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Interestingly, the S4 has the same power trucks as USAT's first loco, the NW-2. As always, I strongly encourage you to read the "basic" USAT stuff on the main page, and the cracked axle repair/prevention and quick and dirty DCC pages.

Interestingly, the NW2 and the S4 never came with traction tires, and also the typical brass bushing/bearings on the axles are not present, the axles ride in a plastic saddle that is part of the motor block.

The wiring follows typical USAT wiring.


Note: following pictures courtesy of R.J. DeBerg

There are 6 screws to remove the shell.

Two by the rear truck:



And 4 more by the front truck:




Kadee couplers:

As usual, the Kadee recommended mounts use the stock pedestal, a large offset coupler that also swivels the coupler box. The standard Kadee mess that is not needed for 8' and greater diameter curves. This mounting is not good at all under stress, and it looks terrible.

In 2019, CamPac has started making a custom mount that not only close couples, and uses a center set Kadee coupler, but also they provide the "filler" piece around the coupler.

This coupler mount is still in beta testing, so wait for it to go on the CamPAc site.



beta testing notes:

As installed, coupler way too low, and the angle is wrong. A 0.034" kadee shim (the thick black ones) brings the end of the coupler to the right height, but the angle is poor. The mount needs to mount the coupler level and higher. I have not determined whether the pedestal or the coupler box is the culprit.

Height of platform. Lay a straightedge over the rails, and then use calipers to measure walkway height. I used a file across the rails, and was able to hold it with one hand, and measure with the other, you might need to affix the straightedge. Also, you hang the assembly out over the end of a counter so you have clearance for the calipers. Works very well.

20191005 182136






Describe the wiring, relate to standard USAT practice.



The wiring of the trucks is typical USAT diesel.

There are two sets of wires ending in two connectors coming from each truck from underneath "topside" to where the circuit board is under the hood.

If you look at ends of the trucks, there are 6 pins, the inner 2 pins are to the motor(s). These two wires come up "topside" to a female plug.

The other 4 pins are the "skates" and the wheel pickups. These 4 wires come up "topside" to a male  plug.

It's easy to tell which wire sets are which "topside".The track pickup set has 4 wires in a male plug. The motor wires have only 2  wires terminating into a female plug.

Both the front and rear trucks are wired the same. The trucks are normally mounted so the connections/pins face inwards, so one truck is effectively mounted "backwards" from the other.

Looking at the front truck on the S4, of the track pickup wires, the red wire is the right rail, and the black wire is the left rail.

Since the rear truck is "backwards", the pickup wires are connected to the opposite rails, right rail is black, and left rail is red. (It's turned backwards, remember?).

Likewise, the motor leads are reversed.

So, when wiring, you need to keep all of this in mind when connecting the wires for track pickup and the motors.

So, for DCC, you connect the red lead from the front truck and the black lead from the rear truck to the decoder "right rail".

You connect the black lead from the front truck and the red lead from the rear truck to the decoder "left rail".

For the motor leads, connect front red to rear black, and vice versa. To determine which of these 2 is "motor plus" to the decoder, power these leads with a DC supply. Set the polarity of you DC supply so that the wheels on both trucks turn "forwards". When you have this set, then whichever is positive voltage from your DC supply goes to the decoder "motor plus", and the other to "motor minus".

The important thing is to check your wiring with a voltmeter set to measure continuity. There should be no continuity between any track pickup wire and any motor wire.

If you have done all this, you have effectively 4 leads now, right rail, left rail, motor plus and motor minus. You connect these to the decoder leads with the same names. If you are using a standard decoder with NMRA color codes, then the leads are:



Here is a closeup of the board. The cab is to the RIGHT in this picture, so I could show the silk screened letters right side up. Motor 1 is the REAR truck, and Motor 2 is the front. 


On the bottom edge of the board is an unused 2 pin connector labeled "sound1", to the left where there are just solder pads is "smoke 2" and to the right of the connector, the solder pads are labeled "sound2". Weird, maybe trying to make a more universal board?

Light wiring:

In the lower right corner is a small 3 pin socket, you can see pin numbers 1 (yellow), 2 (black), and 3 (red) on the board. This is the headlights and number lights. Black is ground, red is always on for the number lights, and yellow is only on in the forward direction. They are LEDs and run at a regulated 4.3 volts. But since I need to measure the current, do not exceed 4.3 volts. 


Rear lights:

The next socket to the left for the cab lights and backup light, with the same pin numbers and same wire colors, except that pin 1 is the backup light and 3 is  the cab light.

The smoke unit gets track voltage, and the polarity reverses with track polarity reversals. 

current measurements:

  • backup light 18ma
  • cab light 7 ma
  • marker lights 9ma
  • headlight 12 ma 

calculating resistors for DCC:

Using 20 volts as a reference voltage, and R=V/I, the resistance is 20/I so, let's assume the backup and headlight should be pulling 20 ma, or .02 amp....   20/.02 = 1,000 ohms, 7 ma will take 2,800 ohms, and 9 ma will take 2,222 ohms...

1,000 ohms is a standard value, need 1/2 watt resistor, the other 2 can use 1/4 watt.. 2,200 (2.2k is a standard value, as well as 2,700 ohm (2.7k)

So buy TWO 1k (1,000) 1/2 watt resistors and ONE 2.2k (2,200) ohm 1/4 watt resistor, and ONE 2.7k (2,700) ohm 1/4 watt resistor. 

1k 1/2 watt is 271-1118

2.2k 1/4 watt is 271-1325

2.2k 1/2 watt is 271-1121

2.7k 1/4 watt only found in 500 piece assortment 271-312



Click the links below to go "deeper" into details on the S4

  DCC install on the S4     "Quick & Dirty" USAT DCC install