Alternator warning on, but battery doesn't discharge?

Hi all,

So it looks like I need a new alternator since my warning light comes on and stays on even at high RPMs (though it does start to dim.) Voltmeter says the system is putting out 13.5V max. A new regulator didn't change anything.

My question is this: I've actually been driving the car around for nearly a month since I first noticed the warning lamp. But I've never had trouble with starts or noticed any dimming headlights. Could this be because my batteries are new and maybe the deficit in the system is so small that it's just taking a while to notice that the batteries are draining? I'll be getting the alternator rebuilt for sure, but I just wanted to understand why my batteries don't seem to be affected so far. Shouldn't I have had seen some symptoms by now?

thanks!
1971 911E w/ Webers

I have seen cases where, if driving was always in daylight hours, and few heavy loads were placed on the system during short trips, that discharge of the battery would take several weeks, with intermittent warning lamp on. In one case it was caused by worn brushes, and the other, by blown rectifier diodes ... two to be specific, which reduced current output by 2/3 of rated capacity! But, driving at night with headlamps drained the battery in just a couple of hours of a late night trip!

So, my suggestion would be to monitor system Voltage with a digital Voltmeter during a nightime drive. A steady decrease in Voltage is indicative of insufficient current capacity by the alternator! And, the headlamps and instrument lamps will grow dimmer and dimmer, too!
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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 04-22-2001).]

I just pulled my alternator out because of the exact same problem its down getting rebuilt right now. I also had a battery light that was staying dimly lit but I did not want to take any chances and worry about my car. I talked to the shop that is rebuilding my alternator on Friday they told me my stator was bad and it would be ready on Monday. It is relatively simple to take the alternator out it took me a little over 30 minutes, feel free to email me if you have any questions or if I can be of any help.

Have had this in the past and it was a bad diode inside the alternator. Still worked but the "trigger" to the light went tits up and showed that the unit was not working even while it was still putting out power. Overhauled the unit and it worked fine.

BTW, had a local shop OH my alternator six months ago for around $75, so do not get one from a shop for three times this amount unless your core is bad! Local shops can work on them with no problem.

JA

Thanks guys!

Geez - where would I be without this BB??

What is the normal current output for the alternator?

Mine is a new Valeo, not rebuilt and about 2 months old. The voltages I am getting during daytime driving are between 13.5 and 13.9 volts with few accessories on. At night, the voltage is around 13 volts.

Is this sufficient to keep my battery charged? If not, is there an adjustment on the regulator, because I would be surprised if the alternator wnet bad that quickly, plus these voltages have been consistent since it was installed.

Olivier
82 SC

Olivier,

Your alternator sounds fine, since your readings have not changed since installation, and presumably, you haven't had your battery run down or have to be charged externally! There are several factors at play that could cause the variations in system Voltage you note.

First, any two multimeters you pick at random will not necessarily agree at the same time measuring the same car's system Voltage. It could be due to differences in the different meter's stated accuracy, or calibration. Also, stated accuracy is not usually obtained until a meter has been turned on for at least 30 minutes, in a room maintained at a contant room temperasture of 23 degrees C ... not your typical 'mode' of operation for a portable, toolbox multimeter! Typical under-$50 meters have stated accuracies of 2% when measuring DC Voltages, yet have the same displayed 3-1/2 digits that 30-year old meters of 0.1% accuracy that cost close to $1000! My 25-year old analog Weston 660 VOM has a stated accuracy of 2% on DC Voltages, yet would be viewed as obsolete by many!

Then there is a phenomenon many are not aware of ... alternators 'put out' a higher Voltage when cold, versus operating at normal engine temps, due to the higher Voltage drops across the rectifier diodes at 200+ degrees F ... the difference is typically 0.5 V to 0.7 V.

Finally, under heavy loads, an automotive electrical system has higher Voltage drops across the various connections in the system, including ground connections, intermediate bulkhead connectiions, an terminal connections at various devices such as motors and headlamps!

In summary, the readings you cite could be perfectly normal, with a 13.9 Volt reading first thing in the morning, 13.5 Volts at lunchtime on the way to a restaurant in heavy traffic, and 13.0 Volts at night on the way home with your 100 Watt H1, H4, or H5 headlamps illuminating the way!

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Warren Hall
1973 911S Targa