[W126 Coupe] Problem 560

[Vance Rowley]

Hi Bellamy,

 You might be lucky and find that the alternator is OK. Don't forget that an alternator needs power from the battery in the first place to power the field coil. So if this power is compromised (i.e. this bad battery voltage drops considerably due to a high current draw) then the output from the alternator will also be reduced. Although 2 X 100W headlights will only draw around 16.6 Amps, this battery you are talking about sounds like it's completely shot in the first place. Especially as you mentioned that even after jumping and running the car for a while even the interior light wouldn't come on. 

Therefore this battery might have a lot more than one shorted cell and just been on borderline to run the engine electrics and alternator of the car after being jump started which could be why you got only 13.8V on such a low battery. This voltage from the alternator would be quite normal with a reasonable battery even if it was a little drained, but it sounds like this battery has a few more than just one shorted cell and was also placing quite a high load on the alternator as well (which is why it will get hot) which in turn would lower the voltage to the field coil which in turn drops the output from the alternator and the vicious circle continues until everything grinds to a halt and the engine stops. All this triggered by the 16.60 Amps when switching on the headlights and taking this bad battery just over the edge.

Obviously the best and most obvious thing to do (if you haven't already) is place a known good battery on the car and see how things go. From your description of the condition of this battery, I would suspect the alternator could be OK. Let me know how you get on.

Good luck,
Cheers,
Vance.

----- Original Message ----- 
  From: Mister McGoo 
  To: mbcoupes at mbcoupes.com 
  Sent: Wednesday, May 03, 2006 4:58 PM
  Subject: Re: [W126 Coupe] Problem 560


  Revisited the 560 problem on starting and running (when it was not raining and 7ºC).  The problem IS the combination of dead unchargeable battery and marginal irratic alternator, nothing more complex.  (The latter likely having contributed to the former.)  

  The lights (headlights 100w) appear to draw such significant power suddenly that it is enough to put the challanged altermator over it's limited capabilities.

  I have not tested yet, but I suspect that the alternator quites completely from time to time after it gets hot, and there is no battery to help it over these little seizures.  A test with a new battery in place will likely show this.

  Learning every day.... "Don't look for zebras when you are in a horse pasture."

  -Bellamy

   




----------------------------------------------------------------------------
    From: "Mister McGoo" <eelploot at hotmail.com>
    Reply-To: Mercedes Coupes Mailing Lists <mbcoupes at mbcoupes.com>
    To: mbcoupes at mbcoupes.com
    Subject: Re: [W126 Coupe] Problem 560
    Date: Mon, 01 May 2006 14:51:10 +0000


    Thanks guys for input on the 560 electrical problem.  I will not have access to the car again for a few days and will be a bit more prepared for what I'm looking for.

    -Bellamy 




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