I'm using this motor shield for arduino from adafruit. It says it can drive 4 DC motors with it. However, two terminals M3 & M4 (as marked in the image) seem to be supplying lesser voltage.
To avoid this becoming an XY problem, some more details:
The entire setup consists of an arduino, shield, raspberry Pi 3 & the motors being powered with a 2200 mAh 12v supply. The raspberry pi is powered from the 5v supply of this shield.
Now the problem as mentioned is that even though M1 & M2 get the 12v, M3 & M4 don't. When I measured they give varying degree of voltage (3v - 6v). I tried disconnecting & connecting various combinations, with & without motors connected, pi removed, etc. All are giving same result. When I supplied 5v as external power, these pins in question dropped to 2v-3v.
As per this documentation, there seems to be a difference in M3, M4 in that they run at 1KHZ, to save power. But my knowledge to make of this info seems to be limited. Can anyone point me to the right direction here?