Fitting DCC to Wrenn OO Locomotives – Horizontal Motors

This week I’m going to share with you a simple way to add DCC (Digital Command Control) to older Wrenn OO locomotives.

Wrenn locomotives date back to the 1960s but don’t be fooled by their age.  They’re very good models and are still widely collected and run.  If you find one in its original box it may even be worth a lot of money, depending on the model inside.

One of their main advantages is they’re all metal, making them very heavy.  This gives them a lot of tractive effort compared to models produced in later years.  The mechanisms are simple but well-built which means most of them are probably still running well.  However these were all designed well before the concept of DCC came along so the motor wasn’t isolated from the chassis.  In fact one of the motor brushes is connected directly to the chassis which makes converting these to DCC a problem.

But to overcome that problem I’ve come up with a simple way to easily make the conversion.  The Wrenn locomotives I’ve come across have one of two types of motor; horizontal and vertical.  This week I’ll cover the horizontal motor which is in the 8F 2-8-0 as pictured bellow, the Castle 4-6-0 and the Rebuilt West Country 4-6-2 which is the locomotive I shall be working on today.

The Rebuilt West Country has the motor and all the wires located under the shell.

With the shell removed you can see a single black wire, which runs from the right hand wheels, connecting into the green wire and going to the right motor terminal.

The left terminal is connected to the chassis by a metal bolt.  Both terminals are linked by a capacitor which acts as a suppressor to prevent interference with televisions etc.  Each terminal also has a spring which keeps pressure on the motor brushes inside the brush holders.

The brush holder on the right is isolated from the chassis and is only connected to the green wire.  The brush holder on the left is the one which gives us the problem.  In the image below I’ve released the spring and the brush has fallen out.  Be carefull not to drop the brush as they are made from carbon, just like a pencil lead, and can easily crack.

The brush holder is made from brass and is fixed directly into the chassis, making a perfect electrical connection.  The brush holder should pull out with a pair of pliers as I have done below.  If not, it will need to be drilled out; if you have to do this dismantle the whole motor first, because you don’t want to damage the inside or get metal filings in the armature.

With the brush holder removed it’s a simple matter of replacing it with something which works as an insulator.  And the answer is a 3D printed brush holder.

These have been designed to be a direct replacement.  They are 3D printed in Shapeways’ Frosted Ultra Detail material and should fit into the hole with a push.  It’s important to check first that the brush slides freely inside the holder.  Any print residue inside may cause the brush to stick and this will prevent the locomotive from running.  Any residue can be removed with a drill, the same size as the brush, or a round file.  If the brush holder is a loose fit in the hole simply fix it in place with some superglue.  (Superglue is made from acrylic and so is the Shapeways FUD)

The black wire from the right hand side wheels has been cut and will be joined to the DCC decoder.  The capacitor has also been removed.  Under the left motor terminal is a bolt which also connects this side back to the chassis; this needs to be removed and left out.

At this stage a continuity test using a volt meter is a good idea to ensure the two terminals really are isolated from the chassis and both left and right wheels.  If they are, then the brush can be re-fitted and the spring clipped on to hold it in place.  The wires from the DCC decoder can now be soldered to the motor terminals.

The power feeds can now be connected; one goes to the black wire and the other to the chassis.  I connected the chassis wire to the screw holding on the weight at the front of the loco.

And that’s it, the loco is chipped and ready for testing.

Next week I’ll share with you how to isolate a vertical Wrenn motor and where to get the 3D printed brush mounts from.