I have a P2K GP7 in which I had a decoder installed few month ago. It ran fine for months. Yesterday night it pushed a string of cars into a siding. Today when I swhiched the layout ON it shorted the layout completely. Every other locomotive I tried ran fine but as soon as the wheels of that particular locomotive touch the rails, they short the layout. I don't know what happened during the night with the layout OFF. Not being very good with electronics and electricity, I wonder where I should begin after I would have removed the shell.
Can someone help?
Guy
Modeling CNR in the 50's
You did not mention any smoke so I would assume nothing burned.
Do a visual inspection. Look for a wire that might have been pinched when the shell was installed and it took time for the insulation to be cut. I am guessing a metal frame and a wire pinched against the frame.
This may take some time. Shorts are not always easy to find. No idea if this a a shrink wrapped decoder or light board type decoder.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
It takes an iron man to play with a toy iron horse.
Are you sure it's the locomotive and not a piece of derailed rolling stock? If you ran the train onto a section of track that could be turned off with a toggle switch and think the locomotive is shorted out because it tripped the system when you turned the toggle switch back on, it may be something other than the locomotive that is causing the short.
Yes I am sure. I tried this loco and three others on different parts of the layout. The GP7 is the only one that shorts the layout wherever it is put. It acts like a quarter put on both rails, a solib block of conductive metal.
Do what Rich suggested, a visual inspection.
Remove the shell and check for pinched wires, stray wires touching each other on the decoder.
Alton Junction
Guy,
I had the very same thing happen to me,with two P2K GP7's,that I bought at the same time, although they were DC,the drive line's the same. One would run great and so did the other, at first, then, every time, I put it on the rails dead short. Turn the loco upside down wheels up and gauge the wheels, if you can even take the bottom plate off and look at the wheels, axle's and gears and make sure,they all line up, I took mine out, one did not look right, as it turned out that one, although in gauge, one wheel axle,was pushed in to the gear further, than the other and that made,them touch slightly,creating a dead short. I would not be surprised,if that was the case with yours, sounds like it is..
Cheers,
Frank
Frank, you made me think of something with your comment.
The P2K GP7 is one of those diesels that is notorious for split and cracked gears.
There was a thread on the forum awhile back about how those broken axles can pull the wheels out of gauge and short the loco.
One more thing to consider.
Rich,
Sad, but true, they come from the factory like that. I got mine from Bud at GP and he was going to send it back, until I found,the problem. Lot easier,to try that first, than take the shell off.
use an ohmmeter on a multimeter to see if there is indeed a short between any pair of opposing wheels on the locomotive. A short is something close to zero ohms. Measure a working loco to see what typical resistance is.
if you find something visually, the ohmmeter will no longer indicate a short.
if the wheels are shorting against the frame, the ohmmeter will indicate a short or no short by moving the wheel away from the frame. Try to isolate where the short is. Be methodical.
greg - Philadelphia & Reading / Reading
Besides the axles, check that the truck "L" bracket is not touching the weight. It is a pretty close fit.
Jim
I checked the wheels set with an ohmmeter, no short. I removed the shell and inspected all wires. No unsolder wires, no pinched wires, no exposed wires. The locomotive is still shorting the layout as soon as two opposed wheels touch the rails.
Remove the decoder and see if it runs on DC.
Rich, you were right.
It runs fine on DC. I figure that means I have to change the decoder.
Thanks to everyone
Well, kudo to you for trying that, Guy.
Looks like you found your problem.
Could be that a stray strand of wire shorted the decoder.
Send it in, might still be fixable.
P.S. You might try reinstalling the decoder to see if removing it eliminated the shorting problem, especially if it was a stray strand of wire.
I took some readings with a Ohmmeter. Here are the results:
A : no contact
B: two probes touching each other (short)
C: red probe on red wire, black probe on blue wire.
All other readings were like A.
Is this meaningful?
#1 shows you how an open circuit, no continuity, reads. #2 shows how a short circuit would show.
#3 is pretty much meaningless, if you measured between the red wire and the blue wire - that just is showing the internal resistence of the rectifier diodes in the decoder, or as best as can be measured using the ohms setting. The decoder track inputs are the red and black wires. Unless there's a dead short in one of the recifier diodes, you'll get an equally random reading measuring ohms across the red and black wires as well., If you get a dead short indication - well, there's the problem.
DO NOT connect the meter set on ohms to the orange and grey motor wires, the meter actually uses the battery to send current rhough the circuit under test and under no circumstances do you want to feed power back in to the motor connections on a decoder. It probably won't hurt anything - after all, the turning motor is generating back EMF (even if the decoder doesn;t do anything with it) in voltages up to the motor rating, and the meter should only be putting 9V or less out to the probes, but better safe than sorry.
==Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Randy,
When I have made the readings, the decoder was not hooked on the motor. So it was safe.
If you read the post, you already know that the decoder is faulty.
With your comment, do you mean that the ohmmeter is of no use to know what is the problem?
Regards,
Not of no use, but just using the ohm setting isn't all it can do. It's valid to use the ohms setting and touch one wheel on each side with the red probe and one on the other side with the black and make sure there are no shorts - you shouldn;t get the short reading, even with the decoder still connected. With the decoder disconnected, it shouls always read as the open circuit example, unless a pair of sheels got pushed together too far and are shorting (but they'd also be grossly out of gauge), or the truck to decoder wires got pinched and are shorting together. Witht he decoder connected, it should be some odd reading depending on the circuit details of the decoder, but never open circuit or short circuit - either indicates a problem.
You cna use the meter on AC volts and check power at the decoder's input (red and black), and you can use the DC volts and check the output at the motor terminals as you increase the throttle. You cna use DC volts to check the functions outputs, blue is positive, the function connections are the negative. Obviously need to turn the function on t get a reading.
Lots of things you cna test with a multimeter.
--Randy
I have measured the resistance of decoder at the red and black input wires. Several thousand ohms for a good decoder. Around 100k. The red and black wires are connect to the full wave bridge rectifierr. There is a lot after the bridge. Never had a bad decoder to compare to.