Is it the battery or the alternator that's at fault?

[Doyt W. Echelberger]

For the record:

A new battery sitting outside of any circuit and fully charged would read
about 12.6 volts at room temperature.    Under a test load, if the voltage
drops below 10.6 volts, consider replacing it.  All the above with the
engine off and the alternator not complicating the problem.

Alternators charge at voltages above 13 volts.....between about 13.6 volts
and higher, up to 14.2 in one case that I saw. So if your alternator is
only making slightly above 12 volts, then something is wrong in the circuit.

Your reading of "just over 12 volts" would indicate a good battery, if the
reading was taken with the engine off.  But, I thought that the reading was
with the engine running, so I figured that the reading was with the
alternator operating (which would indicate a malfunction in that charging
circuit.)

On reconsideration, it looks like the alternator isn't charging, or the
charge isn't getting to the meter you are reading.  And the battery voltage
is probably OK, but at risk because of the murky brown color of the
electrolyte (assuming it is a conventional lead-acid battery.)




 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
At 11:27 PM 1/27/2002 -0800, you wrote:
>Fellow Listers,
>
>I got an interesting fault on my typ 89 coupe on Saturday evening.  I was
>driving with dipped headlights when suddenly my stereo and dash lights lost
>power.  About twenty minutes later the power resumed and the autocheck
>computer returned the OK message.
>
>As soon as I used the indicator stalk I lost power again.  When I got home I
>turned off the ignition and cranked the engine.  The engine started without
>any problems.  The autocheck computer displayed the amber battery symbol.
>
>I switched on the the dips again without any decrease in brightness.  The
>car's voltmeter was reading just over 12 volts.