I have a old Max Gray Loco which uses an open frame motor. I use DCC for control and usually deal with can motors. However for this install I'd like to try using the original Open frame motor. I insulated the motor from the frame using nylon fasteners. But the motor shaft which goes into the metal gearbox obviously overrides my effort causing the frame and motor to become shorted. Not sure how to approach this issue. Your thoughts appreciated.

Thank you


Original Post

I have installed decoders in a bunch of locos with open frame motors including  many USH and probably 4-5 Max Grey.   

I have never found a motor that is a problem.     I don't think there is a connection to the brushes through the armature.    Mechanically it should be the same as a can motor.    Remember your Can motors also have metal shafts.

I assume this is a permanent magnet motor, not series wound.    Look at the brush holder on the motor.     It has to be a non-conducting material, otherwise the brushes short out.    There should be a wire soldered to each brush.   One goes to the frame and one goes to the connector to the tender.     If you disconnect these two wires from the frame and connector and then connect to the Decoder the same as  usual, it should all work.  

Now some HO locos I have seen have a metal strap from one brush to the frame or motor frame.    On some motors this is a pretty subtle tab connection.     I have never seen this on an O scale loco, but it is a possibility.   If that is your case, you need to remove that strap.    I have done it on an old HO loco for a friend and then it worked fine.   I think the idea that this is a problem comes from this HO practice.

I use NCE D408SR decoders in these locos.    They have 4 amp continuous and 10 amp stall ratings.     That is plenty for an open frame motor.    Mine usually draw about 2 amps under very heavy load and less than an amp running light.



Hmmm, I have never seen one like that, that I remember, in an O scale loco.   Is this in a 4-4-0 maybe?     That is a typical design I have seen on most older HO motors prior to cans.   

In the photos, it appears that both brushes are mounted through insulated fittings.    The top one obviously has no connection to the frame.    I can't see the bottom one well enough to tell.    I can see the insulted fitting however.    So Check to make sure the bottom one has not strap to the frame of the motor or the loco frame.    I'm guessing it does not.

I have a Max Grey PRR 4-4-0 that may have a motor like that.   It has been a few years since I worked on it.    I remmber it draws very little current and I was able to use a real small decoder (by O standards) in it.   I think it is a Digitrax DH123.   The loco draws well under 2 amps at all times.

If you want to discuss this in more detail, you can contact me via email from  my profile.

Last edited by prrjim

You don't have to insulate the frame ... just make sure both brushes are insulated from the frame .. a multimeter can tell you ..

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