Question DELL Latitude E6440 automatically turns off shortly after starting up ?

Mar 28, 2021
Hello everyone!
I have a problem with my brother's DELL Latitude E6440 (i5 4300M, 8GB RAM, Intel Integrated Graphics)
So, when I push the startbutton, the laptop turns on (fan and HDD starts) for about 5 seconds and then turns off automatically. During the start, the display remains black.
After that, the LEDs for the WLAN and HDD lights up permanent, meanwhile the LED for the battery flashes every second.
According to the manual, this pattern shows that there's a problem with the memory, but as you'll see in the list below, i already changed the RAM without success.

The list of things i already tried but with no result:
  • Started with no battery, just the AC Adapter connected
  • Cleaned the laptop completely
  • Started just with the essential parts (just one RAM-Stick, no WLAN-Adapter, no keyboard...)
  • Checked the voltage of the PSU (Achieved 19.5V)
  • Changed the CMOS-Battery
  • Changed the Hard-Drive with a 100% working one
  • Changed the RAM with a 100% working one
  • Disconnected the CMOS-Battery for 24h
  • Checked the Power-Jack with a multimeter
  • Checked the Power-Jack connector to the Mainboard with a multimeter
  • Bought a new used Mainboard
  • Changed the CPU with a working one from a friend
Yeah, so I did a couple of things...
But until now, it is still worth it. Those latitude are still being sold for over 200€ here in Germany and I just spent about 60€.
What else could be the source of the problem? I don't know what to do now.

I thank you in advance. :)