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HO Scale Walthers GP9M Decoder Installation

These series of pictures show how to install a T1 decoder in a HO scale Walthers Trainline GP9M locomotive. The pictures were submitted to Train Control Systems [TCS] (http://www.tcsdcc.com) for use on their webpage, so the same pictures may appear there. (I retain copyright, but they are granted permission to use the pictures for their business.) This inexpensive locomotive was produced before DCC gained popularity, so the decoder has to be hard wired.

For this installation, I used a TCS T1 two-function decoder. Since I do not plan to have more than one accessory, I chose this decoder. If extra lights or sound is desired, a more fully featured decoder may be required.

The first step in the process is to remove the weight bar and motor. Be sure to mark the motor to remind yourself which terminal is which. There's no distinguishing marks on this motor to indicate which terminal is which.

Marked Motor

Next, since the pick up wires are both black, I used some heat shrink tubing to indicate which ones pick up from the right-hand rail (respective to the front of the engine). I verified this by placing the locomotive on a piece of track and measuring the resistance with an ohm meter. A reading of close to 0 ohms means the wires connect. The meter in the picture is showing 6 ohms. (Not perfect, but close enough.)

Pickups and multimeter

I followed the directions as to which wire went where, placing heat shrink tubing on any solder joints. After everything was connected, I then thought about where the decoder would mount. Big mistake. I had tried to mount the decoder to the top of the weight bar, which would not allow the shell to fit back on the chassis. Fortunately, I was able to move the decoder to the bottom of the weight bar and complete installation.

Connected Wiring

Initial decoder placement, decoder is on top of weight.

Finished Installation, before shell is reinstalled.  Decoder is now under weight bar.


1. All wire-to-wire solder joints were made using a Western Union Splice. Simply overlap the exposed wire at the halfway point and twist the left wire around the right side, and the right wire around the left. This method makes a very smooth joint, allowing heat shrink tubing to easily seal the joint.

2. Trim your wires, but leave at least an inch extra in case something goes wrong. I had a difficult time with making the motor power connection and wound up having to extend the orange wire.


TCS - Train Control Systems Website
Western Union Splice

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