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I was looking on some of the auction sites at Atlas engines and I see several units advertised with TAS smoke units.

Can someone give me a heads up on their performance??  Where would it rate with the comparable units being MTH/Lionel Legacy. All I know about the TAS unit is they have a thermsistor that shuts the smoke unit off when fluid is low??

Thank You!

Original Post

I have a couple of Atlas CN units with these smoke units.

In order of smoke volume - MTH - Lionel Legacy - Atlas, and not too far behind Lionel.

I like my smoke and find the TAS units very satisfactory.

And yes, there is a thermistor right next to the element.

I never let my smoke units run out so I can't confirm their operation but I'm confident they work.




the TAS unit was designed to be an improvement over what was available in the market 15+ years ago. It uses a thermistor combined with a microprocessor to monitor the heating element and keep it within the optimal operating range. When the element temp increases, due to low fluid, poor heat sinking, etc. the thermistor cuts power to the element until it drops back into optimal range. 

FWIW, we designed that unit using what is now Lionel Premium Smoke Fluid (this was the TAS smoke fluid we sold "back in the day"). 

Let us know your thoughts when you receive the model and have a chance to run it!

Thank you,


I have an Atlas O GP-35 with the TAS smoke unit control board,  no smoke with the slide switch is on, the fan runs with no issue. Placed 10 drops of smoke fluid in the wick reservior, the resistive element is not heating, what should I look for to resolve this issue. This engine was purchased new and has no active run time just removed from the box, I used a continuity tester to check the wiring from the TAS control board to the 4 solder connections on the green circuit board of the smoke unit continuity was okay for all connections. From the Atlas O parts drawing for this engine no circuit boards(TAS) are available for replacement.  

Last edited by John Ochab

I wondered why this semi-old thread had been revived. Units like those that TAS produced are still made somewhere because 3rd Rail still use them. But where you might get your hands on one is hard to say although they do come up from time to time on the auction site. 

But based on my own experience with these units, I think that while it may be the case that a component on the PCB board has failed that seems unlikely with an engine that has no run time. I wonder about your switch and whether you are getting any power to the resistor - you mention checking continuity but what about voltage at the resistor terminals?

I had a look at a spare 3rd Rail smoke unit I have and it was clearly designed to be controlled by TMCC functions and is otherwise “dumb” with no logic on the PCB. So another option may be to get an old Lionel TMCC smoke unit and try that.

Hopefully one of the experts who knows exactly what components are on the PCB can chime in about whether one of them might need to be replaced. A photo of your unit would help.

After my previous posting I did a web search on the TAS smoke unit and some some information. On the Atlas O GP-35 engine that I have the control unit circuit board is seperate from the smoke circuit board, base, fan, resistive element and thermistor. This was done because of space limitations, the smoke control board is located on the F End whereas the smoke circuit, base, fan, resistive element and thermistor are located near the mid-length of the engine. The four wire leads I checked is the wiring between these two boards, I will check today whether there is voltage across the resistive element. I will fluff up the wick material so that the resistive element and thermistor is embedded in the wick with the smoke fluid . If there is sufficient fluid in this wetted wick the thermistor should allow the resistive element to heat up to create the smoke. At the present time the fan works , if the fluffed up wick and smoke fluid does not correct this issue, then I will have to check the input wiring to the smoke control board on the F End or shut off and not use the smoke unit. This engine with the exception of no smoke works fine in the conventional and command mode.   

Last edited by John Ochab

 ??Thermistor??  not properly covered with fluid, as Mike R mentioned above, that is design to inhibit Smoke resistor burn-out.   With the fiber wadding, in the smoke fluid chamber, sometimes the Thermistor is not positioned properly.   Too close to the Resistor, another possible problem.   The blue thing. 


Last edited by Mike CT
Mike CT posted:

 ??Thermistor??  not properly covered with fluid, as Mike R mentioned above, that is design to inhibit Smoke resistor burn-out.   With the fiber wadding, in the smoke fluid chamber, sometimes the Thermistor is not positioned properly.   Too close to the Resistor, another possible problem.   The blue thing. 


That is a very good point. At least a couple of the 3rd Rail steamers I have required the spacing between the thermistor and the (ceramic) resistor to be adjusted - as in increased to something like 1/8th inch. Many Legacy smoke units now have thermistors and the same problem arises with them, exacerbated by the fact that their wiring is not as robust as in the unit shown above so the terminator is pretty easy to get in the wrong location when pressed into the smoke wadding.

I may be repeating what other people already know but some while back Jon Z. explained that the thermistor senses heat through the fluid in the wadding so if it's dried out that may cause smoke output effectively to stop - although there will always be some power to the resistor. 

P.S. My TAS unit doesn't look exactly like yours - it's bigger but I'm aware, as GRJ mentions, that some of these didn't have all the electronics on the unit itself. Somewhere I have seen reference to cutting part of the PCB off to split the unit into two parts.

Last edited by Hancock52

Hey Gunrunnerjohn,

I just picked up a project Atlas GP-35 with Train America Studios EOB electronics (factory installed). The seller noted it didn't run and didn't have an idea why. I took a risk and picked it up cheap. I found a 6.3V 1000µf capacitor on the mother board had blown out. I replaced it and everything works except for the smoke unit. The fan motor was slowly spinning backwards while twitching. My thought was an issue with the wiring so I traced them back to the mother board. The hot and ground from the motherboard to the smoke unit board appeared to be switched. BUT, everything looked to be factory pinned? I switched the pins and the fan works properly but the heating element doesn't work. I checked the thermistor? and that's good. I checked all of the components on the smoke unit's board (it's separate from the smoke unit's body) and found nothing wrong. I can't figure it out. I jumped 2 of the 3 solder pins for the semiconductor (triac T2322BG next to the 4 pin plug) on the board and the element heated up. Do you think the triac is bad? How robust are they? Thoughts?

***I saw that you have the wiring diagrams in PDF on another post. Would you be willing to email them to me?***

Last edited by DanVW

Hard to say what the issue is, the triac is controlled by the uP on the board, so there may be something else going on.  I haven't spent much time trying to get these to work, I've just popped a Lionel PCB on them and used them as a dumb unit.


I bypassed the triac and in turn the thermistor circuitry as well. The heating element and fan unit turn on and off in command mode as normal but it no longer has the thermistor function as a fail safe. Do you have the wiring diagram for this loco by chance?

Nope, no wiring diagrams, but using TAS, the wiring is pretty set I would imagine.  How they route the wires, that's another matter.

@DanVW posted:
***I saw that you have the wiring diagrams in PDF on another post. Would you be willing to email them to me?***

Not sure what you're referring to.  If you mean the TAS documentation, that's in the TMCC/LEGACY REFERENCE MATERIALS & MANUALS thread pinned in the TMCC/Legacy sub-section.


I got everything working again. The PIC chip in the TAS EOB board originally had a prong bent/folded to prevent it from engaging the PIC base. I had straightened it thinking I may had accidentally bent it. Nope. It’s supposed to be that way. I pulled another PIC chip out of another EOB board and the last prong was folded inwards. I straightened it out and the EOB board is no longer causing a short at the transformer. I also fixed the smoke unit. I replaced the triac and it solved the problem. Thanks for your help!

Hey guys,

In answer to where the Sunset 3rd Rail smoke units can be purchased would be Roys Trainz - $40.00 for a new unit and postage is $5.00. I think he carries them exclusively.  Sunset 3rd Rail, even though they have ERR, don't have these smoke units on their site and that is probably why, as Hancock52 noted above - they are hard to find.  They are supposed to be an improved version of the older TAS unit.  They are supposed to smoke just a little less than MTH or newest Legacy equipment but more than anybody else. They do not have the thermosistor in their design.  I hope this can help someone.

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