Philreilly

Right.. Thought I would share my experience. I'm new to mercedes and also new to this forum.. But hope this will help some of you out there with similar problems. I drive a 2008 w204 2.2cdi estate. It is a beautiful car of excellent build and finish but recently my confidence in the electronics has gone south!
All of a sudden the car lost (wait for it!!) electric boot not operating, rear indicators showing a fault on the display, but operating correctly when outside the car, lost my fuel gauge, lost the heated seats and rear de mister, lost the use of my key fob to open the car (unless standing right at the drivers door) and my lights would not switch off as well as my front and rear fog lights not working..!!!!
My first reaction was, this is gonna be expensive and after checking multiple forums for answers I couldn't find either a reason or a solution... Most topics covered one or two of the issues, but not all at once.. And even at that they seemed to relate to low voltage or battery issues. I suspected the rear SAM and after checking all the connectors and fuses everything was fine.. Even some local MB specialists suspected the rear sam but I was hesitant to get a diagnosis until I was happy that I had done what I could..
My last resort was to disconnect the battery and reset the ECU, so I tried that and hey presto the car was back to normal!!!! I still have no idea what the cause was, but there must have been a few hidden gremlins in the electrical system which it has cleared..
I still have a parktronic fault, but I had that previously anyhow so thats next on the list! But hopefully this may help some of you out there with similar issues!!

MJ50

moved to W204 forum...
glad to hear battery disconnect / ECU reset fixed the problem...

Carsy

Phil,

A very useful post . Thank you.

You battery is a few years old . I wondered whether it would be wise to have it load tested by an auto electrician ?.

I enjoy driving my little diesel each time I take it out.

Earths being loose to chassis can cause some weird symptoms too.

c300white

I would check battery voltage after standing overnight with multimeter on battery terminals , then voltage reading at idle and reading at 3000 rpm ...

Overnight voltage should be 12-12.8 range (if a lot lower battery should be load tested)

Idle should be 13-14 v (without lights/accessories on) and 3000rpm should be a little higher but not over 14 - else you might have voltage regulator problem which could mess up your ECU .

At idle with lights and seat heaters on what does it drop to ? Again if lower than 13 - get battery load tested at auto parts store for free - ground cable issues or alternator diodes could also cause problems - so that would be next place to check if battery is good ...

Philreilly

Thanks for replying guys. I appreciate the feedback, so I took your advice and got the multimeter out today. Heres the readings.
At rest, after a night and day sitting 12.04V
Idle 14.1V
2000rpm 14.54V
3000rpm 14.64V
I'm not sure what this is telling me, but as far as I've read voltage should not exceed much over 14V as this could lead to erratic system failures due to a dodgy voltage regulator. (According to discussion on https://mbworld.org/forums/subscription.php?do=addsubscription&t=487314)
Whats your thoughts?

c300white

14.6 could be a bit high but maybe nothing to worry about - issue is if there are transients much above that it could mess with your ECU - I'd go to local alternator rebuild shop and get an opinion - maybe time to change the regulator - should be a cheap part -

I had my porsche bosch alternator rebuilt for <$200 recently in San jose due to bad diodes - in that case it was causing my battery to drain overnight ...

Carsy

G'day Phil,

I took a few readings with my digital multimeter & logged them for my own reference down the track .

At battery terminals :-

Standstill = 12.18 volts.
Idle = 13.9 volts average.
2000rpm = 13.88 volts.

My battery was put in the car Oct 2007 so it is 7 1/2 years old.

Why your voltage is over .6 volts higher than mine at 2000 revs is a mystery to me. It could be battery condition. It would be interesting to ask an expert.

c300white

Auto Battery is 12.8v nominal but will typically read lower under load and/or after standing . To charge it needs 13-14v from alternator - so any higher voltage you are seeing is from alternator. A regulator on the alternator should limit maximum voltage to around 14 ...13.8 is probably ideal under normal loads with a good battery. Much higher than this probably needs checking out ....

jpminou

Hi, i was checking the voltage in my car so i noticed when driving the car the voltage is 12.7v and when i remove my leg from pedal it goes to 15.1v and stay there till i hit the pedal again it go back to 12.7v so this normal?