Adding a decoder to an old MDC loco

DKRickman's picture

I thought this might be of interest to some folks here.  I am working on installing a decoder in an old, very non-DCC-ready MDC 2-8-0.  As it turns out, it's not difficult, but it helps to have some direction ahead of time.

As with most old steam locos, the challenge is insulating the motor from the frame.  The open frame motor in the model has one brush insulated, but the other is in direct contact with the motor frame, and thus with the model's frame and right hand rail.  The simple thing to do would be to replace the brass bushing that holds the brush with an insulated one as used on the top - but where do you get one of those these days?  The next best thing is to replace the mounting screw with a plastic one, but I think it's metric, and I don't have any on hand.

Since I am a believer in keeping things simple and using what I already have on hand, I had to come up with a different solution.  What I did was to glue a piece of styrene tube into the hole in the frame, through which the motor mounting screw passes.  Make sure that the tube does not extend beyond the bottom of the hole - the top is okay, since it can be trimmed off easily.  When the glue is thoroughly cured, drill the hole out carefully so that it will just pass the screw - take your time because there won't be much plastic left, and if you deviate a little you can cut through it and let the screw short against the frame.  Using a chisel blade, cut off any of the tube which extends above the frame.  Make an insulating washer by soaking a piece of paper with super glue (might as well do it while gluing the tube in place), and when it's dry poke a hole in it and cut it out somewhat larger than the head of the mounting screw.  Insulate the bottom of the motor with tape (kapton if you have it, scotch tape if you're cheap like me, and poke a hole where the mounting screw goes.

I chose a TCS M1 decoder because it is cheap, tiny, and I REALLY like their warranty.  As it turns out, the decoder is almost exactly the same side as the end of the motor, and there's a little room between the motor and the inside of the firebox.  I just used scotch tape to tape the decoder to the back of the motor, with the wires facing up.  The orange wire is soldered to the tap on the top brush (or anywhere on the brush, but the tab is easiest).  The gray wire is soldered to the bottom brush, or anywhere on the motor frame.  Mine is a little tight, as I cut it a bit short, but it works.

With the decoder taped to the motor and the wires soldered in place, screw the motor to the frame as usual.  It will be a little harder getting the screw started, but it should go in and seat properly.  Make sure to put the insulating washer under the head of the screw.

If your model is more or less complete or at least has some way of picking up power from the rails, you can solder the red and black wires in place as best suits you.  I'm having to re-make the pickup on mine, and it came without a tender, so for now I'm clipping the wires to the rails for testing.  MAKE SURE TO TEST THE DECODER ON THE PROGRAMMING TRACK FIRST!  If there are any shorts between the rails and the motor brushes, you must correct them before putting the loco on the track.  I have fried a decoder that way (why do you think I like the TCS warranty?) and very nearly fried this one.  If the decoder responds to programming properly, takes an address and reads it back properly, etc, put it on the layout and make sure it runs.  Assuming you did everything as described, you should have a decoder-equipped model for little more than the cost of the decoder and about 30 minutes of time.

Here is my model on the work bench, showing the wires soldered to the motor:

And on my layout, with wires trapped in switch points for temporary testing:

I am happy to report that the little beastie runs fairly well.  Of course, there are a number of places to mount the decoder, including the tender (if you have one).  That's the easy part.  The tricky thing is figuring out how to isolate the motor, and I think the method described above works remarkably well.  One thing I like about it is that it requires no modification to the model, with the exception of adding an insulated sleeve to the mounting hole.

Motor isolation

I have done something similar. Both frame halves contacted each motor half. I use Kapton tape and nylon screws.

Kapton tape is thin and tough.

I am quite sure the new Roundhouse open frame motors would fit if you ever desire a motor with flywheel. They snap right into a plastic motor holder. My 1998 and 2008 Roundhouse loco's have the motor with flywheel. The NWSL 45 to 1 replacement gears are very nice. They all run very nice with a Tsunami Micro.

Yes, I saw, keep the cost down.


I hope you don't mind...

This is EXACTLY what I need, so I copied it to have as an instruction sheet!  I hope that's OK with you.


DKRickman's picture

I demand royalties!

This is EXACTLY what I need, so I copied it to have as an instruction sheet!  I hope that's OK with you.

Absolutely not!  I demand payment for every loco, by the running minute! cheeky

Of course it's okay with me!  Why do you think I did the installation now, when I have so many other projects on the shelf?  By the way, on your loco which runs backwards:  If you do this conversion, you can leave it alone.  With the motor isolated, it doesn't matter what the polarity of the frame is (unless you use metal couplers in metal boxes and couple to other similar models).

On replacing motors, gears, etc.:  Yes, there are a lot of ways to improve these models.  My goal for this one is not to make it the best engine ever, just to add a decoder to the model as-is, without a lot of money or time invested.  As such, I wanted to re-use as much as possible.  Besides, with the conversion done as described, it can easily be reversed and all the other improvements can still be done in the future, if you like.

I should have stressed one point.  It is imperative that the ENTIRE bottom of the motor be insulated, from the brush back to the magnet.  If it makes contact with the frame anywhere, you will let the magic smoke out of the decoder.  I don't know how I got away with it - either the decoder has a safety built in, or being temporarily wired backwards saved it, or I'm just lucky today.  I do NOT recommend trying your luck with the same thing.

Ken Rickman

Danville & Western HO modeler and web historian

  "just to add a decoder to


"just to add a decoder to the model as-is, without a lot of money or time invested"

                                                                                                                     - Ken                


That's my way of thinking too.  I'll worry about improvements later if I can do the basics first!

Very cool!  My issue is I have the entire learning curve ahead of me in DCC - I read one book and did not understand much!  Most of the conversation here goes about a foot over my head.  This is basic enough that at least I can get it.  


Now... programming track.... hmmm.... ??!


DKRickman's picture

Update - The decoder will not fit as shown

As the title suggests, I made a mistake!  There is not enough room behind the motor for a decoder.  Because I had already cut the motor leads short, I was somewhat limited in my options for placement.  There is a fair bit of room ahead of the motor and above the worm, so..

I cut a scrap of .010" styrene, taped the decoder to it, and taped that to the top of the motor so that the decoder fits just ahead of the motor frame.  It's not rigid, but it is well clear of any moving part, and the body now fits properly.

The wires are just snuck through a hole in the frame for now.  I'll route them properly when the new pickup solution is finished.  As always, it is important to check that no shorts were introduced with the modification.  Here's the loco reassembled, just to prove that it actually does fit together.

Now... programming track.... hmmm.... ??!

Any DCC system should have a programming track output.  The power is lower, and will not generally damage an improperly installed decoder.  It also allows programming that cannot be done on the main line.  In my case, I'm using a garden track outside the (as yet unbuilt) roundhouse.  Eventually, I'll put a DPDT switch in the leads, so that it can serve as a normal track or a programming track.

Ken Rickman

Danville & Western HO modeler and web historian

DKRickman's picture

DCC is pretty simple, really

My issue is I have the entire learning curve ahead of me in DCC - I read one book and did not understand much!

There's not that much to DCC, to be honest.  Like you, I was more than a little lost and confused when I first started thinking about it.  I decided to jump in and just figure it out, so I bought a Digitrax Zephyr, sat down with the manual and a locomotive, and slowly worked through the features.  I had made the rather risky decision to install a sound decoder in a steam loco without having a DCC system (risky because I could not test the installation) and that gave me the opportunity to play with all sorts of CV's.

One thing I highly recommend:  Keep a notebook with a page for each loco.  Write down any CV you change and it's new non-default value.  Of course, it's also handy to include things like the manufacturer and decoder, so you can refer to it for similar future installations.

Ken Rickman

Danville & Western HO modeler and web historian

New Roundhouse

A picture for comparison for those not familiar with the present loco's. Same size frame used for the 4-4-0, 2-6-0 and 2-8-0.


Hi Ken, DCC really is quite

Hi Ken, DCC really is quite simple. The same  principles apply that are used in DC. Two suggestions if I may. I would replace the metal screw and plastic sleeve with a plastic or nylon screw. Make sure you have a supply for next time. To me it is just too risky and you stated you have fried one already and almost this one. Not for this reason by why tempt fate? Also, I encourage all who have a PC or laptop to GET DECODER PRO! No need for sheet of CV's and notes, etc... It will make DCC that much easier and fun.   


DCC controller

I use the NCE Power Cab and set it to program track, first. I then flip the switch to power the program track. I had mis-wired a decoder. The red and orange wires looked almost the same under the lighting I was using and swapped them. Right away, the Power Cab let me know there was a problem. The Micro Tsunami was not damaged. That could have been a $90 error.


Been there, done that.

DCC is simple when you understand the basics of DCC. Until then, it is not plug and play. I remember my humble beginnings.

Having a basic knowledge of digital devices is a good help.

I belong to at least twenty MRR forums and see a lot of new people jumping into DCC with little knowledge and making sometimes, costly mistakes. Some make assumptions based on DC which many times are different when using DCC. I have lost track of how many have smoked a decoder or burned out a LED because they thought it worked like a light bulb.

I see many discussions about using a PC with the controllers and many have issues. Not to scare you but there is a learning curve. Remember how you felt when you first learned how to drive and how you drive now is a lot different.


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