Yesterday I had the basic driving robot with two large motors, but I could not get both motors running at the same time. I had the tank control programming block, with both motors set to 50% and when running the program both motors would start and one would continue spinning, but the other would stop after only 1 or 2 degrees of rotation.
First I thought it was a problem with the motor, so I replaced it with another motor from another set, but the problem remained. When I switched the ports it was the other motor that would not keep running. I tried different ports, changing the cables and restarting the brick but nothing helped. Checked the rechargeable battery to see if it was a power issue, but that was full. I did this in a class with several other EV3 sets using both large motors and none of the others had this problem.
I then used the programming block for a single large motor and tried both separately and that worked fine. But when I put two of those blocks after each other to start the motors sequentially again the second would not run after an initial click.
When I wanted to troubleshoot it further after I got home everything worked fine, which makes it even stranger. That also suggests it was not the battery box as mentioned in this post.
So I hope maybe someone has seen this before and can tell me why this happened and what to do should it happen again.
EDIT: I found the faulty EV3 (it was number 9, not number 6....). So I was able to do more troubleshooting. Attached a screenshot of the various programs I tried with comments.
It looks like port C is consuming more power than the other ports (motor runs at a significantly higher rpm). And this shuts down any motor running at the same time through one of the other ports. combining two of the other ports (e.g. A and D) works fine.
Any thoughts on how this can happen?