When just one specific motor is turned on it doesn't work propely

[mlamounier]

Hi Jan,
I'm try to using but I've a problema when I try turn on just a M3 motor. It start very slow or not start. When I turn on the four motors, everything is ok.
I'm using raspberry pi 4 model B

Other issue, when I turn on the raspberry and motordrive together some motors start work unexpected

coding working:

amspi.run_dc_motors([amspi.DC_Motor_1, amspi.DC_Motor_2, amspi.DC_Motor_3, amspi.DC_Motor_4])    
time.sleep(3)

amspi.stop_dc_motors([amspi.DC_Motor_1, amspi.DC_Motor_2, amspi.DC_Motor_3, amspi.DC_Motor_4])

Not working propely :

amspi.run_dc_motors([amspi.DC_Motor_3])
time.sleep(3)
amspi.stop_dc_motors([amspi.DC_Motor_3])

[Zogoo]

From blog comment:
I also found that (as was written in a previous post) there is no regular ground connection between RaspberryPi and the motor shield. The only ground connection is used to pull pin 13 of IC 74HC595 to LOW. It would therefore be advisable to connect, for example, pin 6 (GND) from the RaspberryPi to one of the pins next to the 5V connection on the motor shield.

So you need to connect pin 6 (Raspberry) to GND of 74HC595 chip.