Motor shield R3 does not supply full power

Other Hardware Motors, Mechanics, Power and CNC
1

Hey ,

here's the story:
I'm currently trying to build a small independent rover from my old lego technics. Crucially I power two 9v DC motors (2838) using an arduino motor shield R3 (http://arduino.cc/en/Main/ArduinoMotorShieldR3) 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 the arduino plug-in jack which also powers the board.
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 had 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 current was down to about 7,4 V. Is this a normal behavior 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.

Best,
Jannis

2

That seems reasonable. The datasheet says that there is a voltage drop with the L298.

3

I thought so. Unfortunately I only measured the voltage only after the problem occurred and not before so I have no means to compare values.
But this still cannot explain why it worked in the beginning and stopped doing so.

4

If its putting out 7.4V to the motor and the motor's not running properly (it should be a bit slower than at 9V) then the motor might be damaged? Overloaded mechanically? Or have I misunderstood "not running properly"?