.  Some mux are on chassis, other in boiler but lights on chassis etc...

Not all 10 wires have to be used it is just a jumper so boiler and tender can be separated.  Some signal up do not go through mux like fan return and element heat return.  They are PS-2 board controlled.  But they have to get from chassis connection up to smoke unit.   G

@GGG: Got it.  I'm pretty confident that I've got all the loco connections and wires correct now.

I only have one lingering question, and it has to do with the reverse lights on the tender.  As I mentioned before, I stole this PS2 card from a RK diesel, so the "reverse" lights are not wired form the card to really reflect the S1.  I'm confident that the backup light (the lamp that's centered on the middle of the tender) is run from the purple PV wire (at pin 1 on the auxiliary tender board) and through the green wire back to pin 4 on the 12-pin connector.  I would assume that the S1 sound file will activate this wire when running in reverse.

But there are also two corner "marker" lights at the top corners on the rear of the tender.  They are small red LED's.  I'm wondering if anyone knows which wire those can be driven from, and are they supposed to be lit continuously or only when reversing the train?  I imagine they can be driven from one of the pins on the 8-pin connector, but since they're LED's and not lamps (light the backup light) can I still just run the positive side from the purple PV connection (see my chicken scratch sketch in red).  And which pin will the S1 sound file activate properly for those? Will either of the yellow wires work?  This is the last connection I have to figure out, then I'm pretty sure I've got it wired 100% per original plan.

I also tried my hand at making a complete harness with a 10-pin Hirose connector to connect the tender shell mounted stuff to the auxiliary board, and it took me a while to figure out how to crimp those tiny little things properly (yes, I have a decent crimping toop for the open crimp terminals, but they're so **** tiny...)  But, I'm happy today I've got a harness with the smoke switch, volume pot, and charge port (even though it's not being used) plus the Molex pigtails for the rear light and rear markers!  I'm quite proud of myself!


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  • mceclip0
Last edited by Jeff_the_Coaster_Guy

Look at the PS-2 or 3 Steam upgrade wiring or even PS-2 diesel in the upgrade instructions.  The markers are run off the 8 pin, you can choice either front or rear.  It uses a 5V signal off the 8 pin along with the return pin which has a current limit resistor on the PS-2 board for LED markers.  Markers are series wired with PS boards.  G

So after satisfying myself that I got all the connections right (thanks @GGG and @gunrunnerjohn) I taught myself how to properly build a wiring harness that I could use at the auxiliary tender shell connector. It's a 10 pin Hirose connector that allows the electronics mounted in the tender shell to be disconnected when reviving the shell.

It was not easy manipulating those tiny terminals and figuring out how to use the crimper properly, but after ruining a couple dozen terminals, I finally got the hang of it and built myself a harness worthy of the MTH factory!

I also had to rig up a mount for the smoke unit since the package of stuff I got from the auction for this engineering sample didn't include it. As fortune would have it, they DID include two extra "seat" brackets that the engineer and fireman sit on. They worked perfectly (almost too perfectly) as a smoke mount after a little epoxy work:


Also had to build a custom tach mount since the pittman motor I got from Lionel didn't have the plastic mounting ring.

So finally after triple checking everything, and mounting and plugging in all the boiler electronics, I hooked up to my track power Molex connector and stood back and... Success! Video below.

The Diesel sounds are a remnant of the E9 that I stole the board from, and predictably the incorrect sound file meant no chuffing smoke. But that'll be fixed with the correct file when I find someone local with a programming track.

Thanks to you guys I now have a fully functioning gorgeous PS2 loco from the MTH archives. All that's left is to finish applying the brass adornments.  We decided not to paint, the flat gray and brass look is just too unique.

One final question: I didn't have enough connections from the MUX to hook up the number board lights. Can I run a third bulb from the same circuit that runs the firebox and cab lights? Can the board handle the current of three bulbs?

Thanks again guys!



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