Ah, yes. Just looked at the Mega schematic, and Vin does feed INTO the 5V regulator. Which means the 5V regulator won't be outputting 5V (it has a minimum drop of perhaps 1.5V, resulting in about 3.5V output, which then reduces the output of the 3.3V regulator as well.. not good).

Vin needs to be at least 6.5V, or probably at least 7V.
Using the USB connector bypasses that, allowing a 5V feed.
This method has the advantage of also being compatible with out-of-car usage. smile




Edited by mlord (09/01/2018 03:03)