The motor must be electrically isolated from the track, how do I test for this using a voltmeter what parts do I touch and what results show that the motor is isolated, thanks in advance for your help.
I am assuming that you have installed a DCC decoder and want to test for shorts. I normally power the locomotive up and if it doesn't work I remove it from the tracks quickly. I suppose you could test it by placing one lead of your voltmeter on one motor tab and checking both rails with the other, then testing the other motor tab in like fashion. I haven't done this because I wasn't sure if the DC from the Ohm meter would feed back through the decoder and/or possibly damage it. Reflecting on it now, I think it probably wouldn't damage the decoder, which operates on +/- 12v, but current through the bridge diodes might be a possibility that would give a false reading. I'll be interested to see what others have to say.
Remove the loco from the track. Place one test on the top (+) motor lead and put the other test lead on the locos frame. If the motor is grounding to the frame you'll know it instantly. Neither motor lead must contact the frame.
If it is a split frame design do as Jeff suggests, Pierre - it's the simplest way. Otherwise, similar to what Lownen recommends but from the motor tabs to the pick-ups if an open style frame. You can also do it with the loco sitting on the rails but would only recommend it if you haven't powered it up. Look for a high reading around (from memory) 10K ohms plus between the motor and rail/pick-up.
I think you need to give us a bit more info, Pierre. A normal DC loco has both sides of the motor connected to the track - else it won't work. The assumption we'd make is that you are installing a DCC decoder as this sort of issue arises then, but can you be more specific about where you are in the process? If a specific model is involved, tell us; someone on here will probably have done it and can tell you what needs to be done for it specifically.
Powered "it" meaning the track. My goodness, the thing's I'm taking for granted that people will know these days. Thanks Gats!
More properly, the motor should be isolateable from the frame so that the track input connection to a decoder (red and black wires) won't short to the motor output connection (orange and gray wires). As supplied, most all locos of typical split frame design will connect the right and left rails to the right and left halves of the frame respectively, so measuring resistance (use a multimeter set to Ohms or a continuity checker) between the motor brushes and the frame halves will show that they are connected, normally via metal clips or straps). Do this off the track with no voltage applied. What needs to happen is that the motor connections can be disconnected from the frame so that the motor brushes can be connected to the decoder outputs only. Look at how the brush caps are connected to the frame and determine how to disconnect them. Do that, then measure resistance between the brush caps and the frame. It should be high/infinite (i.e., an open circuit). Most newer "DCC ready" locos are set up this way, and the stock light boards serve to connect the track inputs to the motor. Removing the light board should result in motor isolation. Light board replacement decoders squeeze themselves in to this open connection and take care if sending all those tiny electrons to the right places.
