[s-cars] Alternator Musings

[Brian Powell]

Dave,

Thanks for the info. I just did a test. I fired it up, turned on all 4
seat heaters, rear defrost, AC, high beams, and radio. I tested the
alternator and it was at 13.44V with the car idling and everything on.
I tested at the battery and it was 13.1V during the same period. I let
things go, turned things on and off, and retested. The largest
difference I got was the battery being 0.45V below the alternator. Is
this enough of a drop to cause it? It seems like less than 0.5V would
be in the "acceptable" range, but I really don't know.


On 7/13/05, Djdawson2 at aol.com <Djdawson2 at aol.com> wrote:
>  
> 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 
> 
>  


-- 
Cheers,
Brian