GP38 Wiring & Final Assembly

Part 4 of our “Let’s Build a Train!” series wraps up the GP38 wiring & final assembly. This will get your engine ready for riding the rails!

If you have not already done so, in our three previous articles, you should have built your gearbox, assembled your trucks, and printed out all the remaining needed components out from both Depronize’s original design (Designer of the GP38)  and ours.

We will add one additional component in a download which will complete the GP38 wiring & final assembly: A suitable magnetic coupler. This will connect your GP38 to the rest of the “rolling stock” that we will be releasing during this series.

Like to see how the train runs? check out the video:

Here is what you will need to complete your GP38 project:

Here is a list of things you will need:

  1. M2 sized screws (8mm and 10mm in length). We chose this from Amazon because it gives us a good assortment.

      2. M3 screws (12mm,20mm and 8 mm (coupler)in length)

3. A couple JST style connectors (if you want to make your motorized truck removable). We used these.

4. Some flexible wire. We like to use Hansen Hobbies 22-gauge servo wire. Its flexible, holds up well, and takes soldier quite well.

5. (1) AA Battery holder. This quad design works well and fit perfect into the GP38.

6. A slide switch to turn your GP38 on and off. The slot in our forward chassis is designed to fit this sized switch. We used these.

7. Some 5mm diameter x 1mm tall magnets. We used these for our custom coupler. There are many vendors that come and go for this item, but we are sure you can find one that will work. We used these.

8. 3mm LEDs. (2) Red and (2) white. We used these. They are very bright!

9. (2) 220ohm ¼ watt resistors. Something like this should work.

10. Some heat shrink tubing for small gauge wires similar to this.

11. A few 8-inch-long tie wraps. You can obtain these cheaply at your favorite dollar store or Amazon.

12. Some weights for the GP38 to give it some pulling power. Self-stick ones would make it easy for mounting like these, but you can use just about anything for weight (bolts, nuts)

13. Soldier and Soldiering iron.

14. Our download for the magnetic coupler. We suggest you print this coupler at 100% infill. You can download it here.

Getting Started with the GP38 Wiring & Final Assembly

The circuit we are going to be building looks like this:

Everything will turn on when we activate the switch. We will be covering each part of this circuit.

To get things started we will first prepare our motorized truck. While adding a connector to the motor assembly is not necessary, it does make things a lot easier for both assembly and for any future maintenance.

Adding the JST Connector to the Motor

As mentioned above, adding a connector to the motorized truck has its advantages. Start by soldiering the connector wires to each terminal of the motor.

Even if your connector cables are flexible, we recommend adding an eight-inch tie wrap to hold the wires to the motor. This will act as a strain relief, helping to prevent the wires from breaking at your soldier points on the motor due to repeated flexing back and forth.

Wiring up the headlight

With the motorized truck prepped, the head light is next. You want to take two clear 3mm LED’s and place them into our front headlight assemble, just to verify that they fit.

Once fitted, you want to orient both LEDs so that the short legs (that’s the negative polarity ones) are next to each other.

We cut 2 strips of our flexible 3 conductor wire 10 inches long, and strip away the white wire, so that we are left with just the red and black leads. One strip will be used for the front lights and one for the rear lights. You will need to take each section of wire and strip off the ends.

Take one of the sections of wire for use with the front LEDs.

Both negative (the shorter LED leg) inputs need to be connected. A small piece of heat shrink is slid over one end of the black wire and it is soldiered to the two negative (short leg) terminals of both LEDs. It is easiest to do this if you place the LEDs in the front light fixture, and then mount the fixture in a vise or something like it to keep it stationary, as shown above. You do not want to forget to use the heat shrink in this step, because you do not want to short out the connections later between the long and short legs of the LEDs when you place the fixture in the GP38 body. Repeat this same process with the red (positive) wire.

Now before going any further, you may want to test that your front LED wiring is correct. To do this, you can take one of the 220-ohm resistors, twist it to one end of the wires like this:

Then connect your battery pack with batteries installed, one battery pack wire to the other end of the resistor and one to the other wire. If you did it all correctly, or front lights should light up.

You can now glue your front light fixture assembly into the GP38 body. We would suggest applying a small amount of glue to the back of the fixture assembly to hold both LEDs in place. Then once this dries, slide the wires into the GP38 body and glue the front fixture into place.

Wiring up the taillights

Wiring of the taillights is handled in a similar fashion. First, you want to make sure that the taillights fit in the body.

Once fitment is verified, you want to soldier the wires and terminals as you did with the front LEDs. The short legs get connected with a black wire and the longer legs with a red wire. Use heat shrink tubing on both.

Again, if you wish to verify that your wiring is correct, you can use one of the resistors and battery pack to check it.

Once this is done, you can glue the rear LEDs into the GP38 body.

Extending the Battery Pack Wiring

If you used the suggested battery pack, you would want to extend the battery pack wiring. To do this, take another 8-inch strip of wire and older each of the red and black wires to the battery pack wires. Again, use heat shrink tubing.

Wiring up the Switch

To wire up the switch first route the red battery wire thru the slot in the forward chassis as shown below.

Look closely at the forward chassis. You want to route the wire in this way so that the wire is coming out the “bottom” of the GP38 chassis.

Once this is done, you want to secure your switch into a vise or something similar and soldier the wire as shown.

You will also need another 8-inch strip of just red wire. Connect this to the center terminal of the switch by soldiering it as well.

Once this is done, you can glue the switch into place.

Connecting it All Up

Now you are ready to bring it all the wiring together. First you want to collect all three of the black wires from front and rear LEDs and the matching JST connector (you have not yet used this other connector). Twist them all together and soldier.

To the end of this collection of wires, soldier the black lead of your battery pack. This is the location of the circuit we are completing:

Soldier a 220-ohm resistor to the end of the red wire coming from the front lights. Do the same for the rear lights.

This is the area of the circuit we are completing:

Add some shrink tubing over each of the Resistors, so that just the other end of each resistor lead is exposed. Twist these two resistors leads together.

Now connect the twisted resistor leads to the other end of the red JST connector (that will go to your motor) and the red switch lead, applying heat shrink to all of it after you soldier this connection. This the area of the circuit we are completing:

When you are all done, you will have something that looks like this:

Now is a good time to test things out. Put batteries in your pack and turn on the switch. If you have done it all correctly, your front and rear lights should turn on.

Chassis and Body Assembly

With the wiring done, it is now time to assembly the GP38. First start by laying out the front and rear chassis over the tank like this.

Next secure both chassis to the tank using (4) M2x10mm screws.

Note: As you do this, you may wish to add weights to the center tank. This is because, we found later that adding about 98 grams (3.5 ounces) of weight to the center tank was needed to give the GP38 traction to pull cars. This may seem like a lot, but once you start adding cars to the back of the GP38, you will really notice the difference. Also, due to the choice of motor we made, without this amount of weight, the rear truck tended to lift the wheels at the front of the truck because it has so much power.

Once both chassis are secured, you should route all the wires outside the top of the GP38 body.

After you do this, pull the top body slight off, so that you can install the rear motorized truck.

Installing the Rear Motorized Truck

The rear motorized truck has a ball which fits into the “socket” of the rear chassis. Its rather tough to push in, so its best to install the rear truck now so you are pressing on the chassis and not the top of your GP38. If the fit is too tight, you can use a small file to slightly enlarge the socket hole. It will snap into place when you get it right.

Installing the Front Truck

The front truck is held in place to the front chassis by (1) M3x20 screw.

Front truck installed:

Adding Weight for Front Truck

Now is a good time to add weight to the front of your GP38 as well, before securing the GP38 body to the chassis. Without it, your GP38 is likely to derail quite easily. We found that adding about 56 grams (2 ounces) of weight worked great for our GP38.

Securing GP38 body to Chassis

To secure the GP38 body to the chassis, you will need (8) M2 x 6mm screws, securing it at these points:

Cleaning up the Wiring

You can use some small tie wraps to tidy up the wiring a bit. Leave the battery wires long as this makes it easy to get your battery pack out when its time to refresh your batteries. Also now is a good time to connect the motor JST connectors if you have not already done so.

Vent installation

With the wiring done, you can now install the roof vent fan covers and roof vent hatch. This can be glued in place.

All the side vents can also be glued into place.

Adding a Coupler

We chose to go with a magnetic coupler design for our GP38. This will mate up nicely with our other “Let’s build a train” series we will cover.

The coupler uses (1) 5mm x 1mm neodymium magnet and is held in place using (1) M3x6mm screw

Guide Rails

We found the guide rails a bit too fragile, so we did not install them. If you wish to, they will go into the holes in the sides of the chassis and body.


After a period of use, we discovered several things you may run into with the GP38.

First, with our unit, the GP38 front wheels began to “lock up” randomly, slowing the GP38 down significantly. What we found was happening is that there were very slight imperfections on both the inside of the truck and wheels themselves which occurred during the printing process. As a result, these tended to “catch” which was locking the wheels up due to the close tolerance between the face of the wheel and the truck.

So, if you run into this problem, the solution is to take the front truck apart and file both the wheel face and truck insides down slightly to remove these imperfections in the areas shown below.

Secondly, we discovered that the rear truck bearing slots on our unit were a little too loose. As a result, what would happen is our motor would sometimes stop if we repositioned the GP38 on the track between uses. What we found was happening is the axles were “riding up” and almost out of the slots because the bearings move up and down in the area shown below:

To fix this, you have a couple of options. We just made sure the axles were pushed back in place and this worked fine as a solution for us. But if you have “younger engineers” using the GP38, you may want to print several small squares, about the size of the slot, and glue them into place to keep the bearing from moving. A simple solution, but it will do the trick.

We hope you have enjoyed building the GP38 with us in this series. Now that you have your engine ready to go, we suggest you stay tuned, as we will be covering, decals, track, and cars to complete your train!

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