openinverter forum

Hi everyone,

I'm looking to find some information for my project which is not Openinverter specific, but it seems like this forum is the place with the highest concentration of Mercedes B250e knowledge.

I've recently bought two Mercedes B250 Electric Drive with different issues:

No. 1 has a coolant flooded Tesla LDU, still drives jerkily, but needs the motor overhauled or replaced.
No. 2 has a mint LDU, but doesn't drive and has several fault codes from the BMS (ECU number N82/2) (U1C47F1, U1C03F1, U1C6BF1, U1C61F1, U1C62F1, U1C07F1).

I'm now contemplating what to do with these. At the moment, my favored option is taking the battery and BMS from No. 1 and putting it into No. 2. I could then keep the rest (dead battery/BMS, dead motor) for a possible future EV conversion that i've been thinking about.
However, I cannot find any information about the necessary steps for swapping the BMS from one car to the other. Whether it's possible at all, whether I need to reprogram the BMS before it will work with the other car, nothing.

I know the BMS is Tesla made... so I wonder if anyone has any knowledge about it? @asavage maybe? I've read many of your B250e/RAV4 posts across various forums.

Hi,
I have a B250E that I filled with coolant. The engine electronics were also flooded. The windings are new and have been checked. Now I have a BMS error: U1C3BF1 - "High Voltage Battery Module Voltage" sensor malfunction.
Did you fix the problem?

No, unfortunately not, but I didn't have time to continue looking into it yet.
I'm still trying to figure out how to switch the batteries from one car to another without a lift, to see whether they just work without having to reprogram the BMS ECU.

Try swapping the batteries and see if you still get errors in the BMS.
I don't think the BMS will need to be programmed.

You can also swap LDU, and replace immobiliser from original LDU...
Most of the time moisture/glycol in immobiliser connectors

My board was soaked in glycol, the socket was damaged, and the connections fell off. Does this LDU board have an immobilizer?

hanskraayeveld wrote: ↑Wed Oct 08, 2025 5:08 pm You can also swap LDU, and replace immobiliser from original LDU...
Most of the time moisture/glycol in immobiliser connectors

I'd prefer to switch the batteries because the car with the batery issues is in much better condition than the one with the flooded LDU.

It's just really difficult to find any kind of information on the W242, so I'd have to switch the batteries without knowing whether I would need to code anything. If I remember correctly, there is a CBF file called something like "242BMS", which might point at the possibility of flashing something.

Does anyone know whether I can use the Tesla Diag Software to read out errors from the Tesla controller in the trunk of the car? The stuff I can read with Xentry is very basic, it seems like the Tesla system doesn't hand over all the information it has to the Mercedes ECUs.

Anyways, excited to see some interested in this car from other tinkerers... I felt like I was alone trying to fix this car until now.

hello mate . Di dyou get to the bottom of this ? I work on mercedes b250e and I had this fault code U1C3BF1- sensor voltage of high voltage battery modulehas a malfunction . I replaced the HV battery - it dont need programming . I opened old battery to find 1 module faulty . I got tool for Tesla batteries to check voltage + voltage difference in then module .

Good news is that the specific module you connected to is reporting voltages across all 7 cell groups - BMB 1 should read 4 cells, with BMB2 reading the remaining 3 cells. The module is a 7S configuration. Couple of thoughts:

- I have no idea what the upper voltage cell in-balance threshold is, but the 3.964V cell group seems like an outlier. Try bleeding it down gently by carefully connecting some kind of load to it
- how does the 7s module voltage compare to the 11 other module voltages? Is it different?
-Are you certain fault code U1C3BF1 points to the module you are measuring? Do the other modules all report well balanced cell groups?

You may have an intermittent fault due to a loose balance lead. My B250e pack suffered this problem. Before you do anything, check that the readings on your CanOpener gauge matches readings on your multimeter for each cell

sokisoki3 wrote: ↑Wed Dec 10, 2025 9:52 pm I replaced the HV battery - it dont need programming .

Hey! Sorry I didn't reply earlier, I hadn't enabled e-mail notifications for this forum and didn't see your post until I found my own thread on google. Totally forgot about it.

I finally gathered the courage to swap my batteries solely because you wrote that it's possible withou reprogramming and... it works. Really appreciate your post, that was a lifesaver. Couldn't be happier.

Unfortunately I can't help with your question as I haven't opened the battery, sorry. I swapped it into the car with the motor damage and plan selling it as a donor car.

This thread (in German) might be helpful: https://www.goingelectric.de/forum/view ... 82&t=98178
A Smart ED expert wrote that the fault (voltage too high in one cell, voltage to low for the adjacent cell) might be a cold soldering point between two soldering points. Might be worth checking out.