TCS CN-GP shorting problem -Atlas N GP7

[Centercab]

I installed two CN-GP decoders in two GP7's and they both have the same problem.  They run full speed in reverse.  I took them back apart and used my multi-meter.  Both motor brushes show continuity between...both motors. 

I've installed plenty of decoders and have only come across this on some old open frame motors.

I thinking i'm just overlooking something, but this really has me confused.   Any ideas?

[MK]

Both motor brushes show continuity between...both motors. 

Can you clarify this statement?  Both motors?   

[Centercab]

Can you clarify this statement?  Both motors?   

That did come out a bit weird.  If i put one meter lead on one brush holder and the other lead on the other brush holder, it shows up as a dead short.  It is this way on both motors.  I checked a third motor and it doesn't do this.

[peteski]

Motors would have a non-zero ohm resistance. Depending on the position of the commutator and number of poles, and the resistance of the winding I would expect them to show between around 50 to couple hundred ohms.

Are the motors out of the model when you are measuring their resistance?  If not, then take them out, or at least isolate them from the rest of the model. One thing that makes no sense is if they were really zero ohms they would not run (and would overload the decoder's motor output).

[Centercab]

They are out of the frames.  I did measure them and they are in the range that you pointed out.  Plus, they make my meter beep at me!

These two ran fine on DC.  I'm thinking of trying a different decoder just to see if they work.  Hard to imagine i have two bad motors or two bad decoders.

[woodone]

Are you checking with the decoder still hooked up?
The two motor leads will have resistance between them ( like Peteski stated). Even with out the decoder hooked up.
On these Atlas locomotives there is not much room between the motorbrush caps and the frame so watch for a short in this area!

[Centercab]

I unsoldered the decoder from one and took the readings.  Since it sounds like the motors are OK according to that, could it just be a programming problem since BOTH locos act the exact same way?

Thanks for the help so far!

[central.vermont]

Could be. Might I suggest that you reassemble them both and then do a factory reset to both and see what you get for throttle responses. Also what are you using for programming JMRI or direct throttle use?

Jon

[woodone]

What was described in the OP's first post , it sounds like a motor to frame short.

Very easy to do on these Atlas loco's

[peteski]

They are out of the frames.  I did measure them and they are in the range that you pointed out.  Plus, they make my meter beep at me!

These two ran fine on DC.  I'm thinking of trying a different decoder just to see if they work.  Hard to imagine i have two bad motors or two bad decoders.

I don't know the specs of your multimeter but the continuity beeper will sound even with some resistance in the circuit (non-zero value), so it doesn't just indicate a dead short.  Usually the digital display shows the resistance in the continuity mode, so you can see how "shorted" the circuit is.

When you have everything hooked back to the decoder, and the chassis is reassembled, test continuity of each motor lead to both sides of the frame. There should not be continuity from either motor lead to either half-frame.  Also make sure that each decoder's half has good contact with both frame halves.   Lack of contact with the frame on one or more of the decoder's contact pads can cause strange behavior.

Your problem description is also not very clear. Does the loco take off full speed in reverse as soon as you place it on the DCC-powered track, or only after you select its address on your throttle and then crank the throttle knob to speed step 1 and higher?

[Centercab]

After screwing with these this afternoon, I figured out the problem.  The rear circuit board was not making good, continuous contact with the frame.  I shimmed the board to make it fit tighter and both locos work like they should.

Thanks everyone for the help!  Boy, I feel kinda dumb...

[peteski]

After screwing with these this afternoon, I figured out the problem.  The rear circuit board was not making good, continuous contact with the frame.  I shimmed the board to make it fit tighter and both locos work like they should.

Thanks everyone for the help!

Very good!  All it took is some investigative troubleshooting. 

[Ngineer]

I usualluy put additional solder on the TCS decoder contacts that touch the Atlas frame. I have found out that in most cases there isn't enough solder in these spots to make good contact.

   Javier