It turned off out of the blue, and now no signs of life.
Things to try
It’s listed as: 19.5 V output, 3.34A
Testing the power supply with the multimeter, it actually was reading out around 19.5. And lo and behold, the LED is actually back on again now.
So let’s see if it’ll power the laptop again OK.
Wait, weird, it’s gone again. Is it a loose connection? Or the laptop shorted it? Yeah, the laptop shorts it out it seems? The light goes off when you plug it into the laptop.
My laptop charger turns off as soon as I plug it in to the laptop. It’s not t…
It sounds like it might be a short circuit on the motherboard… oh dear, not good.
Dell power supply light goes out when plugged into the laptop computer #short… ‎Laptop shorting out charger | DELL Technologies laptop adapter light turns off when plugged in - YouTube
Possibly the laptop overheating could have caused it?
laptop motherboard "shorts" when connected to adapter - Super User
This post suggests a couple of things to try:
‎Latitude E7450 Power Failure | DELL Technologies
check the voltage of the internal power cable if possible
This guy has a great video, same symptom:
Dell Latitude E7450, no power, charger cuts out - LFC#318 - YouTube
He shows how to check the internal DC jack.
I think he’s checking the 2nd and 3rd pins there. If I do that, no continuity.
Here’s a handy teardown video for replacing DC jack:
Dell Latitude E7450 DC Jack Replacement Video Tutorial - YouTube