The two motors are mounted to the underside of the chassis and the roller chain drive is installed.  Although it’s not marked, it looks like the wire leads coming from the motors are 10 Gauge wire.  I purchased a ten foot length of 10 ga wire in red and another in black.  It’s important not to get the polarity reversed accidentally when connecting to the Sabertooth control unit, as it can damage a component that costs over a hundred dollars.  For that reason, I’m color-coding all of the wires to help prevent an incorrect installation.

And on that note, when looking at example wiring for the Arduino programmable chip, it seems to be a convention that the ground wires are white.  The ground wires in automobiles are by convention black.  So I’m using black for the ground, because that’s what I’m accustomed to.

I installed wire disconnects between the motor leads and the wiring harness to make it easier to remove the motors at some point for maintenance or repair.  I had to ensure that the crimp-on disconnects were rated for 10 ga wire so I don’t overload them.  Then I ran the wire from the motors to the Sabertooth control unit, ensuring that I didn’t get the polarity reversed.  So the motors are wired in and ready to go.

The next step was to wire the batteries together in series and connect them to the power terminals of the Sabertooth.  I’ve been waiting on the delivery of a fuse link component before wiring in the batteries.  If something goes wrong, I want the circuit to blow a fuse instead of frying something expensive.

Well, the fuse link finally arrived, so I wired it into the battery circuit.  You can see it in the picture between the two batteries on the red wire.  It contains a 40 amp fuse to protect the circuit.  The Sabertooth control unit has the six wires (three red, three black).  There is a red/black pair on each side that feed the two motors (one pair per motor), and the red/black pair in the middle provide the power from the batteries.

The loop of red wire is there to eventually connect to the kill button that hasn’t yet arrived.  When it arrives, I’ll mount it on the firewall in the cab and splice it into the red wire.  That way, hitting the stop button will interrupt the power flowing from the batteries, effectively turning everything off.  Because the amperage of the circuit exceeds the amp rating of the stop button (29 amps vs. 10 amp rating), I’ll also have to install a relay.  So the button will actually control the relay, and the 40 amp relay will control the circuit power.

One other note about the control wiring:  the previous version of the circuit included both a radio receiver, from which I can control the motor remotely with an RC control unit, and an on-board potentiometer allowing me to control the motors on board.  For now, I’ve removed the potentiometer from the circuit and simplified the computer code to listen only for the RC signals.  I’ll add the on-board control back when I get the control levers installed and replace the potentiometer with a slider pot.

Running the Tests

Just to be sure I assembled all of the moving parts correctly, I ran some test to see if the power and controls actually work, and to see if the motors are wired backward.  I lifted up the cab to get the wheels in the air and wired the unit using just a single battery at twelve volts.

The first thing I noticed was that when I inserted a fuse into the fuse link, the wheels started turning.  That’s not supposed to happen.  When I turned on the RC unit to control the motors, everything stopped, as it should.  So there’s something missing in my control code that I’ll have to track down and correct.  I’m guessing that when I took out code related to the on-board potentiometer, I either removed something that should have initialized the state of the motors, or should have added something to do the same.  I’m sure it will be an easy fix when I open the code.

Using the RC controller with the wheels off the ground, I cranked it up to full power.  I counted the number of revolutions of the wheel during a thirty-second period to see how fast it ran.  Without boring you with the math, it works out that the wheels were turning fast enough to run the train at 1.67 mph.  Then I turned everything off and wired in the second battery.  Running the same test, the wheels turned nearly twice as fast, running at 3.2 mph.  I don’t know how much slower this will be under load, but it’s nice to see that the reality is so close to my original calculations.  I had sized the sprocket to get a gear ratio that would run at about 2 mph with the motors at about half speed.  So that’s encouraging.

Still to do:

Now I’m at the point that I can build a few feet of test track and run the hill climbing test on the ground.

I’m also getting to the point where I need to do some metal work.  I have some steel rods (two for the piston rods, one for the control levers in the cab, and one to connect the two brake shoes).  I’ll need to cut some threads on the ends of the rods so I can attach brackets with half inch nuts.  Also will need to fabricate some brackets for the siderods and control levers.

Before I can install the siderods, I’ll need to cut the axle for the drive wheels so they don’t protrude.