I'm currently trying to build a small independent rover from my old Lego Technics set. Currently I power two 9v DC motors (2838) using an Arduino motor shield R3 in combination with an Arduino Uno. I connect the motors independently to the two outputs of the motor shield and use an external 9V power supply to power the Arduino via the barrel jack. All this worked fine in the beginning with both motors doing their job at full speed. (The software should be OK - I did set all needed pins on their desired values (brakes off, directions on, PWM to 255)).
Now here's the problem: After having tinkered around a bit with my design it seems to me as if I have broken the motor shield. As I said, the software worked in the beginning. I also double checked the motors by directly connecting a 9V battery to them, they both work just fine. Also, the Arduino board itself is unharmed; I checked it with other designs just to make sure. But the motor shield does not supply enough power to the outputs anymore and so the motors don't run properly. I used my multimeter on the motor shield's output terminal screws and voltage was down to about 7.4v. Is this a normal behaviour or is the shield really broken? If so, what could be a possible cause for the lack of power on the output?
I would be very thankful for any comments or hints, since googling the problem did not show any useful results.