Decoder Isolation
In An Older HO Brass Diesel

I have a few older brass HO locomotives that are not "DCC ready". The motors are not isolated from the frames.

If you've ever tried installing a decoder in an older brass or other metal locomotive you know it can be a real challenge to isolate the motor and the motor must be isolated for DCC.

One of the engines I have is an Alco S2 diesel switcher. There's not a lot of room inside for a decoder and adding sound would be really difficult.

I opted to try installing a silent TCS decoder that was left over from an earlier sound conversion in another locomotive. The engine had a can motor but the electrical path for DC used the frame. I didn't want to remotor the locomotive if I could avoid it.

Bill Payne, fellow member of our Nottawasaga Model Railway club is far more knowledgeable than me about solving these kinds of problems. He came up with a novel solution that may help you with your decoder isolation challenges. Follow along as I show you what he came up with. 

The key was using the vertical motor support. The motor is screwed to the support and floats above the floor.  The usual trick of disconnecting it and floating the motor in silicon didn't seem like the best way to go. We began by disassembling the motor from the frame.

The motor support in the S2 brass diesel
brass motor support

We used a thin piece of styrene and cut a barrier that could be placed between the motor support and the motor. 

The decoder isolation styrene shield
brass motor shield

The holes for the screws had to line up but the size of the hole in the middle wasn't critical. It only had to clear the shaft.

The metal screws that held the motor in place needed to be replaced by nylon screws. We discovered by trial and error that a 1-72 screw would fit but it would have to be shortened. We could have isolated the motor by using the screws that held the mount to the floor of the frame but that metal is very thin and we didn't want to interfere with the swing of the trucks.

The motor mount
brass motor mount

The nylon screws were too long to insert into the motor so we cut them with rail nippers. The trick to shortening screws is to feed on a nut passed the point where you will cut.

Shortening the nylon screws
brass motor mount screws

After cutting you back off the nut. This cleans the threads at the cut end of the screw. As you can see in the picture Bill used a metal 1-72 nut on the nylon screw to clean the threads. Here is the barrier lined up on the vertical brass support.

Setting up the decoder isolation shield
brass motor mount barrier
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We used some liquid electrical tape to isolate the motor shaft bushing. It was carefully applied with a toothpick to ensure that it didn't foul the shaft itself.

This isolation may not have been necessary but we wanted to make sure the bushing couldn't contact the brass support when the motor was reinstalled.

Isolating the motor shaft
liquid electrical tape

Also for added insurance we installed a piece of black electrical tape on the floor of the frame beneath the motor. The motor floats above it. This was just an added precaution.

Isolating the motor from the floor
brass motor floor isolation

Engine reassembly

Now we were ready to reassemble the locomotive. The first task was to put the gear train back together. This has to be done before the motor is screwed into place.

Reassembling the drive train
brass engine reassembly

The top screw was loosely attached using a screwdriver that could "catch" the head. A magnetic screwdriver wouldn't work, obviously, because the screw was nylon.

Tightening the nylon screws
brass engine reassembly

The bottom screw was more difficult to line up and reinsert. Bill used a smaller screwdriver and slid it under the handrail.

  • A trick is to use a piece of putty or something sticky to hold the head of the screw to the screwdriver temporarily. 

After a little fiddling we got it snugged home. It was time to check the continuity with a meter to make sure the motor was completely isolated from the frame.


Checking electrical isolation

Electrical continuity test
continuity test

We used my small RadioShack meter for the continuity test.

Meter reading
meter coninuity reading
  • You want to see that zero number!

Good. We were set for decoder isolation. The TCS decoder had an NMRA plug on one end. As there wasn't going to be room for it I cut off the wires. The decoder would have to be tucked up into the cab. Not an ideal solution but there was no room above the motor or the gear towers.

Decoder hookup
decoder wiring

Before proceeding we put the engine on the programming track. In Service Mode on DecoderPro I wrote in the engine's number. 

It accepted it at the first attempt confirming that there was no short circuit.

I planned to use only the standard headlight wires. I cut the wires short that I wouldn't be using to limit the number of wires that would have to be grouped in the shell.

These need to be kept clear of the gear towers and shafts. We carefully soldered on the orange and gray wires to the motor and the red and black wires to the trucks.

We had to check which side of the rear and front wheels origianlly went to the DC motor. The standard is the red wire is connected to the right rail (engineer's side in North America).

The next step was to drill out the chassis for the headlights. I used a Dremel to slowly increase the size of the holes starting with a #56 drill bit and worked up to 1/8".

A little cutting oil helped. Bill showed me how to sharpen a drill bit on a belt sander. Never tried that before! I had a mini 12v bulb that I glued with ACC into the rear of the cab.

I need a second one for the front so this project will have to wait for a few weeks until I buy one. I could have used an LED but I didn't have any in my supplies.

You'll notice in the photo that we inserted a 150 ohm resistor into the blue common wire for the lights.  This makes the bulbs less bright and will help them last longer.

Decoder hookup
decoder installation

After getting the wires for the rear light hooked up we tried the locomotive again on the track. It ran in the wrong direction with the headlight on.

 I used DecoderPro to change the CV for direction so the diesel would go forward on my Lenz throttle when the arrow was in the normal UP position. However, the light was coming on at the wrong time. It was easier to resolder the headlight to the white wire instead of the yellow.

 Sometimes older brass locomotives were wired using the red wire for the front truck to the left rail instead of the right. That may have been our error. This S2 diesel doesn't pick up on all 8 wheels. The front truck is used for one side and the rear is used for the othe. There's no easy way to get all 8 wheels powered.

The last step was to hook up the other headlight bulb to finish the installation.



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