Digital Command Control

Installing Plug-N-Play Decoders in N Scale Locomotives

Just as Plug-N-Play doesn't always work as advertised on computers, sometimes some extra effort is required to make Plug-N-Play (PnP) decoder installations work just as they should in N-scale locomotives. The following are some problems encountered and suggested solutions for the various PnP decoders, as well as other problems related to good operation of specific locomotives. There is also a discussion on relacing the amber LEDs that come with some decoders with the white LEDs that are now appearing on many new locomotives.

Digitrax DN121IP Decoder
The Digitrax DN121IP decoder is the first N-scale decoder equipped with the 8-pin NMRA plug. The decoder's printed circuit board has been made longer so the pins of the NMRA plug can be attached to the end. This should allow installation in locomotives equipped with the NMRA plug to be very simple — remove the dummy plug and plug in the decoder — and it is, at least this part of the process. What may become the harder part of the installation is the disassembly and reassembly of the locomotive.

Installation of DN121 IP Decoder in Con-Cor S-2 4-8-4 Steam Locomotive
The Con-Cor S2 4-8-4 Steam Locomotive is the first N-scale locomotive to be equipped with the NMRA 8-pin plug. The circuit board and NMRA plug are mounted in the tender. Power pick-up for the decoder is from both the locomotive driving wheels and tender trucks (all 12 wheels). There are four wires between the locomotive and tender — two are for power pick-up and connect via the drawbar; the other two are the motor power leads and are wires between the motor brushes and the circuit board in the tender. The headlight is hard-wired to the locomotive power pick-ups and cannot be controlled through the decoder. Whenever the locomotive is on DCC-powered track the headlight is on; fortunately it is not too bright.

The following steps make up the install process:

• Remove the tender shell

• Remove the NMRA dummy plug and plug in the decoder

• Test the locomotive, program the decoder and test again

• Reassemble the tender

• Do a final test

Remove the Tender Shell
First you have to remove the tender shell. Not quite as easy as it looks, but if you look carefully the exploded tender diagram that came with the locomotive you will detect there is a small clip at the front, another clip at the rear, plus you have to pull the two rear ladder supports out of their holes, because they are actually part of the tender shell that comes off.

Install the Decoder
Gently pry up the dummy NMRA plug in the tender and replace it with the DN121 IP decoder. The sheet that comes with the decoder clearly shows how to position the decoder.

Test and Program the Decoder
Because the headlight is hard-wired to the locomotive power pick-ups rather than being powered through the decoder, the current drawn by the headlight will prevent the DCS100 Command Station from reading back the CV values at all or either the DCS100 or DB150 reliably programming CV values.

(Skip this step if using a DB150.) Place the DCS100 in program mode and read back the value in CV01. The display should be Ad:03, but will probably be Ad:FF, which indicates the Command Station could not read the CV value. However, you have confirmed the presence of the decoder to the Command Station.

Place the locomotive and tender on a DCC-powered track and check operation of the locomotive with the decoder, using address 03. The locomotive should operate exactly as it did using DC power.

Put the locomotive back on the programming track, and program the decoder CV values (e.g. address, CV29, etc.) as desired. There is a good possibility the programming will work, especially if using a DB150.

Place the locomotive and tender on a DCC-powered track and check operation of the locomotive with the decoder's new configuration.

If the programming attempted above does not work, remove the decoder from the plug in the tender, and hook it up using test clips to the program track and to a spare motor. Program the decoder to the desired configuration. Place the decoder back into the tender and test on a DCC-powered track.

Reassemble the Tender
This, again, is not quite as easy as it should be. First you must make sure the metal mount for the circuit board and decoder is seated properly over the two plastic lugs sticking up from the tender floor, all the while making sure the wires go where they should go. The idea is to get the assembly as low in the tender as possible. Then replace the front steps in position on the tender shell and lower into place. As you do this make sure the rear ladders go our over the rear of the tender, and the wires to the locomotive are routed properly and not pinched. Once all is in alignment push down until the front and rear clips engage. Having 3 hands helps a lot! Then push the ladder supports back into their holes in the end of the tender.

You may notice that the tender shell is bowed up very slightly (you have to look closely to see it) as the decoder assembly is not quite low enough. It appears this cannot be fixed by simply moving the decoder/circuit board inside the tender. It is virtually unnoticeable when viewed from more than a few inches away from the tender, but there is a rib along the top of the tender shell that can be carefully cut back to help seat the shell completely.

Do a Final Test
Place the re-assembled locomotive on a DCC-powered track and do a final operational check. This locomotive looks and runs very good.

Digitrax DN141K2 / DN163K2 Decoders
There appears to be no problems with the installation of this decoder in Kato EMD SD80/90 locomotives. Just follow the directions included with the decoder.

Digitrax DN144K / DN163K1A Decoders
There appears to be no problems with the installation of this decoder in Kato GE C44-9W locomotives. Just follow the directions included with the decoder.

Digitrax DN145K / DN163K0A Decoders
There is one potential problem area with the installation of this decoder in Kato PA-1, E8/9 and P42 locomotives. Step 10 (DN145K) or Step 6 (DN163K0A) of the Digitrax instructions tell you to "bend both motor brush leads towards the motor pads on the decoder. Press the retainer clip into the hole between the motor pads clamping the motor brush leads to the motor pads of the decoder."

The problem is that in bending the motor brush leads up so you can remove the original light board and insert the decoder, you cannot bend them back to the exact same tension. Thus when the retainer clip is inserted the motor brush leads may or may not be making a good contact with the decoder motor pads. Even if they are when first installed, simply replacing the shell on the frame or the vibration of running the locomotive may cause the contact to go bad.

The solution is to spot solder the motor brush lead to the decoder motor pad, using a very small amount of solder. (You may sometime in the future need to disassemble the locomotive for maintenance.) Be sure the soldering iron is 15 watts maximum and has a small round tip.

Digitrax DA120 / DN146A / DN163A0 Decoders
The DA120 is the decoder supplied by Atlas in their decoder-equipped GP40-2 and some other locomotives, while the DN146A and Dn163A0 are the Digitrax FX decoders that are sized identical to the DA120.

The potential problem here is poor or no contact between the decoder motor pads and the motor brush leads resulting in no or intermittent locomotive operation. (The motor brush leads can move.)

The best solution is to solder wires between the decoder motor pads and the motor brush leads. Use a very small gauge stranded wire for this purpose, such as the wire used in Digitrax non-PnP decoders. About ¾" long is enough. Strip 1/16" insulation from the ends of each wire. Cut the motor brush leads back about ¼" so they cannot interfere with the solder connection to the decoder motor pads. Solder one wire to the left motor brush lead, then to the left decoder motor pad. Repeat for the right side. Be sure there are no solder bridges between the two decoder motor pads.

You must be sure that neither the installed wires nor the copper brush leads can touch the metal frame or you will destroy the decoder. Use small pieces of tape to prevent shorts.

Digitrax DN147A / DN163A1 Decoders
The decoder supplied by Atlas in their 6-wheel truck (C-C) diesels (SD50/SD60/SD60M) is manufactured by Lenz. The Digitrax DN147A and DN163A1 replacement decoders appear to be slightly thinner than the Lenz decoder, which causes a potential problem of poor electrical contact with the locomotive's frame. The same problem of poor or no contact between the motor brush leads and the decoder motor pads as described for the DN146A/DN163A0 above can also exist. The blackened wheels of the Atlas diesels have some kind of coating which can cause problems which look very much like a decoder problem, but aren't.

To solve any contact problems between the motor brush leads and the decoder motor pads, follow the steps described above for the DN146A/DN163A0 decoders. Be sure to insulate the wires and motor brush leads so they cannot touch to the locomotive frame.

Please note that these motor contact problems can exist with the factory supplied Lenz decoder as well. Use the same solution.

To solve problems of poor contact between the decoder and the locomotive frame you will need to build a very small solder dome on the power contact points at the four corners of the decoder. Do this very carefully, as you want to end up with a snug fit, but remember that the decoder must slip into the slots in the frame. Use a soldering iron with a small round tip and 15 watts maximum. Heat the decoder power pad with the soldering iron while applying solder until a very thin dome forms, then remove the soldering iron and allow to cool. Very carefully insert (do not force) the decoder in the Atlas frame and re-assemble the locomotive. If a great deal of force is required then the solder dome is too high; remove some solder with the soldering iron.

Atlas SD50/SD60/SD60M locomotives have blackened wheels, but the coating easily comes off the wheels treads. If you clean the wheels of a brand new, out-of-the-box SD50/SD60/SD60M with Goo Gone or 409 on a paper towel, they will blacken the towel. Run the locomotive for a few minutes, then clean the wheels again. They will again blacken the towel. Don't expect real good operation until all the coating has come off, or been removed from, the wheel treads. Until the coating is all off, the locomotive may run in a hesitating manner, which shows symptoms typical of the two previously described problems.

Digitrax DN149K2 / DN163K1A Decoder
There appears to be no problems with the installation of these decoders in Kato EMD SD40-2 or SD70 locomotives. Just follow the directions included with the decoder.

Digitrax DN163K0B Decoder
There is one potential problem area with the installation of this decoder in Kato F3 locomotives (new production). Step 6 of the Digitrax instructions tell you to "bend both motor brush leads towards the motor pads on the decoder. Press the retainer clip into the hole between the motor pads clamping the motor brush leads to the motor pads of the decoder."

The problem is that in bending the motor brush leads up so you can remove the original light board and insert the decoder, you cannot bend them back to the exact same tension. Thus when the retainer clip is inserted the motor brush leads may or may not be making a good contact with the decoder motor pads. Even if they are when first installed, simply replacing the shell on the frame or the vibration of running the locomotive may cause the contact to go bad.

The solution is to spot solder the motor brush lead to the decoder motor pad, using a very small amount of solder. (You may sometime in the future need to disassemble the locomotive for maintenance.) Be sure the soldering iron is 15 watts maximum and has a small round tip.

The various conversions listed below provide detailed descriptions covering installations actually performed by model railroaders throughout the world, and reflect the experiences encountered during those installations.

Click on the author's name(s) corresponding to the desired locomotive to go to the detailed installation description.

Many new locomotives are being equipped with white LEDs for headlights on their DC light boards, whereas Digitrax Plug-N-Play decoders continue to be supplied with amber LEDs. Once you have seen the realistic appearance of a white headlight, the amber headlights no longer seem to pass muster. When installing a Plug-N-Play decoder the amber LEDs on the decoder can be replaced with the white LEDs from the locomotive light board. You can also use a Golden White or Warm White LED from Richmond Controls.

The process is not difficult, but requires extreme care when using the soldering iron to remove the amber LED and connect the white LED, as it is very easy to damage adjacent components on the decoder or cause such components to become unsoldered. The use of a magnifying device such as an Opti-Visor is strongly recommended.

Use a 15 watt maximum soldering iron with a very small tip when soldering/unsoldering the LEDs on the decoder board. You must also secure the decoder so it does not move around as you are attempting to solder/unsolder the LEDs.

Depending on the decoder, the LEDs may be held in place by a lead soldered through a hole in the board, or held in place solely by the soldered connection.

Matching the polarity of the LED is very important if the LED is to operate correctly. The polarity of the white LED should be matched to the polarity of the amber LED it is replacing, rather than to its polarity in the original light board. If the LED is installed with the reversed polarity it will not be damaged, but it will not light when you want it to either.

Following is the process to change the LEDs:

• Compare the length of the LED leads on the lightboard with the length required on the decoder, as well as the method of securing the LED to the decoder (wire through holes or just soldered).

  • If the light board lead length is longer than required for the decoder, simply cut the LED leads close to the light board.

  • If the light board lead length is the same, then unsolder the LED from the light board. Do this one lead at a time, using the 15-watt maximum soldering iron.

• Anchor the decoder so it will not move while soldering/unsoldering the LEDs. Be very careful that the method of anchoring does not damage either the circuit board or any of the components on the decoder.

• At this time check the polarity of the amber LED in the decoder. Do this by looking at the metal plates within the LED as this is a more reliable method than the length of the leads, flat side on the LED, etc. Make sure the white LED is aligned the same way, keeping in mind whether you are looking at the decoder from the bottom or the top, and which way you will looking at it when soldering in the new LED.

• Unsolder the amber LED.

  • If the leads are bent and pass through a hole in the decoder board, carefully heat the solder point where one of the wires passes through the board. When the solder melts, use a knife, small screwdriver or small needle-nose pliers to pull the wire out of the hole. Remove the soldering iron and let the remaining solder harden. Repeat for the other lead.

  • If the leads are held in place only by their soldered connection, using a small pair of diagonal cutters, cut one of the LED leads at the end of the decoder board (or right at the LED itself if the LED is against the end of the board). Unsolder the remaninig lead with the LED still attached and remove the LED, then heat the other lead and use a small screwdriver, etc. to remove it when the solder flows.

• The next step is to cut the leads of the white LED so it matches the amber LED, both in length, and whether it needs a right-angle bend to go through a hole in the decoder board. When making the bend, keep polarity in mind in determining which way to bend the leads.

• We are now ready to solder the white LED to the decoder circuit board. Proceed as follows depending on the method to be used:

  • If the leads have a right angle bend that goes through the decoder board, use the solder iron to heat up one hole until the solder melts, then while it is still melted insert the appropriate lead of the LED through the hole. Remove the soldering iron and let the joint cool. Repeat for the other LED lead.

  • If the LED is held only by solder, hold the LED in place (watch polarity) against the location on the decoder board and heat one lead until the solder melts. Sit that lead down into the solder then remove the soldering iron and let cool. Heat up the other lead until the solder melts, then sit that lead down into the solder then remove the soldering iron and let cool. If necessary touch up with a little additional solder.

• Repeat the appropriate steps above for the LED at the other end of the decoder.

• This completes the process for replacing the amber LEDs with white LEDs.

Now install the decoder in the locomotive, and test the installation. Make sure the LEDs light as they should when your turn F0 on and change directions of the locomotive. If the LED does not light as it should (especially if one lights and the other doesn't) check the polarity of the LED carefully in case it was installed reversed.

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Last updated: Thursday, February 12, 2026

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