[s-cars] Alternator Musings

[Djdawson2 at aol.com]

In a message dated 7/13/2005 5:00:25 PM Mountain Standard Time,  
powellb at gmail.com writes:

Dave,

Coming home today, the meter was a little above where it  can get to,
but it was well below the 13.7V mark. Once I get home, I pulled  out
the alternator and checked the reading as the AC was running and  it
was 13.7V coming out of the alternator.

So, the question is: is  there a problem? Like Rokas, when it is acting
this way, I can sometimes  get lights to dim and other symptoms of low
system voltage even though the  alternator is pumping out 13.7V...



My experience...  In a car, the electrical system is fed by the  alternator, 
but high demands require the battery to function as a  capacitor.  In our 
cars, the battery sits a significant distance from the  power source, as well as 
those things to which it provides power.  This  doesn't have to be a problem, 
provided appropriate sized cables... which we  have.  The problem, over time, 
are the connection points.
 
Try this first...
Start the car, and pile on the accessories... the lights, the A/C (fan on  
high), the rear defrost, heated seats.  Now measure the voltage at the  
alternator.  Without changing any of the conditions, go back and measure  voltage at 
the battery posts.  Is there a difference?  If there is,  that is where your 
problem lies.  Clean up the battery posts, and every  other connection point of 
the cables that lead (eventually) to the started and  alternator.  Once you've 
completed that, run the same test again, and I  think you'll find the same 
voltage throughout the system... allowing the battery  to do its job when 
heavily loaded, and allowing it to receive an adequate charge  rate from the 
alternator.
 
I have run into several Audis over the years, whose batteries were going  
dead when the accessories were used heavily, and extended idle periods were  
experienced.  Battery great... alternator great.  But the contact  points on the 
cables were corroded... typically at the starter.
 
I think if you clean these things up, you'll see a marked improvement...  and 
no more dimming lights.
 
And even if you get it all working right... STILL don't trust your  voltmeter.
Let us know how it turns out....
HTH,
Dave