Mercedes W203 Parasitic Battery Drain

[GT160]

Sooooo, sorry to bring back an old thread, by its reappearance you've probably guess i havent got to the bottom of it.

Auto Electrician had a look and as others have advised said leave the Remove the alarm unit and Unplug - IT definitely not working anyways. So have done that with and then put the wire in a plastic bag and tapped to try and prevent water creeping up and into it.

With that Unplugged - the drain is still there all be it at a better acceptable level of approx 200, which drops to about 80 Amps when the dome light is disconnected. All seems to be pointing towards something to do with the lights, especially the fact they do not come on with opening and closing of the doors. Chap seemed to think it might be one of the SAM's, and wanted to change the front SAM, but at the cost of them etc im not keen on the idea of changing parts out without a definitive diagnosis. Unfortunately, he didnt have star i think.

Soooo Im guessing STAR would be able to tell me about the SAM modules Central gateway etc for sure. Going to speak to some local independent specialist probably MBS, but they have quite a wait i believe so if anyone else can help, Im happy to travel up to two hours for the right person to give me a proper diagnosis really. My gut feeling is if there was something wrong with the same SAMs front or rear or central gateways i would be having more issues than just the lights. Everything else works spot on.

Thanks in advance sorry if its a repeat questions but its annoying me now

 

[GT160]

Hi,

Sorry to resurrect an old thread, but I always think its nice to post something that may help others and give the outcome

So very important thing to be aware off when chasing a parasitic drain on the merc w203 anyways and maybe others is you must must leave the car at least 15mins with all the doors shut etc to allow plenty of time for the computers to go to sleep. I found this out the costly way...after finally giving up and sending the car to a merc independent specialist, who said they were getting different readings. I did the test again today after leaving the car for 20 mins and confirmed as expected the garage readings were correct!!!!

I have a total drain of about 80Ma, the alarm has been disconnected. Removing fuse 8 alarm, and fuse 9 interior lighting no longer changes the drain. Removing fuse 23 from the drivers side fusebox drops the drain to 10Ma. Therefore fuse 23 is draining approx 70ma. The garage did further checks and have told me on there xentry or diagnostics its showing that the gateway control module is the cause of the drain and is faulty draining the 70ma.

Does anybody know a rough normal range of expected drain for the w203. To be honest if it was a total of 80Ma i would have probably not even have bothered to investigate further, its just sometimes the car does sit for a week and i thought it was draining 260Ma.

Also where is the gateway control module in the car, the quote i have been given to replace and code is eye watering!! Also what does it actually do as previously i have removed fuse 23 and not noticed anything different in using the car for 3 days? Is there any way of repairing the module or installing a second hand one or will it always need coding etc?

Many thanks in advance for any further help.

 

[Ted]

Pretty sure the maximum permitted drain is 50 mA but Tbh, if you have a drain of 80mA, whilst theoretically excessive, will drain around 2Ah from your battery per day.
Given that your battery is probably at least 60Ah, and likely more, you can work out how long it would take to flatten the battery e.g. from full charge to half charge - about a month (assuming your battery gets fully charged on a run and is in good condition).
I think you might be chasing shadows, and potentially spending a lot of money trying to track down an issue that is marginal at best.
That having been said, the nights are getting longer and colder so it might be worth keeping a close eye on things to ensure the battery doesn’t go too flat over time. If it were me, I would make sure that I carry a set of jump leads, and keep a general eye on the battery voltage with a plug in voltmeter or bluetooth monitor.

 

[GT160]

Pretty sure the maximum permitted drain is 50 mA but Tbh, if you have a drain of 80mA, whilst theoretically excessive, will drain around 2Ah from your battery per day.
Given that your battery is probably at least 60Ah, and likely more, you can work out how long it would take to flatten the battery e.g. from full charge to half charge - about a month (assuming your battery gets fully charged on a run and is in good condition).
I think you might be chasing shadows, and potentially spending a lot of money trying to track down an issue that is marginal at best.
That having been said, the nights are getting longer and colder so it might be worth keeping a close eye on things to ensure the battery doesn’t go too flat over time. If it were me, I would make sure that I carry a set of jump leads, and keep a general eye on the battery voltage with a plug in voltmeter or bluetooth monitor.

Thanks Ted for the reply, to be honest we are thinking very much the same way as long as the car gets a drive once a week, it should be fine really. It is a newish battery less than 6 months old. Whilst it is not really causing a problem as it doesnt effect the use of the car it maybe best left alone....it is my curiosity that is causing the problem as I do not know why fuse 23 drains, when removed it does not drain but also does not seem to affect the cars functioning. I have a jump start power pack, but whenever that is attached, it always says reverse charge and wont jump from it ( works fine on other cars). Also ive seen posts of problems post jump starting i think if i remember correctly to do with the rear sam so have always been a bit nervous to jumpstart it. Instead i tend to disconnect the battery charge it up using the charger then put it back in which is easy enough to do but a bit of a faff.