Voltage for SMT ditch lights on Railking imperial 30-20316-1 GEVO- what am I missing?

Are you sure you have continuity to the LEDs? This PCB is KNOWN to rip and crack the solder joints to the plugs for the ditch LEDS.

@Vernon Barry posted:

Here is the lighting distribution PCB style many engines use. This is where I've seen bad or broken solder joints from the wires and connectors flexing and ripping the relatively thin traces. The Ditch lights are 2 of the red connectors.

Closer picture of board as installed- again noting that 2 of the 3 Red connectors are the ditch lights.

The backside of that PCB after the center mounting screw and short plastic standoff are removed.

Again, the failure I have seen, the thin copper trace on this single sided copper PCB can rip or crack if force pushes on the pins of the sockets pretty much in any direction. That will then cause either an intermittent connection or complete open circuit of an LED output. I've had to use wires to rebuild traces on my fair share of this specific PCB.

What I'm getting at is- to your question- a white LED is pretty much the same voltage as any other white LED.

And, LEDs have 2 values- a voltage drop across the LED- or put another way- the minimum source voltage to turn on the LED to begin conducting and generating light AND a current limit. That voltage is called forward voltage and is sometimes shown as Vf.

And, I test LEDs using a CR2032 coin battery (either one for 3V or stacked pair 6V) because an interesting side effect of such a small battery is it limits the current due to internal resistance.

Example, if you have a set of series typical green LEDs for class lamps or "markers" then because of series- you may need to stack the 2 coin cells to get 6V to have enough forward voltage.

However, a red set of the same markers- may light on a single cell or also might require 2 stacked.

Or most single and even dual LEDs in parallel test fine with one CR2032

https://youtu.be/7IoyYj6BJlc?t=66

Another option is a Ngineering N8021 LED Tester. Design allows any color LED to be tested. No need to worry about choosing the proper resistor for protect the LED. simply snap the tester n a 9V battery and start testing. Includes spring-loaded contact block to hold wired LEDs.

Many thanks @Vernon Barry

I will check for the cracked pcb.  I tested the LEDs apart from the pcb, using an individual plug like in this picture and using a 3 volt DC supply from a buck converter.  I also just tried the CR 2032 with one and two stacked- nice idea and easier to do.  Still can't get these or the ditch lights to turn on- puzzling.  Neither these below or the ones on the loco front respond.

You should not be attempting to light bare LED's with a power supply with no current limiting.  If you want to test LED's, I suggest a 1K series resistor and 9 to 12 volts DC.  That's a safe way to test LED's.

You have to remember that LED's are current mode devices and they don't act like resistors.   A white LED will typically draw it's rated 20ma at 3VDC, however by the time you get the voltage to 4VDC, it's already over 60ma, three times it's max current rating.  The bright flash and darkness that soon follows spells the end of that LED.