Hello everyone,
I saw a few old posts asking about the output voltage of the M1-M4 ports on the MKR Motor Carrier, and it seems that the questions were not answered. I was wondering the same and did a measurement (the official documentation did not mention it and I don't have the knowledge to understand the schematics.). I powered the carrier from the power terminal at ~7.3V and set M2 to run at full speed. The M2 had a DC motor load and the voltage difference measured at the + and - screw terminals of M2 was ~7.2V. It seems that the carrier does not boost the voltage to 12V, which Nano Motor Carrier does based on its documentation.
Just hope that this info could be useful to someone in the future.
Kang