Intermittent battery drain W220

[Ratel]

I can see battery drain is a common problem. My S400 CDI is in daily use and on four separate occasions over the last yearI have found the car dead. usually overnight, but one incident actually happened after work… Seems to be somethnig happening randomly, but the powerdrain must be significant to drain a 100 Ah battery in less than ten hours. Charging the battery and starting the car leaves everything like normal until Next time it happens. I have yet to find any predicting signs of this.
However, two days ago I found the car dead but this time I noticed no effect of charging. Disconnecting the battery let it charge normal. When reconneting I noticed that there was a significant current, and My amp-meter maxed out at 60+ Amp!!! Ignoring this and starting the car seemed to reset back to normal.
My question then is: does anyone know what could have stayed active that would drain that much? I am now in the process of furter fault finding as the car now reports no communication with the front right SAM, and door control module in the front right door. (As I live in Norway, this would consequently be the passenger door).

 

[Blobcat]

Hello and welcome,

Could be a faulty diode in the regulator pack on the alternator.
Could be Comand not shutting down (or waking up again)

 

[Ratel]

Thank You

I find it hard to believe though that the Command unit is able to draw over 60 Amps… That is a massive current and wouldn't that cause a lot of heat somewhere?

The regulator pack on the alternator is a likely culprit though. Tonight I had a bit longer drive and the battery went dead while driving... No error message telling me this, but quite interesting behaviour on many things as the voltage went down…

Next step replacing the alternator. Seems to be a bitch to get to. Anyone who has done the job and can give any pointers on how much else need to be removed to replace it?

 

[mersum1es]

Electric Heater booster

 

[mioba]

Be sure to make sure its your altenator.
Where are you based as the w220 was not sold in the UK.
My W220 altenator failed after 16 years but it was accompanied with a red battery warning light on the dash. A check with the voltmeter was pretty conclusive it was the Alty.

 

[Ratel]

Thank you all for your replies.

The alternator seems to be guilty in this case. The 150A prefuse was blown, and between the charging lead from the alternator and ground I read 4 ohms. I believe this is indicative of a shorted diode...

I am now in the process of removing a lot of parts to get access to the alternator. So far I have established it is a watercooled unit and quite pricey. Hopefully a new regulator will fix the problem

 

[Ratel]

Update.
After all the work on removing the alternator and checking it, it turned out to be just fine. Further tedious investigation finally identified the radiator fan control box to be the offending item... this displayed obvious signs of past inferior repairs and was full of water. Not sure if this is a problem to be aware of for others or just black swan in my case. This is what it looked like:


I must admit that I have never worked in an engine bay with less space available for hands, tools light or even finding a line of sight to wherever. This engine in this body does not leave much space for anything. Just getting to the alternator required removing quite a lot of other parts. The intercooler providing the biggest challenge. While I had some accessibility anyways, I decided to replace the engine mounts as well. The left one requiring even more intricate manouvering of parts, tools, hands and light... Glad I'm not doing that again in the foreseeable future... water pump and serpentine belt was also replaced for good measure needless to say.
Now I'm just happy I found the root cause