Apologies all as I haven't set up notifications on my phone for this app. Will do soon.
Its the consumer battery (located in the rear boot) that's low on charge.
I believe the original battery is an AGM battery. If it was not, I'd be a bit concerned about the different charging voltages for the two. AGM battery would accept the charging voltage for a normal battery but might not get fully charged. Perhaps still behaves better, specifically at cold weather where it is most important.
You mention 'the original battery is a AGM'... are you referring to the starter battery or the consumer battery? The rear battery (original MB battery) mentions 'VRLA' on it but not sure if its a AGM VRLA or non-AGM VRLA...
We replaced the starter battery in 2019 with a specific Bosch unit from ECP (ECP recommended the correct battery according to their database) and it is not a AGM battery... Is this something of a concern - i.e. should both front and rear batteries be of the same type? The rear battery says VRLA on it (original MB battery) so should the front also be VRLA?
Back to the main problem, if you look back to my 1st post regarding the yellow engine fan light and engine fan on when car is cold started - we have been using the car over the last couple of weeks now and noticed that once the car is started, the fan stays on until the engine is warmed up - once engine reaches 90C, fan goes off and never comes on again. I have read elsewhere that there is a sensor in the screen wash reservoir and the coolant expansion tank, which are both connected in series. If one sensor fails, it can trigger this engine fan light etc.
I would like to know for the W221 Pre-facelift 2008 model:
1) Is there actually a sensor in the coolant tank? (I was told there isn't but wanted to confirm this). If there is, what could the part number be (assuming it is a replaceable part)?
2) Part number for the sensor in the screen wash reservoir (again, assuming it is a replaceable part)
3) Part number for the engine fan unit - my local MB specialist told me it is A2219066500 but this appears to be for the facelift models when I search it in eBay.
Star Diagnosis says: 1636-004 - Check component M4/7 engine fan - discontinuity of signal line.
I am told it most likely could be the fan unit but would like to try the sensor(s) first before spending £££ on a new/used fan unit.
Thanks.