DC/DC not charging 12v battery (Gateway Comm Issues?)

Howdy folks,
I've got quite a bit of experience working on the B-Class (rebuilt lots of drive units, replaced OBVCs, etc.), however I've come across one that has a bit of a puzzling issue that I've never encountered. When the car is started, the DC/DC converter does not power on to charge the 12v battery. Other than this, the car functions normally, charges just fine, drives fine, etc, until of course the 12v starts to get too low, then the car starts freaking out...

While it *could* be a DC/DC problem, my thought is that there is a communication issue with the Powertrain Gateway, as I've been unable to connect to it with my scan equipment, and the car also throws several intermittent loss of CAN messages related to the Gateway.

Curious to see if anyone has encountered this issue before and fixed it, before I start throwing parts at it...

Even the B-Electric has a OBDII port, but it doesn't help since it is not an ICE engine. Once the 12 V "Vehicle Battery" is low on Voltage, there are a lot of error messages generated not even reflecting actual problems. I had a message showing to visit shop for gear problem on an early morning. I was thinking to dump this car since at 80K, I would not spend a few K's to replace the motor drive. However, after reading several messages from the forum. I decide to check the 12 V battery which just replace 5 months ago. It was low 11.6V, looking like a bad one under 36-month warranty. I charged it by standing alone for a day to get fully charged, while no power providing to the entire care to reset other electronic control devices. At the same time, I check the speed sensor to see it is clean, not messy as those inherited the problem with cooling leak. To my surprise it is clean, giving me a piece of me the this car still can run. Why it was my surprise? since I had Mercedes dealer replace the motor drive at 40 K, not it is 80K, I thought that it was time again. I put every thing back and it ran fine, no error message. I have a USB cigarette charger with voltage display plug in. Next morning, before the start the Voltage displayed 11.5V. As soon as I started, the voltage displayed 13.6. Besides knowing needing to replace the battery, I could see that as soon as the car start the DC/DC converter will take care of the Alternator functioning to maintain the Voltage level for other control modules. Therefore, in your case, it is definitely a DC/DC converter issue. I see a used one on eBay around $250 in Nov 2024, not sure that a new one could be obtained from Tesla.

Does a new battery solve these problems? Can we do something to reset the car? We have a B, replaced the engine €11k. And it stopped working again after 1,5 month. Sick of the car and the problems, we had some 'diagnosis' but no-one seems to know anything about this car.

At this moment the car hasnt been driving since april 24. and the engine malfunction was from june 23 til feb 24.

today we put in a new battery, but the 'stop switch engine off' was directly visible.

ajbessinger said:

Howdy folks,
I've got quite a bit of experience working on the B-Class (rebuilt lots of drive units, replaced OBVCs, etc.), however I've come across one that has a bit of a puzzling issue that I've never encountered. When the car is started, the DC/DC converter does not power on to charge the 12v battery. Other than this, the car functions normally, charges just fine, drives fine, etc, until of course the 12v starts to get too low, then the car starts freaking out...

While it *could* be a DC/DC problem, my thought is that there is a communication issue with the Powertrain Gateway, as I've been unable to connect to it with my scan equipment, and the car also throws several intermittent loss of CAN messages related to the Gateway.

Curious to see if anyone has encountered this issue before and fixed it, before I start throwing parts at it...

Click to expand...

It could be that you are using the wrong cable. I had thin cable supplied by the previous owner . It didn't work on any dc/dc public charger. So I brought a very thick 3 phase cable type 2 cable. Had no problem with the new one whatsoever.