Too much amperage from adding a dummy??

I would try the dummy by itself.  Check it for a stuck roller against the frame or other short.  A dummy shouldn't pull much current at all.

You either have a dead short in the MTH dummy or what I like to refer to as a "resistive short", which is some type of conductive debris in the mix that allows current to flow through but has enough resistance to prevent a breaker from tripping right away. Check for wire strands, frayed insulation or the tip of the roller kissing the frame.

What happens if the dummy is the only one on the track?

Well, first the good news.  The problem, as you suspected, is in the dummy.  

Now the bad news.  After about an hour and a half of detaching and reattaching connectors, 

wrapping electrical tape around what seemed to be the slightest frayed area, repositioning wires to fit better within the shell, nothing seems to help.

So I'm at my end point.  I need and will seek out professional help!!  One thing I did notice when I placed this opened dummy on the track and gently throttled up.  I could hear a very faint sort of buzzing/crackling type noise coming from what appeared to be this area.  It was hard to determine because the noise was so faint and did not seem to get louder as I powered up (and the circuit would trip again anyway before I got too much further.

Thanks guys for your helpfulness.  If and when I learn the root of the problem I will try to report back what the fix was.

- Mike

Same thing...the ammeter went wild.

Turn off the dummy's lights and see what happens. If the short goes away, you have a bad lighting board. As a practical matter, making a replacement circuit or using batteries (simplest approach) is probably cheaper than buying replacements from MTH.

Do you mean by pulling all of these?  Actually, there currently are no lights when I place it on the track.  I think the very first moment I tried it with the Atlas engine, they both had lights for a second and then the circuit tripped and that was it.

Do you own a volt-ohm meter? I would start tracing from the rollers inward to and through the various circuit boards.

Not being familiar with the loco in question, since you say it has lights, can you remove the lamps from the sockets and see if you have a shorted lamp?

Also, forgive my ignorance, but why all those wires for a "dummy"? Does it do things that require all that stuff? Sounds, etc?

Art, beats me.  This is what the MTH description mentions.

Features

• (2) Operating Metal Couplers
• Illuminated Number Boards
• Constant Voltage Headlight

Plus, one odd thing.  There's a smoke switch on the underside ; I set it to "off."  I wonder whether this had been a powered unit originally that someone converted to a dummy .  The shell has the correct dummy # 151.  But there's the smoke switch and gears on the underside.

Does power come from two wheel- trucks via a pair of red and black wires that go into twist wire-nuts like on their powered-units?  If so, you can un-twist one wire nut and separate the wires.  This isolates the trucks.  Then with an open-end piece of track or whatever you can apply track power to one truck at a time to see if the short is in the truck.

I didn't catch if you said you had a multi-meter but you could also test each truck individually for a short that way too once you un-twist the wire-nut and separate the red or black track power wires.

Which model?  Does the dummy have a 2R and 3R switch?  Otherwise sounds like a bad CV board.  That should be an easy fix.  No smoke unit on this model?  G

20- 2871-3 was the box it came in.  But because of the cab number and the smoke unit switch, I'm guessing this was not the original shell.

I just placed it on the track with all the pickup rollers taped.  I then untaped just a single roller towards the rear of the engine and applied the power, ammeter soared to max.  I retyped everything and uncovered a single roller on the front-end pickups, same result.  Does that mean the problem is not with one particular set of pickups but rather is located somewhere "in between" for lack of a more technical term?

The rollers are connected internally.  So your last experiment is inconclusive.  

Per before, you need to separate that internal connection to separate the two power sources from the boards/electronics.  That's the way to isolate whether the short is in the front truck, rear truck, or the electronics.

Sounds like the pickup wire for the front roller is shorted

Mike, the FET on the right of this picture has a screw through it to the frame. The backside of the FET may be the issue. Take the screw out and make sure the back side does not tough the metal

Mike,

If you want I can take a look at it for you. We can meet at our usually Ramsey Eating Facility Parking Lot if you want?

Hello Matt, and thanks.  I tried.  Didn't seem to help.  (Although I'm a little confused...doesn't the FET sort of sit perpendicularly on the metal when its screwed into place? ) I took the screw out so the board assembly was sort of "floating" a bit, not touching anything..and placed it on the track but got same result.

This doesn't seem to have any 2R/3R switch.  The only thing I can see is a smoke switch, yet this unit did not have smoke, nor is there any funnel or stack.

Some of the FET's need to have a little plastic insulator between the farm and the back of the FET

I read the title and it reminded me of blowing my top when working with a knucklehead...

Any particular instruction or chance you could explain  how to do that, based on what you see here?

Overall view

Front end

Back end

I believe you will find a red and black wire from each wheel truck supplying power to the electronics.  This is circled in yellow from your photo below.

Can't quite see this on the right side of your dummy but it probably has the same hookup there.  In greatly magnified view as below I believe you'll see them screwed into the truck.  I have seen these connections loosen where the wires or lugs rotate or get nicked as the wheels trucks are moving relative to the chassis particularly when going around curves.  So carefully inspect these areas on the trucks for shorts between black and red or between red and the chassis.

As far as isolating the trucks, I am referring to those yellow wire-nuts.  I believe the red wire from each truck comes to the red wire nut, and the black wire from each truck comes to the black wire nut.  I don't know what that 3rd wire nut is but may not be relevant for now.  Anyway, you'll somehow have to determine (I realize this can be a challenge with the harness spiral wrap) which is the red nut and the black nut.  Then un-twist the nuts and separate the wires.  Now the two trucks will be isolated and the electronics will no longer receive track power.  I'm guessing that the red and black nuts each have a 3rd wire going to the board electronics.

Now put the engine on the track and see if you still have a short.

If I read the lettering on the "FET", it seems to be 7806A.  The 7806A is a voltage regulator (not a FET) that generates 6V which sounds right given MTH uses 6V bulbs.  Not that it matters what it's called since it doesn't seem to be the issue...

I would take take the red wire nut off and separate the wires form each other and see if you still have a short. If so then your issue lies most likely in one of the trucks. If no short then reconnect the red wires one at a time and test for the short every time you connect a wire