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Dim LH Rear Lights?

Discussion in 'Electrics, Vacuum, Ignition and ECU' started by ernieh, Jan 13, 2012.

  1. ernieh

    ernieh Senior Member

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    Hi All,
    First problem of 2012.......Noticed that the near side rear lamp seems to have become less bright than the right hand one on my 204. It also seems to take a second or two to reach its full brightness as though there is an electrical resistance somewhere. As far as I can see, it seems that there are possibly 3 bulbs in each rear lamp circuit, but the overall brightness is an overall effect rather than down to one bulb. The operation of the reversing and indicator lights don't seem to be affected.

    My first inclination would be to check for a corroded earth in the circuit and corroded connections in the multiconnector to the lamp unit. However, before doing some checks with a multimeter, I wondered if there is anything specific to the 204 that I should be aware of or even if anybody else's 204 does anything similar to mine.

    Many thanks for any help,


    Ernie
     
  2. whitenemesis

    whitenemesis Senior Member

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    No malfunction messages in the cluster?
     
  3. television

    television Always remembered RIP

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    I would measure directly across the bulb, if voltage down, measure from the ground contact on the bulb to a good ground bolt on the car, and there should be no voltage across this, if none do the same from the same ground point to the live feed on the bulb, if low then the fault is further back
     
  4. OP
    ernieh

    ernieh Senior Member

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    Thanks..........No, nothing recently.


    Ernie
     
  5. whitenemesis

    whitenemesis Senior Member

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    Just wondering if the car is substituting a bulb for a failed one.

    Is it the same with the brakes applied?
     
  6. OP
    ernieh

    ernieh Senior Member

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    Thanks for that, Malcolm; before I start checking things out over the w/e, can you say if there are likely to be any SAM or canbus issues as in the 203 or (hopefully) is it plain straightforward, oldfashioned wiring...or is that too much to hope for?

    Many thanks, again,


    Ernie
     
  7. OP
    ernieh

    ernieh Senior Member

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    I wondered about a failed bulb, but thought I'd have a warning up. Havn't checked brake lights or bulbs yet, but will have a look in the morning.

    I think there is more than meets the eye here, as I have a feeling that the rear fog light on the o/s may also double as a tail lamp, but with reduced brightness (if that makes sense!). I say that because I have also noticed that this lamp doesn't 'snap' on, like you might expect, but looks as though a resistor or something is holding things back. Having said that, I'm not having any probs with the o/s lamps.

    Thanks for your interest......any info based on a circuit diagram for the car would be extremely helpful.


    Ernie
     
  8. television

    television Always remembered RIP

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    This is not any CAN fault, it is unlikely to be the lamp module, this is what feeds the voltage to the lamps around the car, the fault is more likely to be local to the lamp holder or the bulb itself
     
  9. OP
    ernieh

    ernieh Senior Member

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    Problem Solved!

    Right folks, thanks for the help in finding the problem......the earth connection on the multiconnector to the lamp unit had corroded and some plastic melted due to sparking. I cleaned it up and with a spot of switch cleaner, it is right as rain for the moment. I don't think this will be the end of this one, though, as the earth wire seems too small for the current it could be taking at times. Or is that modern technology for you?

    However, a point of interest on this car......is that there are 4 bulbs illuminated in each rear light unit when side lamps are switched on; all are single filament and 3 of them are 21w which operate at reduced voltage. Some of these also double, at maximum voltage, for brake lamps, reversing lamps and rear fog lamp. It also means that there must be a relay somewhere for the reduced voltage. I havn't checked yet, but suppose it could be in the rear fuse box. Presumably, this type of system is not uncommon on modern MBs?



    Thanks again for the help and hope that the above info may assist somebody else.


    Ernie
     
  10. OP
    ernieh

    ernieh Senior Member

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    MB Solution

    Just checked with local MB Dealership this morning. It seems that this is becoming more frequent as the 204 series ages.

    Their solution is: new lamp holder, new multipin plug, new pins where / if required and 1 hour's labour. my guess......... about £200 incl VAT. However, they did warn me about the fire risk and to keep an eye on it. But, no recall or updates to an obviouly poor design.


    Ernie
     
  11. Troon

    Troon Active Senior Members

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    It's not reduced voltage, but PWM which is much more efficient and predictable. The rear SAM runs the bulbs at 100Hz and 25% duty cycle for "dim" use.
     
  12. OP
    ernieh

    ernieh Senior Member

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    Sorry for the late reply to this post, but the info probably explains why I had a bit of difficulty in checking out voltages with a multimeter.

    In fact a simple inspection reveals all and I have since done a permanent repair for about £25.

    .....but what is PWM, please?


    Ernie
     
  13. OP
    ernieh

    ernieh Senior Member

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    Should have googled it first - Pulse Width Modulation. Good explanation on Wikipedia.

    Ernie
     

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