Sneakier way to fail - C220 CDi Sport - battery drain

[Botus]

Scantec uncovers parasitic battery drain cause

Scantec Automotive’s Ross Kemp explains how his team came to uncover the culprit of parasitic battery drain on a Mercedes C220 CDi Sport. We were recently asked to investigate an intermittent parasiti

pmmonline.co.uk

car locked, yet wakes itself up and kills the battery intermittently....

wiring diagrams suggested this battery drain could only be caused by the air conditioning (A/C) control module; the fan motor module is a LIN slave and therefore is incapable of waking the interior CAN to flatten the battery. However, the A/C control module can wake the interior CAN and does command the fan module as it’s the LIN master.

Wiring diagrams showed one fuse for the A/C module in the front SAM fuse box, so we removed the fuse, and were somewhat surprised to find the exact same symptoms occurring!

had we missed something? ....Mercedes have a secondary power supply not marked as such on wiring diagrams, to ensure the battery drain works when the AC module fails....

 

[Tony Dyson]

Just goes to show, a simple battery drain isn't always as straightforward to diagnose armed with only a cheap multimeter and a steely determination, I would however be rather suspicious of the following being the product of an undocumented DIY modification rather than OEM! "What we didn’t realise was that there was one wire running between the A/C control module and the front SAM that had absolutely no designation or terminal markings."

 

[domwild]

My C220 CDI W204 switched the RH parking lights on intermittently and this forum suggested switch light switch to AUTO and not to OFF and that seems to work. But I still have problems and will keep on looking!

 

[Botus]

My C220 CDI W204 switched the RH parking lights on intermittently and this forum suggested switch light switch to AUTO and not to OFF and that seems to work. But I still have problems and will keep on looking!

are you sure this isn't parking with the indicator switch still on - to the right ? its a normal german car parking light feature (available for the left or right side)

 

[Botus]

Just goes to show, a simple battery drain isn't always as straightforward to diagnose armed with only a cheap multimeter and a steely determination, I would however be rather suspicious of the following being the product of an undocumented DIY modification rather than OEM! "What we didn’t realise was that there was one wire running between the A/C control module and the front SAM that had absolutely no designation or terminal markings."

which wasn't on the wiring diagram - but they then go on to say it should be - if they can work all this out, I'm sure then can spot the difference between manu build and a pikey bodge up