[W126 Coupe] Problem 560

Sun Apr 30 05:58:05 EDT 2006

Hi again Bellamy,

  As to the question of why a short in the headlight circuit didn't blow a fuse is concerned, one possibility would be if the car has had the headlights modified for higher output. In other words, the feed that went originally straight to the headlights might now only be going to a relay, which in turn is picking up a feed (maybe directly from the battery) and feeding this through the relay directly to the headlights. If this part of the modification wasn't additionally and separately fused and/or if the additional cable required for the feed from the battery through the relay contacts and from the relay to the headlights wasn't up to standard, the short circuit could happen very easily as of course the original headlight fuse is only feeding the relay coil itself and wouldn't be affected at all under these conditions. Obviously this is pure speculation, but would be a plausible reason.

As for not firing up the 3rd time you jumped the car. This could have been that you had also partially drained the battery you were jumping from after the first two times. It is possible to have enough power to turn over the engine but not start due to the voltage drop being too excessive to run all the other electrics for the engine to fire up. If I am not mistaken, if the voltage drop (which you will always get when starting a car) is below 9.70 volts, this is not enough for such as the ECU and so on. Again just speculation,but another possibility.

Good luck,
Cheers,
Vance.
  ----- Original Message ----- 
  From: Mister McGoo 
  To: vance.rowley at tiscali.co.uk 
  Sent: Sunday, April 30, 2006 8:01 AM
  Subject: Re: [W126 Coupe] Problem 560

  Thanks.  I follow that, but I would question why a short in the headlight circuit wouldn't blow a fuse first rather than shut down everything including the ignition. 

  And why the third time I jumped to start, it wouldn't fire at all.

  I have to clarify that today was a very poor day to carry this process out, being a mere 7C outside, raining AND I only had half an hour with the car before the owner turned into a pumpkin. 

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    From: "Vance Rowley" <vance.rowley at tiscali.co.uk>
    Reply-To: Vance Rowley <vance.rowley at tiscali.co.uk>,Mercedes Coupes Mailing Lists <mbcoupes at mbcoupes.com>
    To: "Mercedes Coupes Mailing Lists" <mbcoupes at mbcoupes.com>
    Subject: Re: [W126 Coupe] Problem 560
    Date: Sun, 30 Apr 2006 03:31:13 +0100

    Hi Bellamy,

     I wouldn't exactly call myself an expert, but as you know, I have had battery drain problems and have learned a thing or two. >From your description it sounds like there is currently two problems. The first problem seems that there could be a short circuit somewhere in the headlights circuit. The second problem sounds like the battery itself could be suffering from sulphation or maybe even a shorted cell. With the engine off and all electrics switched off, what voltage are you getting from this battery? A quick 15minute run around the block will not charge any battery fully, let alone one that could be suffering from sulphation, a shorted cell or low electrolyte.

    The first thing I would check is the electrolyte level on this (dead) battery and top up with distilled water if required. Secondly I would attempt to charge the battery with a battery charger over a long period of time at a lower current. A small 4 to 6 amp charger should do the job, but you will need to have it on charge for around at least 12 hours. Best to disconnect the negative terminal while on charge also. After you have attempted this, ideally you could do with a battery hydrometer to test the electrolyte and of course the voltage of the battery after it has been taken off charge for around 3 hours. A fully charged battery should read ideally 12.7 volts. It's unlikely you will get as good as 100% charge and showing 12.70 volts, but around 12.43 should be acceptable. This roughly indicates a battery at around 75% charged.

    Once you have managed to get the battery back up to a reasonable level (if you can, as it might have a shorted cell and not be possible), you need firstly to check the parasitic current the car is drawing. To do this, you will need to measure the current with a meter that has a range at least as high as 10 Amps. With the negative post disconnected and all electrics switched off, connect the meter between the negative post of the battery and the car battery terminal. The parasitic current should be somewhere between 50ma and 80ma. If this is the case then you are half way there. You might get a peak as high as 8 amps when first connecting the meter, but this should drop down and settle to between the figures mentioned above.

    Now you will need to test for that suspected short circuit in the headlights. As the headlights would normally draw more than 10 Amps anyway it's not a good idea to leave the meter connected while you switch on the headlights. The safest way to test for this is to use a 12 Volt inspection lamp which you would normally clip onto the battery terminals for use. But in this case still leaving the negative terminal disconnected, place the connections of the inspection light in place of the meter (one end on the battery negative post and the other on the negative terminal. Then you can normally and safely test the headlights (still all of this with the engine off of course). If the headlights come on but just a little dim and the inspection light will of course come on as well, then the problem might have been just the battery itself all along. If however the headlights don't come on at all and the inspection lamp comes on to full brightness, then you will need to start searching for a short circuit somewhere in the headlight circuit.

    I hope this is of some help. Let me know how you get on.

    Good luck,
    Cheers,
    Vance.

    ----- Original Message ----- 
      From: Mister McGoo 
      To: mbcoupes at mbcoupes.com 
      Sent: Sunday, April 30, 2006 2:02 AM
      Subject: [W126 Coupe] Problem 560

      All;

      I was asked to view a problem 1988 560.  I have limited experience on anything newer than '85. (560)

      I was asked to bring a fresh battery because the battery was dead, and I had about half an hour to determinbe what the problem was.  I wasn't successful.  I'm stumped.  And I ask for comments from 560 experts.

      Symptoms:

      The battery was flat dead.  I jumped the battery and the car started fine.  I disconnected the jumpers and it remained running.  The voltage at the [dead] battery was 13.8v. Obviously the alternator was working.  

      I warmed the engine up, tested the climate control, power seats, wipers, seat heaters, power windows, sunroof, power sunshade.....  all working fine.

      The car had a 3/4 tank of fuel.  It was idling for about 15 minutes.  I drove the car around the block, accellerating smartly when I could (the vehicle wasn't licenced so I couldn't go far).  Then I turned the headlights on and everything abruptly died.  (The owner said, "oh you turned the lights on.,  That's what happens; the alternator is shot.") 

      The battery still was dead.  I jumped it again, the engine started fine, and I disconnected the jumpers.  It continued to idle for about two minutes, then without anyone touching the car, the engine died abruptly again.  This time, when jumped, the engine would turn over but wouldn't fire at all.

      The only possible abnormal condition is the idle speed was a bit high... around 1000rpm.

      Any comments, suggestions? 

      -Bellamy

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