This guide is just meant as a light reference for others who may be trying to fix their own robot vacuum and wanting to replace the main rolling brush motor. It isn't meant to be a comprehensive guide and will probably only make sense to someone who is already familiar with electronics and debugging.
Note: I'm working with a Eufy RoboVac 25C but the Eufy 11S (slim) seems to be pretty much the same vaccuum so this should apply to that as well.
This just started out as my vacuum often stopping and giving 10 beep code. Since, I didn't have the app on my phone (no one wants that vendor spyware), I just thought the cliff sensors were triggering randomly on dark edges in the rug/carpet or transitions to hard surfaces. There weren't any obstructions in the main rolling brush or side brushes for me to notice something wrong either. And unfortunately, their beep sound error code feedback system doesn't distinguish between different reasons for why it stops. My first stab at solving this was covering up the cliff sensors with paper laminated in tape so the beam would just reflect back up to mimic a flat surface and never trigger. But the issue persisted!
After enduring with the problem for weeks, I checked the app which was saying "Rolling brush is stuck". Since there wasn't any obstruction which would normally cause that over-current protection "stuck" message to kick-in, the next logical step is to check the motor.
Before taking it completely apart, I took out the brush and started the robot and could see that the little white square peg wasn't spinning to drive the brush. This suggests that the motor was likely stalling, causing it to heat up until it drew too much current and saved itself by shutting down.
After taking it apart, the motor spun freely by hand but was really weak and I could easily stop it with my hands and observe the same stalling behavior (needed a little poke to get it going). To rule out the power supply as the problem, I tried hooking it up to a separate 9v battery and it had the same behavior.
Popping the gear assembly out, the bigger bearing inside the assembly also felt a little scratchy which probably doesn't help with the starting force required and potentially led to the downfall of the motor in the first place.
Using a flashlight to look inside the motor housing, nothing seemed burnt on the motor windings but the motor definitely seems to be the problem so I decided to order a replacement.
As a note while you are debugging, the Eufy 25c does not work when the side brushes, main rolling brush, or suction fan are disconnected. It will just roll for about 6.5 seconds before it says one of those items are stuck and need to be checked.
In the comments, there are claims that another Eufy will run with things disconnected but that doesn't seem to be the case with mine. You can also see it fault without the motor connected in this YouTube video.
In order to get to the rolling brush motor and assembly, I had to take off the bumper, bottom cover, main PCB, and suction fan. These videos from Al's Auto and Appliance were very helpful to get an idea of what to expect:
It's possible to buy the whole assembly on some listings but the price is pretty high and all we really need is the motor. The gearbox on the side has a couple bearings in it.
Bearing sizes for reference:
(photos from this "Eufy 11S Robot Vacuum Main Brush Motor & Assembly" ebay listing but it looks the exact same as mine)
The motor is supplied with 9v (measured directly with a multimeter). I could observe that when it rose to 10v, it seemed to trigger the over-current brush stuck error. I would assume it actually looks at the current vs the voltage though.
Motor part number:
Standard Motor rp390-st/20143/dv GMCD070717
Which we can find on the Standard Motor website via http://www.standardmotor.net/product-category/home/home_6/?lang=en#top
I was able to find some listings on AliExpress that looked slightly different than mine but they have the same part number so I assumed it would be fine. These motors have a PCB on the back with a connector instead of just exposing some tabs. The listings say that these work for
Eufy Robvac 11S Max 12 15C 15C Max 25C 30 30C Max 39C Max Coredy Robot Vacuum Cleaner R3500, R3500S. My best luck was just searching for
Eufy 11S brush motor.
I ordered one of the main rolling brush motors above. The PCB on the back of the motor interfered with the plastic on the Eufy 25c but I just cut off the connector and part of the PCB to make it fit and soldered directly to the PCB.
I also replaced the bigger bearing in the brush assembly since it felt a little crunchy.
The new motor is much stronger than the defective one! Eufy is once again rolling around picking everything up 🎉
Update 2023-11-10: The new main rolling brush motor is still going strong but the side brush motors are failing now (not causing any "stuck" problems though). Will write a blog post if I ever get around to that but it's time for a new robot vacuum anyway so I'm holding off. Hopefully some more new dual brush options become available now that iRobot's Roomba patent has expired.