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Why does this loco run?

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  • Member since
    April 2003
  • 305,206 posts
Why does this loco run?
Posted by Anonymous on Thursday, September 4, 2008 8:35 PM

I am puzzled.  When putting a disassembled Spectrum USRA Light Mountain back together, I noticed there was continuity between the drivers.  That's a no-go I thought.  But the continuity remained when I checked the loco and made sure the pickups etc. were ok on the tender.  This loco has no decoder at the moment.

Well, that can't be, so I checked my Spectrum Heavy Mountain that I had put a decoder in, and which runs just fine.  There's the continuity between the drivers!  Moreover, there is continuity between the wipers on the tender.  And the drivers have continuity with both sides of the tender wheels.

But as long as I keep the tender wheels getting power from their correct side of the track, the loco runs.  All functions normal, all CVs reading and writing. (Although the light doesn't work.  I think it is just a burned out bulb and I will replace it with an LED next.)

How can this thing run with the left drivers in continuity with the left tender rear AND the right tender front?  And vice versa for the right drivers. And with each other even when disconnected from the tender.?

Does anyone have a Spectrum steamer that is wired this way?

I just checked my Spectrum 2-10-2s (same decoder installed into [I think]the same Bachmann boards) and there is no continuity between their drivers. 

 

 

 

 

  • Member since
    July 2003
  • From: Sierra Vista, Arizona
  • 13,757 posts
Posted by cacole on Thursday, September 4, 2008 8:45 PM

I think you're getting a continuity reading through the motor, and will continue to do so as long as there are wires connected to it.

  • Member since
    April 2003
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Posted by Anonymous on Thursday, September 4, 2008 10:04 PM
Thanks.

Well, yes, I thought this might be the case, as the unit runs fine. But why does this condition not exist in my other Spectrum steamers.......is it that they have different motors? All are new, but perhaps not of the same vintage. The two 2-10-2s were "DCC on boards" variety that I converted. But I think the Mountain was as well. It has a newer looking Bachmann circuit board than the Light Mountain I have in front of me.

  • Member since
    February 2001
  • From: Poconos, PA
  • 3,948 posts
Posted by TomDiehl on Thursday, September 4, 2008 10:13 PM

 Cisco Kid wrote:
Thanks.

Well, yes, I thought this might be the case, as the unit runs fine. But why does this condition not exist in my other Spectrum steamers.......is it that they have different motors? All are new, but perhaps not of the same vintage. The two 2-10-2s were "DCC on boards" variety that I converted. But I think the Mountain was as well. It has a newer looking Bachmann circuit board than the Light Mountain I have in front of me.

What you're calling "continuity" can also depend on the range setting of your ohmmeter. True continuity is zero ohms as read on the X1 ohm range (not Kohms or Mohms). Some motors, if you are reading through them, show a low resistance which could be interpreted as continuity. The easiest way to double check is to disconnect one of the motor leads.

Smile, it makes people wonder what you're up to. Chief of Sanitation; Clowntown
  • Member since
    December 2004
  • From: Bedford, MA, USA
  • 19,778 posts
Posted by MisterBeasley on Friday, September 5, 2008 8:53 AM

To a multi-meter, an incandescent light bulb also looks like a short (continuity.)  It's just a filament, which is a thin wire.  The multi-meter uses a very low voltage to test resistance.  If you give the light bulb more power (voltage and current) then the filament will heat up and start to glow.  The resistance of the filament increases as it gets warm, so it no longer looks like a dead short, but rather more like a resistor.

So, even with no motor, an engine with a headlight wired directly to the pickups, as many DC lamps are, will look like a short.

It takes an iron man to play with a toy iron horse. 

  • Member since
    February 2001
  • From: Poconos, PA
  • 3,948 posts
Posted by TomDiehl on Friday, September 5, 2008 8:53 AM
 davidmbedard wrote:

Every DC motor will have a short (or continuity) between it's motor leads.

David B 

There can be a great variation in the amount of resistance read, depending on the type of DC motor, and the range of your ohmmeter. For example, a can motor will typically have a much higher resistance than the old style open frame motor. If you try to read an open frame motor on the x1K ohm scale, it will probably show near zero (short or continuity). If you switch to (or even have) a x1 ohm scale, the reading will typically be between 7 and 25 ohms, which would show as zero on the x1K ohm scale. Can motors are typically several hundred ohms.

Smile, it makes people wonder what you're up to. Chief of Sanitation; Clowntown

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