Loco causing short circuit

[renard80]

A friend has converted one of my locos to DCC. Whenever I put it on the track, it immediately causes a short circuit. I am reluctant to return it to him as he has been so kind. Is there anything basic I should check? Regretfully, I am not the sharpest tool where DCC is concerned!

[Fishmanoz]

Was the loco DCC ready so the conversion was removing the blanking plug and inserting the plug on the decoder?  Or did it have to be hard wired in with soldering.  If the former, remove the body and check that the plug hasn't been pushed so far into the socket that the end of a pin or pins isn't shorting on something underneath the socket.  If the latter, check there are no bare soldered bits on wires shorting on something. 

 

Or if it is a tender loco loco, does the short go away if you remove the tender from the tracks?  That would probably indicate bent fingers in pickup connections going from tender to loco.

[96RAF]

It could help us to know the loco and if possible its R number as this will tell us the type of motor and installation.

 

It could be your friend as well meaning as he is has not done the installation correctly especially if it is hard wiring an older loco or one of the awkwarder ringfield motors.

 

I understand you say you are not too clever with DCC but if you can use a digital multi meter then there are some simple checks you can do. These can be explained when you come back to us with a bit more info.

Rob

[renard80]

Thanks to you both for responding, and apologies for being unable to reply sooner.

Fishmanoz - The loco was not DCC ready. Unfortunately, I cannot check the wiring as you suggest. I have removed the three screws from the chassis but the body will not lift off. I am reluctant to use force . . . 

RAF96 -  It is Great Western 0-6-0 Pannier Tank Engine 2744, like this one:

http://www.victorianweb.org/cv/models/british/6.html

There is no maker's name underneath.

The chip he put in is Gaugemaster DCC26 OPTI Decoder Small.

I hope this info is what you require.

[96RAF]

I converted a similar one 2728 to TTS sound.

Body was hooked on, see article 1 on my linked website in signature block, which may help getting it apart.

Rob

[RDS]

I can't help but think that your friend would want you to contact him.  (especially if he reads about it on here)

I know I would if I had fitted a Decoder for someone and it didn't work and I am sure I would be quite disappointed if I found out he had been elsewhere to get it fixed.

[renard80]

Rob, you said earlier "if you can use a digital multi meter then there are some simple checks you can do. These can be explained when you come back to us with a bit more info."

Please can you oblige? Thanks.

[renard80]

I can't help but think that your friend would want you to contact him.  (especially if he reads about it on here)

I know I would if I had fitted a Decoder for someone and it didn't work and I am sure I would be quite disappointed if I found out he had been elsewhere to get it fixed.

As he is a very busy man, I am reluctant to bother him further unless really necessary. I am hoping it is a simple fault which I can rectify.

[96RAF]

Rob, you said earlier "if you can use a digital multi meter then there are some simple checks you can do. These can be explained when you come back to us with a bit more info."

Please can you oblige? Thanks.

If the decoder is plugged into a socket (unlikely) then its a simple case of unplugging the decoder and checking continuity between the wheels each side and their respective pins at the socket. Ditto motor connections to the socket. Continuity should only be betwixt these direct connections and not betwixt each other. In effect the meter is checking the integrity of the wire betwixt the chosen connections.

 

If the decoder is hard wired (more likely) then it is more difficult as the meter will be able to 'see' alternative 'routes' within the decoder and motor windings, but the basic principles apply, you are looking to see if there is a good connection from left wheels to the decoder black wire, right wheels to the red wire, and left motor brush to orange and right motor to grey, although these last two may be reversed.

 

Just be aware that some decoders are wired in different colours so if so please refer to the decoder data sheet for those data.

 

There is a picture of the wiring circuit on the Hornby DCC installations page reproduced here.  The instructions below the picture  are part of the Hornby page telling how to install a decoder by hard wiring. The other connections (A B C D) you will not have and the decoder wires should be taped up safely but worth checking they have not shorted against any metal or to themselves.

/media/tinymce_upload/daf588319da97b00c54c94430d717fdd.PNG

 

[renard80]

Rob, I am very grateful to you for going to so much trouble in replying. I will have a crack at it later.

[96RAF]

Just had another thought...

...even with a decoder hard wired in you should be able to check for NO continuity between either wheel set and either motor brush.

If there is then the fault is on that side somewhere.

Nor should there be any continuity from left wheels to right wheels.

Rob

[Chrissaf]

However, since Renard is saying the loco is causing a 'short circuit' this will be a little harder to find using a meter. it is not an issue of lack of continuity, it is an issue of too much continuity. To isolate the location of a 'short circuit' you will have to systematically break the circuit down into smaller sections to see what part of the circuit is creating the short. Since the decoder is 'hard wired' into the loco, this means the use of a soldering iron. I assume that Renard got his friend to do the conversion because Renard didn't feel comfortable wielding an iron (or doesn't own one). Renard, I appreciate you don't want to bother your friend, but in the circumstances he may be better placed to resolve your issue. If he is a true friend, then that shouldn't be a problem for him.

[Fishmanoz]

You won't find continuity between either wheel set and either motor brush, with any luck, else the decoder will now be blown. 

[96RAF]

You won't find continuity between either wheel set and either motor brush, with any luck, else the decoder will now be blown. 

That was the point Fishy. The decoder isolates the wheels from the motor, but if not then there is either a decoder problem or some other alternative current path such as a live chassis, etc, in which case further breakdown work per Chris suggestion will be necessary.

 

Going back to the friend armed with the results of con-testing should help the rectification task.

Either that or cut your losses and take it to a shop for repair.

Rob

[Chrissaf]

Not knowing a bit more about the model that has been DCC fitted. I too was wondering if it is a 'live chassis' model and the friend who did the modification didn't realise and take steps to isolate the motor.

.

I would have thought that the friend would have tested the installation before handing it back ???

[96RAF]

Not knowing a bit more about the model that has been DCC fitted. I too was wondering if it is a 'live chassis' model and the friend who did the modification didn't realise and take steps to isolate the motor. I would have thought that the friend would have tested the installation before handing it back ???

The OP said the model had no identifying marks so I assumed it was a Hornby model and went from there as it is a simple model to convert, but if it is another make or even earlier Triang/etc model then until we have sight of its inner workings we are blind guessing as best as possible unfortunately. I believe there was even a ringfield version as well and we know how many variations of those there are and their little foibles.

Rob

[Fishmanoz]

I've possibly made a fundamental mistake here: I've assumed the person doing the modification knew what they were doing.  It's essential as a first step in modifying any older loco to ensure the motor brushes are isolated from the chassis/pickups. If they aren't, the decoder will blow as soon as power is applied.  And while it may be possible to blow itself short circuit, it would be more likely to do so open circuit. Hence my thinking it is some other problem.