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We are trying to run our Grand Prix racing car on the IR remote control and would like to use two PF XL motors for the drive. One motor works fine on its own, but when both are connected, they stop running. Could it be that the battery pack power isn't strong enough for two motors?

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    Welcome to LEGO Answers. Can you show us a picture of how you have this wired up?
    – chicks
    Apr 5, 2017 at 11:44
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    You need to explain the circuit you have created or provide a picture of it so we can help you.
    – Timtech
    Apr 5, 2017 at 15:23
  • Thanks for your answers. I'll try to upload a picture tomorrow. We tried to let the motors run free without being built in and it's the same problem.
    – user8557
    Apr 5, 2017 at 20:01
  • We tested the motors directly on the battery box and the motors are running fine for 1or 2 sec and then they stop. one motor at a time is fine.
    – LegoFan
    Apr 6, 2017 at 9:57
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    after a few more tests..... it's the battery box. We tested the motors on another box and everything is running as it should.
    – LegoFan
    Apr 6, 2017 at 9:58

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As others have said, it's difficult know what is going without seeing your design. I would expect what you have described to work, but it is possible that your design is overloading the over current protection in the battery box.

PF battery boxes are designed to provide 800mA. Any more than this, and they have protection built in to cut power and prevent damage.

LEGO indicated that the XL motors draw 300mA under normal conditions, and Philo has the stalled current listed at 1800mA. A single stalled XL motor can overload the battery box, and two of them working hard can easily as well. See this answer for more information.

As a quick check, if you disconnect the motors from your car so that they can just run free, are they able to both run as expected? This should work fine and draw only 160mA if everything is connected and working properly. If not, one of your components must be performing abnormally or not be connected properly. As others have said, a photo would help us diagnose connection or build issues.

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