Weak Battery or Weak Alternator?

courtesy of Bob, The Auto Answer Man

I have a '95 Chevy Astro that has a similar problem to the one mentioned in your earlier email. When I run my radio, lights and A/C, the lights go dim, the volt meter goes below 9V, and the idiot light comes on. I have not checked to see if the meter is accurate. However under normal load, my alternator seems to be putting out 12 to 13 volts. This is not a rebuilt engine, nor is the alternator as far as I can tell (I'm not original owner). I've had a mechanic check out the alternator and he says it's fine. How big an alternator should I have (I'm assuming it will be rated in amps)? How can I tell if the alternator matches up with the regulator? Thanks for any help.

Well, what you described sounds like either a weak alternator or a weak battery. If the engine rpm drops below 750-1000, it is not spinning the alternator fast enough to make sufficient current to power the system. However, that is when the battery is supposed to take over and fill the gap. Conversely, when the engine is spinning faster, and the alternator can produce sufficient current, the excess goes into the battery. Then, hopefully, if all is working well, the field voltage to the alternator will reduce, and it will limit its current output. With no load and the engine spinning at 1500 rpm, you should have around 14.8 volts at the battery. If not, then something is weak. Also, during starting, your battery voltage should not drop below 10 volts. If so, manually charge the battery, and try again. If it is still below 10 volts, replace the battery, and then repeat the 14.8 volt test above. If you do not feel comfortable doing those tests, then you could bring your battery in and have it load tested. I think Auto Zone does this for free. Also, you can have your alternator bench tested as well.
Good Luck!
Drive Safely!
Bob, The Auto Answer Man

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