[Cheekymonkey]

It’s a DC-DC converter.

Possibly gone faulty, but you need a diagnostic code scan to further diagnose.

See

https://www.aclassclub.co.uk/threads/charging-the-12v-a250e-vehicle-battery.25624/

Thanks for replying! I did read that thread but it doesn’t make all that much sense to me. Pretty much since having the car I have run it on petrol but since having charging points installed at work I’ve been running it more on electric - could that be the issue? I’m so stressed that Mercedes are going to find a major fault and it’s going to cost me the earth to repair!!

 

[Cheekymonkey]

I’m waiting to hear back from Mercedes…hoping I’ll hear something by today. Will be sure to post on here just in case it helps anyone else with a similar issue

 

[Cheekymonkey]

A bit of an update - MB have conducted initial diagnosis and identified a fault with the high voltage system. They need the master tech to work on it tomorrow to disconnect the system apparently to diagnose further. Still have no clue what all of this means!!

 

[Cheekymonkey]

Right then, so in the end MB suggested they clear the stored fault on the car and charge the battery. I had it back last night, only run it on petrol (EV battery is drained) but the app is again showing that the battery is only partially charged. The system is therefore not charging the 12v battery. As I understand it, hybrids don’t have an alternator. The battery health is fine so it’s not an issue with the battery itself, it’s the system that charges it. MB are warning me that it is going to cost a lot to investigate any further. I’m totally at a loss as to what to do with it now 😭

 

[Cheekymonkey]

I don’t understand why the dealer is being so reluctant

can you ask them for the fault codes that they have read off the car?

that would usually indicate where the problem is.

My guess at this remote distance is the DC to DC converter is faulty.

Well I’m having a bit of a crap experience with the dealer. They told me yesterday that they have no idea what the fault is but when I contact MB directly today for feedback because I wasn’t entirely sure whether it was right to send me away to ‘suck it and see’ this is what they replied with:

Following a diagnostic assessment of the vehicle, they identified a high resistance issue within the high-voltage system. They informed you that they would require an additional hour of diagnostic work to identify the root cause of the issue. Alternatively, they could clear the fault and observe the outcome. This is the option you selected. They also advised that they have previously encountered this fault in vehicles that have not been equipped with a high-voltage or hybrid system.

incidentally, they did not offer me an option, nor did they commit to it being one more hour of diagnostic. Finally, they absolutely did not say ghat they’d seen this fault before!
As for the final sentence, I have literally no idea what that means about my car not being equipped! I’m going to look at the receipt now and see if the code is on there!

 

[Cheekymonkey]

View attachment 13596

 

[Cheekymonkey]

The invoice states: carried out guided fault findings and found fault stored for inter lock circuit. Require wiring checks on high voltage system

 

[Cheekymonkey]

It just means they have come across the same non charging problem on non hybrid cars as well.

Thank you! Makes total sense now re-reading it!!

 

[Cheekymonkey]

How old is the 12v battery?

I presume it’s the same one that was on the car from new. Mercedes have checked the battery and advised that there is no fault with it. They say the issue is with the car generating charge back into the battery

 

[Cheekymonkey]

So, I have the fault code now which is P0A0A00. I have checked and been advised that the battery health was definitely checked and found to be fine. It’s now back on the garage again as, after handing it back to me with the battery charged up, the same thing happened again two days later. Been told more diag work needed now….at £204 per hour 😱