MKR Connector Carrier, low connector voltage when using VIN (non-USB) power

I’ve got a MKR Connector Carrier stacked with a MKR WAN 1310, then an SD Proto shield on top (which has an OLED display soldered to it).

I’m trying to use the “Serial” connector on the carrier, attached to a DFR0077 TTL/RS232 level converter.

If I power the 1310 via USB, then the Serial Grove connector gives me 5 volts and everything is fine, the DFR0077 lights up and I get data from my RS232 device.

If I power the carrier with 12 volts via the VIN screw terminal (and GND screw terminal) but nothing plugged into the 1310’s USB jack, the 1310 runs my sketch, but the Serial Grove connector only puts out about 2.3 volts, the DFR0077 does not light up, and I do not get any data from my RS232 device. I also see only about 2.3 volts on the (empty) 5V screw terminal.

What’s wrong?

Hello Rushtone, I have the same problem, how did you manage to solve it?
I have a MKR Connector Carrier with a MKR WAN 1310. A BMP280/SHT30 sensor is connected on the TWI interface. Values read by the sensor are correct when I power the 1310 via USB, if I power the Carrier with a 9V battery via the Vin screw terminal the sketch still runs but measured values are wrong.
I also found 2.3 V on the empty 5V screw terminal. The 3.7V pin has the correct voltage.