Okay to begin with it is hard to tell anything about the charge on your battery by looking at the volt meter. Years ago when they had amp meters this could be valid. Remember that little black box in the older cars called a voltage regulator? Well it's still there, it's inside the alternator. That can go bad and cause the battery not to get a ful charge too. So your alternator can be putting out like gangbusters and the battery still not getting a full charge. But if the battery reads 12V with everything shut off it is proably fully charged. About the only way you will know for sure is to go have it tested to see if it will handle a load.
When the car starts it uses the battery, after that it uses the alternator for everything including recharging the battery for the bit that it used to start the car.
The voltage you are reading is the output of the alternator. It should read somewhere in the 14's when it is running. With a load on it, i.e., A/C, etc. it may drop down to the middle to upper 13's. I have never looked at mine with the A/C on but I would not be surprised to see it in the mid to upper 13's.
The test that Shadowing gave you will work. Try it.
my 82 had a 70 amp unit, have you ever thought about trying to make a CS series alternator fit your's ? I have one one mine I believe it's rated at 105 amps and it works a heck of alot better than the original one ever did, if you take the alternator off your's look behind the alternator where the top bolt is, the alternator should have the amp rating stamped there.
mine was like 120.00 then you have to buy a special plug and splice your wires (not to hard) its a bit smaller than your original but with a little work Im sure you can make it fit, the CS series are found on 88 up GM car, I have a 94 amp alt. one of my trans am's and its a direct replacement to what you have, my trans am has power everything & air and its never had problems keeping up with charging, I would say look into a 90+ amp unit, Im running a 105 amp alt. in my 82 vette with no problems
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
The bigger the alt the faster the charge... however...
You can run the possibilty of cooking things if the Alt is TO BIG!
Remember VOLTAGE & CURRENT go hand in hand.. so the alternator has a VOLTAGE REGULATOR not a current limiter..
So if your a bit low on VOLTAGE
( EG after a start on a hot day ) the regualtor will open up and allow the full amount of current to flow.. till the battery is back up to 13.5 or so volts.
So far so good..... right?!
Now your battery is spec'ed for lets say a 60 amp alt and you throw a 100 amp alt in there... where do the extra 40 amps of current go to?
THE BATTERY.... and what does it do as it's SURGE CHARGES... genetates HEAT internal of the battery.
You wouldn't change your battery using the 100 AMP setting for Jump starting on your charger....would you?
Something to ponder.....As your battery is stewing away!
Oh yes let's not forget the wiring harness... the wire has a size rating which is equal to the amount of current it can flow.
And as in reguard to the voltmeter test... if the VOLTAGE is low & STAYS low... the ALT is NOT doing it's job.... so you are correct in the reguard that a voltmeter dosen't tell the entire story... but it does give you a piece of the information needed to make a good observation.
More is NOT equal to better!
Except in Ken & Chris's rear wheel horsepower ratings!
Get a reliable shop to test your alternator properly, if the belts are fine.
Get a proper rebuild or new one, as most of the cheapies only replace what is broken, not replace all wearable parts. My guy uses US made parts, which are visibly superior to the Chinese crap the mexican rebuilder for Gm and others use. Plus , they only cost cents more.
Is saving a few bucks really worth the hassle of a half-a---ed job, which will have to be redone?
A fully charged battery emits 2.2 vDC in each cell, so fully charged, + to- s hould read 13.2vDC. For charging current to flow into it, the voltage across these terminals must be greater than the battery voltage; normally about 14vDC or so when engine speed is sufficient, which should be always.
Take a voltmeter and check the battery voltage with the key off. Check it again with the A/C off and on, as you describe. Hold the engine RPM at a fast idle and check for improvement. At less than 13.2, your car is taking juice from the cells, and vice versa. I'd want to know if heavy drains, like headlights drop the voltage as the A/C does. If not, perhaps the RPM drop from the drag of the A/C, at idle,
If the alt. and battery checked out, and the voltages above look okay, I'd suspect the cables, but might have both rechecked. The battery should be out, and tested with a 'pile' to load it heavily. Any good electrical shop can test the alternator properly, inclucing the built-in regulator. I don't have a cable test for you.
May we assume that your battery terminals are clean and that the cables are okay? They do go bad, esp. in northern climes.
Check the wires at the starter, also check the ground strap from the near the starter to the front end (short 18" battery cable). I replaced mine recently and the heat soak type problem has disappeared.
Passenger side, jack up the front and look up at the exhaust pipe coming out of the exhaust manifold.