Hardwiring a Kato SD40-2 early or snoot SD40-2 model

I have been meaning to put this tutorial up for quite a while.

Disclaimer: this is for informational purposes only. If you attempt this repair and mess up your model, it's your fault and I'm not liable.

The purpose is to show how to hardwire a Kato SD40-2 mechanism from either the early or snoot runs.

These units tended to be plagued with problems due to the electrical pickup system that they were designed with. Consequently, these locomotives tended to get a bad reputation for dying or operating jerky.

For this repair, you will need the following basic hobby tools:
small screwdrivers
soldering iron, solder and flux (I use a 30 watt Radio Shack soldering iron, 60/40 rosin core solder, and Tix flux)
A Dremel with cut off wheel
Flexable thin stranded wire (I use Brawa 3108 because it is very similar to the thickness, flexability and appearance to what Kato uses on the truck leads of their other models)

Let us begin by removing the body shell by removing the coupler screws and couplers and gently squeezing the shell and pulling up to remove the shell. You should be left with a mechanism that looks like this.

Next, use a small phillips screwdriver to remove the two screws that retain the motor by flipping the mechanism over. At this time, unclip the fuel tank halves. With the motor removed, you should now be able to remove the black electrical pickup clips...this is the black plastic piece with two bronze strips running the length of the frame and the cause of the problems.
Now take a small flat blade screwdriver and gently pry out to release the lugs on the gear tower retaining clip on each truck. Remove the truck and universal/worm gear assembly from both ends.
You should be left with something looking like this

At this time, dismantle each truck by carefully prying on the truck bottom clip with a flat blade screwdriver to release the bottom of the assembly. With this piece removed, the truck sideframes will pull straight out along with the bearing strips. Gently remove the bearing strips from the sideframes. Notice the location on the bearing strips where the Xacto blade points are pointing...this is where we will eventually be soldering new contact wires to.

Next, we turn to the frame. Although this step isn't entirely necessary, it provides more lateral motion to the front truck. If you don't do this step the loco will still work, but the front truck will not swing as freely.
Using a Dremel with a cutoff wheel, cut two channels into the frame for the front truck leads to fit into. This allows the wires to fit under the cab/subbase assembly on the shell without cramming them down on top of the universal.

Take and cut 4 pieces of flexable thin wire to approximately six inches in length apiece. Strip a quarter inch of insulation off of one end of each wire and tin.

Now solder these tinned wires to the locations pointed out earlier with the Xacto knife blades on the INSIDE of the bearing.

At this point, reassemble the trucks in the reverse order of dismantling. Bend the new wires in towards the truck casing and then up.
Reinstall the motor with the motor screws, but omit the electrical contact pickup screws. Install the newly wired trucks back to the frame making sure to reinstall the universals and snap down the gear covers. Run the new wires up to the circuit board (on the front through the slots, on the rear making sure that the wires are in between the weight. Remove the plastic clip from the top of the Kato ciruit board and discard. Solder the motor contacts to the board using solder and a little flux and cut off the excess motor lead. Now cut the new truck leads to the right length so they line up with the pads in the middle of the board like in the final photo. Unsolder the two bronze clips that would normally go down and make contact with the electrical pickup rails we removed. Strip off an eight inch of insulation or so, tin, and solder to the board in the location of where the bronze contacts were removed.
At this point, your mechanism should resemble something like this. A stock mechanism is next to it for comparison purposes.

At this point, it would be a good idea to double check all solder joints and make sure everything is seated. Give the mechanism a test run. Once everything is known to run satisfactorily, replace the shell and install the couplers and screws and you should be done.


© Copyright 2007 Kelley Duford
Last Updated 6/28/2007