Model Power Pacific DCC short

[woodone]

I have a N scale Model Power Loco with an Econami decoder installed in it.
Every thing works- runs forward-backwards with glitches- sounds all work.
The glitches are caused buy a intermittent short. The short is in the cross head slide on the right side. I can feel the heat from the short but I have no idea has what is really causing it. Does the main connection rod need to be insulated at the wheel (driver) ?
The cross head slide guides are hooked to the front cylinders and they are metal so the guides are hot. And warm to the touch too.
Any ideas.
This is a had me down, so I have no history on the unit.

[mmagliaro]

This intrigued me, so I broke out my 1st-run Model Power 4-6-2 (purely DC).
Indeed, the left-side valve slider is intermittently "hot" to the left rail.

It is intermittent, and I cannot figure out where the connection is coming from.  But undeniably, by fiddling with the engine on its back with two power probes, I could make the engine sometimes run by holding power to the right side drivers or right rail tender pickup, and then touching the other probe to the left size valve slider.

It is very strange indeed in that it comes and goes and I cannot yet see how it is happening.

But I figured this would be helpful to you.  Try it yourself.  Flip your engine on its back and see if you can make it run by powering the right-side drivers and the left-side valve slider.  You need to disconnect/bypass the decoder to figure this out, and get rid of that pathway from the left rail to the left size valve slider, and then put the decoder back in.

[mmagliaro]

I petitioned the admins to move this thread to the general N and Z, since the problem is more general (and maybe more troublesome) than just a DCC issue.

[peteski]

Then the Subject line should also be edited (either  by moderators or by woodone).

[woodone]

Well beyond my pay grade, for an Edit.
It started with a DCC problem, so That is where I first posted.

[woodone]

OK, it has been moved- maybe there will be more feed back here?
I am still puzzled by the short.
The way the wipers get power to the drivers is va wipers that touch the tread only. Therefor the metal boiler part should be neutral.
So some way I have to be getting power to the boiler.
Maybe where the contact wires pass through the frame?
And then is the issue of,  Why is the driver eccentric pin getting power?
Should not the center of the driver be neutral too?
Thanks for the feed back

[peteski]

Well beyond my pay grade, for an Edit.
It started with a DCC problem, so That is where I first posted.

Beyond your pay grade? You are kidding, right?  As you can see, moderators already moved it here (N/Z scale section). All you have to do is edit the Subject line.  If you scroll to the top and look at your initial post, on the top right side of it you will notice 2 buttons: one is "Quote" the other is "Modify".  Click on the "Modify" and you will be greeted by a standard message composition screen, as if you were starting a new thread, except it will contain the text of your post.  All you have to do is delete the "DCC" word in the Subject line field, then hit the "Post" button on the bottom right (just like you do when you compose any post.  It really isn't that difficult, and computers aren't anything to be afraid of. 

I think I have one of those models stashed away, but no time to dig it out.

[mmagliaro]

OK, it has been moved- maybe there will be more feed back here?
I am still puzzled by the short.
The way the wipers get power to the drivers is va wipers that touch the tread only. Therefor the metal boiler part should be neutral.
So some way I have to be getting power to the boiler.
Maybe where the contact wires pass through the frame?
And then is the issue of,  Why is the driver eccentric pin getting power?
Should not the center of the driver be neutral too?
Thanks for the feed back

It is very puzzling, but undeniable.   I don't think the driver pin is where the short is coming from.  I think there's a pathway through the frame to the valve guide, because once I get it in a "mode" where it is shorted, it runs very very smoothly by only touching the guide, and it wouldn't do that if it were depending on that rod and slider to keep making contact while they whiz around and slide back and forth. 

I will confirm this by pulling the rod off and let you know.

I tried fiddling wheels left and right, etc, but cannot yet see a pattern to it.  Sometimes it won't short at all, and then if I just keep tapping/touching a driver with a probe and wiggling things and touching the valve slider, all of a sudden it will start up.  So it's some errant, flakey short through to that valve guide.   And this is a 100% stock, 1st-run DC-only MP 4-6-2 I am testing.  I have never touched it other than to give it a test run on the track.  I stashed it for a possible future kitbash that I haven't gotten around to yet.

[craigolio1]

What if
Something else is contacting the boiler so that's its shorting intermittently (becaowe the boiler is connected to the guide).

Head light maybe?

[peteski]

Same here - I bought few of these at a fire sale few years back and haven't  looked at them yet.  I found my MP Pacific fairly easy - I had one of them out (buried under couple of other items on my workbench). Messy workbench - too many projects.    The valve gear and cylinders are metal, so they conduct electricity. The driver centers are plastic, but I noticed that the main crank pin is very long, extending inward way past the back of the driver. That pin might protrude far enough on your model to contact the metal wheel wiper as it rotates past it. Take a peek behind the main driver to see how far the crank pins stick out. Or just pull the main crank pins out slightly on both sides (so they protrude less in the back of the driver) and see if the short disappears.

UPDATE:  On my loco (and probably on all of them) the metal chassis is electrically connected to the right side electric pickup (probably by design).  So the short is between any of the metal parts of the chassis (including cylinders and valve gear) and the left side pickup.

[mmagliaro]

I scratched some paint off the metal underframe, and touching a probe there, I found the complete circuit was just was intermittent and wonky as when touching the valve slider.

Looking behind the front right driver, I think it's that wiper for the headlight that rides on that front driver.  Depending on how that front driver axle moves and dips, that headlight wiper can just barely touch both the driver and the boiler at teh same time under there.  I think that's where the path is, and that's why it's so intermittent.  I fiddled and pulled on that wiper a bit, trying to pull it a little outward away from the boiler, and then it became a LOT harder to get the engine to run off the frame or the valve slider.   I can still make it happen, but it's suddenly a lot rarer.

Try that.   You can just grip that think with fine pointy tweezers and tug it outward away from the boiler a little.
Of course, the right way to fix it is to pull the drivers out and get back in there to really bend that contact and make sure it can never touch the frame.

I also like Peteski's suggestion about the crankpins.  They do protrude through the backs of the drivers and it looks like they can graze the wiper strip behind the driver, so I'd pull them out and file them a bit shorter and then put them back in to avoid this.  You could probably just pull them out a little and leave them, but that might make the rods sloppy and cause other weird jamming problems.

[woodone]

Well this one does not have the original H/L. has been replaced with a LED. So the bulb or what ever has long been gone.
Will check out the suggestions in the AM.
Thanks for all of the input.

[mmagliaro]

Well this one does not have the original H/L. has been replaced with a LED. So the bulb or what ever has long been gone.
Will check out the suggestions in the AM.
Thanks for all of the input.

That really doesn't matter.  If it has those two contact strips riding on the front drivers, that's all it would take.
They don't even have to be hooked up to anything inside.   I think the strip just touches the driver and the frame at the same time occasionally.

[woodone]

Peteski's idea of the crank pin hitting the wipers was right on. I pulled it out a bit and the short went away.
I removed it and shortened it a bit and re installed- no more short.
Thanks- don't know how I would have ever found that one.

[mmagliaro]

Sounds totally reasonable.  On mine, when I got the short to occur, the crank pins were never located so that their protruding tips could touch those bronze strips.  i.e. I would have the engine sitting there with all the crank pins rotated to where they couldn't touch the strips, and then I could still get a short through the valve slider.  Makes me think this can be caused by multiple problems.  Oh boy.

Glad your problem is solved!