A few years ago, I got an Athearn SD40 in C&O livery:

Unfortunately, the incandescent lights that the Athearn locomotives come with by default are terrible and don’t last very long, so this locomotive has been running without lights for a while. To replace the lights with LEDs I got two replacement LEDs from Athearn, part numbers ATHG67143(standard length 3mm) and ATHG67145(long length 6.5mm). This is what the LEDs look like next to each other:

It’s not clear from the description on Athearn’s website, but you can see that the 7.5mm and 3mm refers to the length of the lens on the LED.

This is the original LED in the locomotive:

As you can see, the 3mm LED is the best replacement for it. However, this is only true for the rear lights. The front headlights work much better with the 7.5mm LEDs, as the hole fro the LEDs is much longer. So if you plan on doing this mod yourself, you will probably need to buy both sets of LEDs.

Now in my SD40, I had replaced the original circuit board with an NCE DA-SR. The directions for the Athearn LEDs say that you should not wire a resistor in-line with the LEDs. However, NCE recommends that you cut the trace around the pre-populated resistor on the board. In the end, I decided to follow Athearn’s directions and not put a resistor in-line with the LED.

I wired the lights up as I thought they should be, and then it didn’t work. I had wired the black to the ‘common’ pin on the DA-SR. After reversing that, the LEDs turned on! Briefly. And then they immediately burned out.

So after burning out a set of LEDs, I decided to do the correct thing and wire a resistor in-series. With the DA-SR, this is as easy as cutting the trace around the resistor. Since the DA-SR has a ~1k resistor, I also did a quick test wiring the resistors in series. This worked much better:

The lights still have a bit of a glow when they are off, so I tried setting CV120 and CV121 to 129, however this did not do anything. According to the DA-SR manual, setting the value to 128 should tell the decoder that this is an LED. What exactly this does is not clear however, as it does not seem to have any change on the LED. I will have to investigate using JMRI later, as that makes it much easier to set the CV values.

The end result looks pretty good, albeit very bright:

So, in conclusion:
* The Athearn system of individual lights for each real light is terrible. The lenses that other manufacturers use(e.g. Atlas, Kato, maybe others) are much easier to work with
* If you’re using a DA-SR decoder in your locomotive, make sure to cut the resistor bypass and wire the LEDs in series. This may not be needed if you use the standard Athearn board that your locomotive came with
* If you want to replace both the front and rear lights, make sure that you get both the 3mm and 7.5mm LED replacements