Posted by: bgtwindad | March 15, 2011

Decoder Install: Intermountain F3A

In this installment, we’re going to tackle another “easy” DCC decoder install.  Not quite as easy as the Kato NW2, but still very much a beginner project.  Today, we’re going to install a TCS IMF4 decoder into an Intermountain F3A locomotive.

This particular locomotive was a gift from a friend.  It’s an excellent runner, but happens to be missing a headlight.  We’ll discuss what to do about the headlight, and I will show its installation in another post as soon as the replacement arrives, but I won’t be able to show that step here.

You will need a soldering iron and solder, and you will need some insulating tape.  Kapton tape would be the better choice, but regular vinyl electrical tape is OK. Do not attempt this installation without a small piece of insulating tape.

OK, let’s get started, shall we?

Step 1: Remove the shell

The first step to remove the shell is to remove the front coupler.  Flip the locomotive upside-down, and remove the screw holding the coupler in place.  Put it somewhere safe!  Then pull the coupler straight out the hole in the pilot.  Store it somewhere safe as well.  You don’t have to remove the rear coupler.

Next, flip the engine upright and carefully lift the shell off the chassis.  Spread the sides apart a bit at the fuel tank with a pair of toothpicks.  Lift the cab end off first, and it will come off rather easily.

Step 2: Disconnect the LED

If your engine has a headlight (it should!), it will be nestled into the black tubing at the cab end of the loco, and two wires will extend back and be soldered to the light board where shown in the picture.  Take a note of which color wire is attached to which terminal on the board.  Better yet, take a photo so you can refer back to it.

With your soldering iron, melt the solder and disconnect the two wires.  Pull the wires back out of the way, but do not remove the headlight.  If you want to replace the headlight LED with, say, a different color or intensity, now would be a good time, though.

Step 3: Remove the light board

In the photo above, you’ll see that the light board is held in place by two screws.  Remove the screws (save them!!) and lift the light board out of the depression in the chassis.

Side Note:  Why you need insulating tape!

When I was preparing to do this install, a friend cautioned me that I would need some insulating tape.  He had learned “the hard way” by frying a decoder and taking advantage of the generous TCS no-questions-asked warranty.  From the install photos I had seen, I did not see why this would be necessary.  After all, wouldn’t the DC light board also need insulation?

Here’s why:  As you can see from the photo below, the back side of the light board (left) has no exposed circuit traces.  The board itself insulates the simple DC circuits from the frame.  The TCS decoder (right), however, has circuit traces and via holes (holes that provide an electrical connection between the two sides of the board).  These via holes would short to the frame if an insulator isn’t provided.

I am not certain why TCS does not provide a piece of insulating material to slip below the board, but it is definitely needed.  Kapton tape would be an excellent choice, but is relatively expensive and hard to find.  You can order it online from various sources (Grainger,, DigiKey, etc.).  Regular vinyl electrical tape will work, as would a carefully cut-to-fit piece of cardstock.

Step 4: Insulate the back side of the decoder

Before installing the decoder, you must insulate the back side from the frame.  Cut a piece of insulating tape to fit between the two posts for the motor brushes and the mounting screw hole on the far end.  The piece should be about 1/2″ wide by 1-7/16″ long.  It is OK if it wraps up around the sides a bit, but you must make sure it does not cover up the mounting hole in the corner nor the motor brush posts.  It must cover all of the other exposed metal on the back of the decoder board.

If you prefer, a thin piece of cardstock cut to fit under the board will work, but it must be thin enough not to interfere with the contact between the motor brush tabs and the posts on the decoder.

Step 5: Install the decoder and attach the LED

We’re almost done, but now you will need your soldering iron.

Before installing the decoder, slip another small piece of insulating onto the frame in the area where the LED wires will be – the cab end, that is…

Drop the decoder in place on top of the chassis and screw it in place.  Solder the headlight LED wires to the board on the two pads indicated in the picture (Note: the picture above is rotated relative to all the previous photos.  The cab end is to the right in this picture).  BE SURE to attach the wires in the same relative orientation as they were before you removed them from the light board.

Step 6: Reinstall the shell and front coupler

Slip the shell back over the chassis.  Then flip the engine over, re-insert the coupler through the front of the pilot and screw it to the chassis the same way it was before you started.

I found it helpful to slip a flat-head screwdriver between the coupler draft box and the front truck to help keep it square to the front of the locomotive while tightening the screw.  Otherwise the box will tend to turn with the screw, leading to a misaligned coupler.

And there you have it!  A DCC-powered Intermountain F3A.  The installation should be almost identical for the Digitrax DN163I1C, and also just the same for the F3B and F7A/B from Intermountain, all of which share a common chassis.





  1. Hey Mark, Nice job on the decoder installs. the comment about kapton tape – sometimes decoders – digitrax for one – come with a piece of kapton holding the decoder to a piece of foam. I overlooked this until someone pointed it out to me. FWIW,

    • Thanks, Gene. That’s good to know. The TCS decoders do NOT include Kapton, which – particularly in the case of the Intermountain drop-in – would appear to be a significant miss, since installing the decoder without an insulator will likely fry the decoder. I should email TCS and see if they have a response…

  2. hmmm, this is perhaps why the two IM F3s (DRg&W) that I acquired seem to be “not happy”. I don’t remember whether or not I installed decoders in them (it’s been a while, and I’m w/o a layout at the moment), but I do remember that they weren’t running. If I did install decoder, I’m SURE I didn’t insulate them, since there was no call to do so…

  3. Does anyone have experience with the “new” (Spring 2014) F7’s from IM?
    The motor is wired to the light board instead of using tabs.

    I tried installing an IMF4 by removing the IMF4 “feet” and soldering the motor leads to the board. I can program and run the engine with the decoder corners alligator-clipped to the DCS51 output with loco on a “dead” track,
    but the decoder won’t respond when the loco wheels are on a DCC powered track.

    Now I may have damaged the decoder when soldering the leads,
    but the loco behavior doesn’t make sense to me.

    • Sounds like you have a problem with the contacts from the wheels to the board.

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