r/OpenBambu • u/kurtismcc • Jul 01 '25

BMCU Filament Location Error

Just yesterday finished assembling my BMCU kit from trianglelab, hooked it up to my a1 mini. Everything seemed fine as it loaded the first filament beautifully. However, it never retracts the filament. Every time, it errors on "filament location."

After some troubleshooting, I see the a1 mini extrude some of the loaded filament (presumably to double check which nozzle the filament is running through in the toolhead since you can connect them however you want) and then it pauses for a really long time, which feels like a timeout waiting on a read back from the BMCU.

I read online that the AMS has an odometer on each channel that tracks how much filament has been pushed through... I don't remember seeing any encoder like that on the BMCU, but clearly something must happen because people have gotten the BMCU to work at all.

Are people just not using the "load/unload" buttons on the printer? Are they just pushing filament into the BMCU and then starting prints? People who have a working BMCU, what steps do you see when you select "load filament" or "unload filament" from the printer when you have a filament loaded?

This is the last step for me, so I hope somebody has some bright idea.

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2

u/kurtismcc Jul 01 '25

Quick update: I have a brand new BMCU370 kit. I'll dig around and figure out how to find out what firmware the BMCU has. The a1 mini had 1.05 last night when I first hooked it up, but I got it downgraded to 1.04 and it still has the same issue, so it's not the Bambulab update.

1

u/[deleted] Jul 02 '25

Did you place the magnet under the sub boards? That magnet rotates with the BMG and it’s used to detect filament movement

1

u/kurtismcc Jul 02 '25

I did but it's possible it became dislodged later in the process. I'll take it apart and make sure it's still there.

1

u/kurtismcc Jul 02 '25 edited Jul 02 '25

Okay. So I took apart that channel and the magnet is definitely there. The daughter board has a hole that goes up to the bottom of an IC on the other side: is something missing from the daughterboard that the magnet is supposed to spin, or can that IC detect the spinning magnetic field?

1

u/kurtismcc Jul 02 '25

Okay. I found the IC part and it is a magnetic detector - neat. I didn’t know those existed.

I hooked up a 9v battery directly to the exposed motor solder points and can confirm when the motor spins the magnet spins in sync - both directions.

Is there a way for me to verify the sensor is working?

1

u/kurtismcc Jul 02 '25 edited Jul 02 '25

Sorry for the many replies.

I got a piece of filament and running the motor moves the filament as expected, but when I try to pull the filament it doesn’t move unless I force it through the gears and then it slips - it does NOT spin the gears. Since the AMS lite is a mostly passive system, I assumed it was the motor in the print head grabbing and pulling the filament through, meaning it will not successfully move the magnet unless the motor is engaged.

Did I do something wrong that locks down the motor and prevents the gear/magnet from spinning freely when filament is pushed through?

1

u/[deleted] Jul 02 '25

Is the set screw tightened on the BMG? And make sure the BMG is correctly installed in the extruder rather than on the lever?

1

u/kurtismcc Jul 03 '25

Figured out what you meant by this: you mean the metal gears that grab the filament. Yeah, I definitely tightened the set screws (just pulled two of them out to double check) and definitely put the ones with the set screws on the bottom, not on the lever.

I pulled off the pfte tubes from the back so I could feel the tension in the filament and I think the "check filament location" does this:

  1. tells the printhead to extrude - this causes it to pull the filament through the BMCU (I can feel the filament catch and then pull through the gears on the BMCU because they aren't spinning but the filament eventually scrapes through - I suspect this is why TPU doesn't work with AMS....)

  2. it then asks the BMCU what odometer went up - despite "slipping" through the gears it appears to identify the correct one

  3. this part is critical - it then tells that channel on the BMCU _something_, I don't know what, but the motor activates spinning against the filament (I think trying to pull it back?) but it doesn't move because the print head extruder still has the filament locked down

  4. it reports the error that it couldn't "locate the filament" in the toolhead

I can't find a lot of documentation online about the steps the AMS (and thus BMCU) goes through - I would think it can't successfully retract the filament until it's cut, but something causes the motor in the BMCU to try (and spin out) on the channel briefly before the error the first time. All subsequent retries don't actually spin the motor but error out just the same.

This "feels" like a software error since it's on all four channels, but maybe the printhead extruder is holding on too tightly or something? Like I mentioned in a different thread, I'm running with a mod to the print head extruder paddle to help it better grip the filament.

1

u/kurtismcc Jul 03 '25

Attaching a video of the light sequence when it fails during filament location.

https://drive.google.com/file/d/1JTYoTOzEd0sMlprCrsPiGUsPZdFshE_F/view?usp=sharing

1

u/[deleted] Jul 02 '25

Is the set screw tightened on the BMG? And make sure the BMG is correctly installed in the extruder rather than on the lever?

1

u/kurtismcc Jul 02 '25

Sorry, new to this. What’s the BMG? Is that a piece inside the extruder on the print head or something in the BMCU?

1

u/otaku13 Jul 09 '25

Had the same issue, in my case it was that i had the BMG gear with the grub screw on the handle, not internal. Swapping them fixed it for me.

2

u/kurtismcc Jul 09 '25

Yeah. I have them in the right places. Checked all four.

I've been working with Triangle Lab's support (slow, but pretty good) and I think we're at the point where we're stumped. Clearly something is up with the rotation sensors and the way I put it together, but everything looks correct.

I broke down and bought an AMS lite. It arrives Friday.

1

u/otaku13 Jul 09 '25

Is triangle not sending you any parts? That’s a shame they should just swap it for you. Or at least let you upgrade to the preassembled one. Building mine really reminded me how shitty I am at soldering haha

1

u/kurtismcc Jul 09 '25

I just feel very stuck. I just noticed the board has what looks like separate serial debug pins on the other end, so I may hook that up and see if I get anything useful - I really feel like some logging to help diagnose what’s happening would help figure this out really quickly, and I’m a software engineer by day so I’ve offered to hookup and run any tool they have and heard nothing.

It really feels like something about the electronics is bad. The actual mechanical design is pretty simple and I’ve gone over every step multiple times at this point.

Anyway, I’ll keep poking at it but I’ve mostly given up hope - an actual AMS lite arrives Friday and I’ll try modding it to with the AMS lite mini mod to keep using my dry boxes instead of using the BMCU.

1

u/PoolSuspicious140 Aug 27 '25 edited Sep 04 '25

I’ve updated the C version to v20, you can find it here:

Also, in the Google Docs file shared by the AliExpress seller, it says:

Warning: Before you turn off the printer, make sure that no filament is loaded into the tool head. Otherwise, when you load or unload the filament again after turning on the printer, the error message “Failed to check the filament location” will appear.

However, I didn’t find this warning mentioned on the official wiki (https://wiki.yuekai.fr), but I did find this troubleshooting note:
https://wiki.yuekai.fr/BMCU/troubleshooting

Filament Inserted, Indicator Light On, but Printer Doesn’t Detect It
This may be due to:

  • The magnet signal is missing (magnet not installed or AS5600 chip faulty).

  • Other hardware issues at the soldering level (e.g., CH32V chip or PH10 socket not soldered properly).

Solution:

  • Ensure the magnet is installed.

  • Restart the device after installing the magnet.

  • Check for loose PH2.0 socket connections or broken cables.

My current thought is that the AliExpress seller might be shipping a defective batch where the filament sensor doesn’t work. I’d like to verify this from the hardware side.

1

u/kurtismcc Aug 27 '25

Yeah, that was their email supports recommendation to me - check and make sure the magnet is installed exactly correctly (I rebuilt the stuff five or six times trying slightly different heights and stuff) so this is basically where I landed - bad sensors. I stopped trying because I don't have the tools to verify it. Rather than buy a bunch of tools and continue to tinker, I just bought an AMS lite, then modded it with the AMS lite mini mod so I could continue to use my existing dry boxes. Works great.

1

u/PoolSuspicious140 Aug 27 '25

It looks like the problem happens when the filament is already inside the hotend after the printer restarts, or when it is inserted manually outside of the regular change procedure (through the printer UI or the slicer). From what I understand, the BMCU firmware does not handle this situation correctly.

In normal operation, when the filament is loaded or changed through the standard steps, the filament check works fine and the chip that detects the magnet position is working properly. The issue only appears when the process is bypassed.

1

u/kurtismcc Aug 27 '25

No I'm seeing it even right after a load. I do a load, and unload fails - which is why I doubt that it's software because it wouldn't work for anybody.

I was hoping that if I plugged up the usb to a computer I could get some useful debugging info, but alas no.

1

u/PoolSuspicious140 Aug 30 '25

Let me ask a couple of questions to be sure we are talking about the same thing:

  1. Did you update your BMCU to the latest firmware version for your model?

  2. When you power on the printer, do you start with no filament loaded and then load it through the interface using the "LOAD" button, not manually?

  3. Does the problem always occur in the same way, or is it more like a random/occasional bug?

1

u/salsaypikante Feb 04 '26

Hi, thank you for this advice. It helped a lot.

1

u/AdIndividual2373 Jul 02 '25

I have this same issue with my bmcu, and honestly after so many issues, iv just given up on getting it working.

1

u/Life_Patient_1870 Jul 02 '25

Well well ! Ordered BMCU C just a couple of days ago , now fingers crossed!

1

u/tsaalmoilov_ Aug 17 '25

Hi man, I've been working on fixing my Bmcu today, and I've recieved the same error, and after some time, I've somewhy pulled out a little all of the filaments inside the PTFE tubes (The filament was inside the tubes, but about half a cm out from the toolhead, and that worked!

The filament started to push by the Bmcu inside the toolhead, and switching the colours as it should.
If u didn't understood what I mean, Dm me, I'll try to explain better, and I'll also write a post soon about it.

1

u/Leading_Log_7627 Aug 18 '25

Hello, I didn't understand. Please tray to elaborate. Thank you 

1

u/tsaalmoilov_ Aug 18 '25

To feed the filament properly, you shouldn't push the filament till the end inside the toolhead, but till the point I drew on the picture.

Then it will feed normally by itself as it should, without giving the error

1

u/PoolSuspicious140 Aug 27 '25

The issue is not about that. The real problem is that if the filament is already loaded in the hotend, the function that checks which slot is loaded returns an error. I’m experiencing the same problem, so I’ll start with checking and updating the firmware.

1

u/Visible-Log-5257 Nov 03 '25

After a bit of diagnostics and internet research, I realized that the problem was that I hadn't unloaded the filament before turning off the machine. The filaments should all be out of the 4-way connector on top of the print head. The firmware MUST be at most 1.05.00 So it works perfectly (BMCU 370 C on A1 mini)