The smoke unit on my Legacy diesel keeps shutting down.

The resistor measures 8 ohms, which I believe is what it's supposed to be.

The thermistor measures 62,000 ohms--but I don't know what it's supposed to be. What is the correct specification?

Original Post

Good question, there is no information on what's in those, so I'll be interested to see if someone actually has some information on it.

why not just order the correct one from Lionel as John has said there is no information on the device ! just look up your engine model number and you fix your problem! then desolder the part and be done with your repairs! 

Alan 

Good question, there is no information on what's in those, so I'll be interested to see if someone actually has some information on it.

Lionel has told me that the thermistor should be 50,000 ohms +/- 2,500 ohms.

Since mine is measuring at 62,000 ohms, it's failed, and it's causing the electronics to think that the smoke unit is too hot and to shut it down.

As it happens, the thermistor is not available by itself. I need to buy the smoke unit board. But heck, I wasn't looking forward to soldering on the PCB, anyway. I prefer soldering jobs that require less finesse. 

Just curious, wondering why you can't order the smoke unit board through Lionel's website, but, instead, you have to call Lionel to order it?

It would be easy to replace if we knew what it was.  50K tells me something, but I still need more specifications.

There’s two type of thermistors,  one type the resistance goes down with heat, the other goes up with heat.  So which one does Lionel use?

Correct, NTC and PTC models.  Probably the only way to figure out which one they use is hit a thermistor with a heat gun and see which way the resistance goes.

I had a early Legacy Big Boy prototype that came in for repair from the Neil Young collection sell off.  It didn't have a thermistor installed, just a straight leads between the connectors where it would go which make me think it could be a thermistor that increases when getting hot.

Jim

Well, you have a 50-50 chance of being right Jim.   I think the circuit design would dictate which type of thermistor was used.  Either temperature coefficient model could be made to regulate the current, so I measure one to find out which it is.

It’s sad no one from Lionel would respond to this thread.  Help us out with such issues.  Isn’t that the point of this section of the forum anyway?

I don't have one of the later Legacy smoke units on hand right now or I'd fire up my meter and heat gun and find out what it was.

@superwarp1 posted:

It’s sad no one from Lionel would respond to this thread.  Help us out with such issues.  Isn’t that the point of this section of the forum anyway?

Lionel told me offline that the thermistor should measure 50K ohms +/- 2,500, which was enough to indicate that the problem with my smoke unit was the thermistor, which was measuring 62K ohms. I reported this above.

I already ordered another smoke unit board. Whether the thermistor is NTC or PTC is sort of taking it another step. I suppose that knowing the ohm rating and whether it’s NTC or PTC, someone else with a failed thermistor could get just a new thermistor from an electronics supply house. But my problem, as the OP, has been taken care of.

@Keith L posted:

Lionel has told me that the thermistor should be 50,000 ohms +/- 2,500 ohms.

Since mine is measuring at 62,000 ohms, it's failed, and it's causing the electronics to think that the smoke unit is too hot and to shut it down.

As it happens, the thermistor is not available by itself. I need to buy the smoke unit board. But heck, I wasn't looking forward to soldering on the PCB, anyway. I prefer soldering jobs that require less finesse. 

If you're saying 62K Ohms is causing the electronics to shut down from heat then doesn't that suggest it's a PTC (resistance rises with increasing temperature)?

50K is an unusually large value for a PTC.  For example, DigiKey has 243 choices for a 50K NTC, and 0 (zero) choices for a 50K PTC.  Can someone post a photo of the smoke unit board showing the thermistor along with whatever supporting electronics?  I realize it's a moot point since replacing the entire board is the only remedy.  It's just I find it curious that a circuit with a 50K +/- 5% trips at 62K which can be as little as 20% variation.

Last edited by stan2004

Right.  Without getting into the muck, a typical 50K NTC (i.e., 50K Ohms at 25 degrees C) that measure 62K Ohms only represents a change of about 5 degrees C....so 20C.  It makes no sense that a smoke unit protection circuit would shut down at 20C (about 68 F)!  I think there's something else amiss with the defective smoke unit... 

Not to mention that it's quite possible that the ambient temperature when you check the sensor could be that much different than the nominal 25C.  Suppose it's 68F, now we're checking at 20C.

Exactly!  It's a rainy day here and my indoor temp is 68 F (20 C).  A typical 50K NTC thermistor would read about 62K Ohms!  LOL.  Good thing the OP bought the entire smoke unit assembly. 

Hard to say what is really going on there, I'm betting the thermistor isn't it.

John, as my father used to say, "When you're right, you're right."

I installed the new smoke unit board. The thermistor on the old board measured 62K ohm; the thermistor on the new board measures 51K ohm.

However, the smoke unit still keeps shutting down. It runs 1-2 minutes, often less, before the cab flashes the dreaded two blinks. This is maybe a tad longer than before--but it sure ain't the solution.

I even tried putting in new batting and using different smoke fluid.

What other variables are there that could affect the smoke unit shutting down? It can't be that complicated, can it? I'm flummoxed!

No surprise there, I bought a couple of typical 50k nominal NTC thermistors on my last Digikey order, they were about 60 cents easy, no big loss.  On the bench in a 76F room, they're around 46K, if I grip them between my fingers, they drop to 30F, and if I hit them with 100C air from my hot air tool (measured with a K-type thermocouple), they drop to 4.5K.  I can hit them with air from my compressor and they shoot up in the 80's and 90's quickly.

Move the thermistor a bit farther away from the smoke resistor, and also make sure both the smoke resistor and the thermistor are firmly in contact with the wick material.

I'd be closely checking the wiring between the smoke unit and the RCMC board, and failing that being the issue, I think you may have a bad RCMC.

I can't find an issue with the wiring.

Poking around on the forum some more, I found that Mike Reagan said a few years ago that the diagnostic codes exist to--

"prevent burning out outputs on the RCMC (the most expensive part of the circuit). However, if you do want to risk burning out the RCMC outputs, you can cycle power on and off and you can reset the fault codes 3 more times, and so on. But be warned! If you keep this up you'll not only be replacing a fan motor or smoke element, but also an RCMC!"

After my SW-7's smoke unit started shutting off, with the cab light flashing 2 blinks, I reset the diagnostic code many times before finally opening the engine up to try to figure out what was going on. Does it seem likely that I damaged the RCMC outputs to the smoke unit and that is now the problem?

On the other hand, if I burned out the RCMC outputs--

-would the smoke unit start running at all?

-would all the engine's other functions still work normally?

If you have smoke, it's unlikely you cooked the RCMC.  My move here would be to INSURE the smoke unit itself was good, there were no shorts from the smoke resistor to the frame, and the smoke resistor is the proper 8 ohms.  I'd also test the smoke fan to make sure it's running with a 1.5V battery and isn't drawing more than around 40 milliamps at 5VDC.

Once I was SURE the smoke unit was 100%, I'd carefully inspect the cable that goes to the RCMC for any issues.  Finally, I'd run it on the bench with the smoke unit exposed without the shell on and see if it still has the error.  It could be something like the shell contacting the smoke unit and causing the short that is giving you the error.

Last edited by gunrunnerjohn

John, right again, nothing wrong with the RCMC.

Here's the bottom line:

The whole time, I've been testing the smoke unit on the bench with the shell off. The resistor is at 8 ohms and the thermistor is at 51K ohms (it's the new smoke unit board I put in last week). The fan motor tested fine. I played around a bit with the five wires than run between the smoke unit and the RCMC. I tightened a couple of screws on the smoke unit.

What was wrong? I still don't know, but after these last steps, the smoke unit is running fine again. I've let it run for quite a while, and it does not shut itself down. I'm thrilled to finally have the smoke back--and not to have to buy a new RCMC!--but it's a little unsatisfying that, in the end, I don't know exactly was the problem was.

In any event, thank you so much for all your help.

Common issue, I'm sorry I forgot to mention it.  The screws get loose on the resistor, and it presents a high resistance that indicates to the RCMC that there's a smoke element failure.  One of the first things I check when I open a smoke unit.

I can only take solace in the fact that the Lionel tech didn't think to mention this obvious failure either, we both whiffed on this one.

One oddity that I still don't quite understand.  Why did the new smoke unit still indicate the same failure?  I'd have thought that one would have the screws tight.

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