c250td window opens when starter motor engages

Discussion in 'Electronics' started by exjay9, Nov 2, 2011.

  1. exjay9

    exjay9 New Member

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    Hi
    My 1998 C250TD w202 has developed the weirdest fault. When I start the car, the rear offside passanger window opens about an inch! Its only been happening for about 2 weeks, I cant think of anything I`ve done that would cause it. I have found a blown fuse in the boot (number 7, 5A) which blows again as soon as I change it, I dont know if this is related or not, everything still seems to work even though its blown. After the car has started, the windows work normally, so it only seems to happen while the starter motor is engaged.
    I have been told it could be an earth fault, but I`ve checked all the brown earth wires under the bonnet, and they all look clean, though I did also clean them all and smear them in vaseline just in case.
    Has anyone come across this before?
    Thanks
    Gary
     
  2. jefrs

    jefrs Hardcore MB Enthusiast

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    It does sound like an earth fault. Not all the earth points are under the bonnet, and some of those there can be hidden e.g. under the air filter.

    It is blowing the fuse for a reason. I suspect it is finding an alternative route to earth, which is triggering the window. Good news is that most of these stupid faults are wiring, not expensive components. The age of the car is also right for this. Bad news is that they are usually intermittent and pita to find.

    Even though the earth points looks good do remember that electricity plus moisture equals corrosion. Undo the nut, lift off the eyelets and scrub them with emery cloth, wire brush the post, re-assemble.
    Vaseline is absolutely the correct stuff there +1 :thumb:

    I suspect your earthing problem is at the door. The door card should be fairly easy to remove.
    Do check around the window motor, I had the PCB come loose from its motor on the headrest, broken plastic clip, it just needed wedging in place.

    Also look for frayed insulation.
    I used shrink sleeve to fix this inside the door mirror, three of these wires had to be cut to get the sleeves on, and re-joined by solder.
     
    Last edited: Nov 3, 2011
  3. jefrs

    jefrs Hardcore MB Enthusiast

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    oops double post - please delete
     
    Last edited: Nov 3, 2011
  4. andy27168

    andy27168 Hardcore MB Enthusiast

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    Hi,
    As these cars have Door control units (DCM) in the rear door, these are prone to moisture ingress and can cause some weird and wonderfull faults which manifest themselves in other seemingly completetly unrelated faults, though I have never heard of one doing what yours is doing.
    If the car was mine I would get it plugged into STAR and take a look at the codes that are being thrown up.
     
  5. OP
    OP
    exjay9

    exjay9 New Member

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    Many thanks for the replies. I took the door panel off today, but there was no obvious frayed wires or corrosion. I will trace the wires further back & also look for some more earth points at the weekend. Will faults like this appear as diagnostic codes?
    I will keep the thread updated if I find the fault!!
     
  6. Dieselman

    Dieselman Banned

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    How old is the battery?
    A dying battery can give spurious faults as the CAN signal is interrupted. On my car when the battery was tired, the fuel gauge would drop to Zero for a couple of seconds once the car was started.
     
  7. BlackC55

    BlackC55 Authorised Forum Sponsor Authorised Forum Sponsor

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    Disconnect the rear window motors one at a time and see if that makes a difference.
     
  8. jefrs

    jefrs Hardcore MB Enthusiast

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    Olly's idea is a good one, to locate a faulty earth point by elimination

    As is the battery condition - the starter puts a huge toll on the battery. There will be a voltage drop when the starter turns and these cars need a tip-top battery - measure the voltage. If there is not enough volts then strange things happen - windows refuse to rise for example.

    Have you reset the windows by holding the button down until it "clicks" at top and bottom. This sets where tops and bottoms are. It may be dropping to its "top position".

    What does fuse No.7 control? - this may well be important.
    It is in the boot.
    Why would the starter/relay circuit be sending current down to the boot?
    No.7 is receiving too much current, it should not, this is bad. This is a fault.

    STAR/WIS is a clever tool but it is also an idiot.
     
    Last edited: Nov 6, 2011
  9. Alex Crow

    Alex Crow Hardcore MB Enthusiast

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    Personally I suspect, as has been suggested above, a faulty rear door controller to be at fault.

    The notion of it being an earthing fault is hard to reason through, with these being CAN controlled (only the actual rear door controller/module itself is capable of opening the window, signals are sent to it via CAN communication). The fact that (again, as has been said above) these rear DCMs often fail giving strange faults should not be ignored - with or without signs of water ingress or corrosion at the DCMs.
     
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  10. jefrs

    jefrs Hardcore MB Enthusiast

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    Again, it is the starter motor sending some signal that is "operating" the window - and blowing fuse no.7.

    Ok, the CAN is sending the command but it is getting a faulty message from somewhere-else. That sort of thing is generally a wiring/connector fault.

    And from the research reading I have done for my 98, it would appear that models of this age are prone to earth faults, which do cause such strange effects.

    And again - What does fuse no.7 control?
     
  11. OP
    OP
    exjay9

    exjay9 New Member

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    Many thanks for all your replies. I thought my 1998 c250td would be too old to have a CAN bus?
    The battery seems to spin the engine over fine, but I will check the voltage when I get a chance. Fuse 7 seems to be part of quite a lot of electronics (see photo), all of which still work! However, I have changed it again & its not blown again yet, but that might just be that I havent operated whatever was blowing it!
     

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  12. BlackC55

    BlackC55 Authorised Forum Sponsor Authorised Forum Sponsor

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    I would be looking at the rear right window motor/module as the suspect.
     
  13. jefrs

    jefrs Hardcore MB Enthusiast

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    re battery - the problem here is the battery can spin the motor fine - and still not have enough volts for the electronics.
    The old adage of if it can spin the motor then the volts are fine does not apply (which beggars the question why make the system so fussy?)
     
  14. OP
    OP
    exjay9

    exjay9 New Member

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    Im thinking it might well be the battery. Just after turning the ignition on, when the glow plugs are on, the battery voltage drops to about 10.5v. When the starter is turning, down to about 7! I`ve had it on an optimate for about 10 hours now & its still charging, though I think once a battery has reached a certain age there is no going back.
     
  15. OP
    OP
    exjay9

    exjay9 New Member

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    Hi all
    Just a bit of an update, the battery died on the first really cold night, changed it and now when starting the window opens with even more gusto!! Fuse has stopped blowing though.....
     
  16. Dave.

    Dave. New Member

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    Any more progress?
     
  17. Dieselman

    Dieselman Banned

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    What, you mean like all 4 windows now open on starting?
     
  18. OP
    OP
    exjay9

    exjay9 New Member

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    No luck in finding it yet, but the starter motor has also started making a really nasty noise, Im going to try to take it off and inspect it tomorrow.....
     
  19. trapperjohn

    trapperjohn MB Club Veteran

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    Good Luck with that. Not really the progress you wanted but I'm sure you will get to the root cause in the end.
     
  20. OP
    OP
    exjay9

    exjay9 New Member

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    Think I might have cracked it. I took the starter motor off & it looked ok, spun over ok on the bench with a battery pack (no load), put it back on the car & it gave up completely, just clicked. I managed to get a secondhand one from ebay, put it on and the car started instantly.
    And the window didnt open.
    So I am thinking, either the old starter motor was drawing so much current that it was causing the electonics to play up, or with the new starter, it doesn`t spend enough time churning the engine over for the problem to show up!
     

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