88 2.5 Charging System

Hello again.
88 Pontiac 6000 2.5 4Tech 127K miles.
Friday, the alternator light came on. Wife stopped by trusted mechanic who fiddles under hood and asks if light went out... no... then it's the
alternator. She tells him I can do that and continues home.
I pick up used one at a quality, respected recycler and slap it in there. Car won't start on it's own but jump starts... but would not continue to run for long. Charge battery a little from another car and repeat experiment and it jumps but stalls again in 5 minutes. I assume battery is low as she drove it home without alternator and go to Auto Zone (last night) where battery was purchased to be safe and get a 40 minute charge... guy says it's good and not low and doesn't need a charge. Heh. Bring it home and put it in another car and it's dead. It's late so I quit for the day.
This AM I put another battery in Pontiac and it cranks but won't start (always an easy starting car) until I try it with pedal to floor as if flooded and it started and after rumbling a moment or five it runs fine.
I went to another parts store (with substitute battery in it and Auto Zone's in the trunk) and asked them to check charging system and they say it's charging fine but the battery in trunk is too low.
Went back to Auto Zone and he checks it and it shows 50A steady at idle... but when he says to tach it a ways he shows me that digital readout jumps around wildly from maybe 50A down to maybe 30A up to maybe 45A down to maybe 11A up to maybe 20A and down to maybe 9A and back up to maybe 40A again and so on. Says this indicates the alternator is bad even though the light is no longer on. <SIGH> Checks his battery in trunk again and says it's good but needs pro-rated and warranted anyhow. Huh? Why? Because it might be good but not good enough. Huh? What the heck, I figure,... it's a good 875 CCA Dual Terminal 8 year battery and for not many bucks I can get a new one, so I did it.
Should this Alternator show amperage fluctuating wildly?
Should I return the alternator to the Parts recycler?
Is the Auto Zone guy full of crap, trying to cash in, or diagnosing correctly?
Why can't anything be simple?
Thanks for any input.
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My guess is that you no longer have a charging system problem, and that the Auto Zone dude might be incompetent or has faulty diagnosis equipment.

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Will the parts recycler check the alternator out for you?
The amperage should not be varying wildly BUT the Autozone digital device could be giving you false indications too.
Have you carefully cleaned all battery cable terminals, grounds, etc? That can start you out on a firm foundation.
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Will have to call them and ask... hard to get there from work on a weekday, though.

I wondered if it were the equipment or the operator as the operator didn't seem to make too much sense in anything he said. I'm not a mechanic, but I've been working on my own jalopies for over 34 years since I was 15 and he just seemed to be talking in circles.

Yup. Even went so far as to put a set of adapters on to change the side terminals to posts on it after using my twister brush on the posts. Actually like it better that way and will go with them when done.
Thanks for the replies... .. .
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sensitive to voltage levels, and I suspect you may have experienced this.
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So... here's an interesting thing I thought about along these lines:
Used to be that we just disconnected a battery terminal after a jump to see if the issue was the battery or regulator/charging. Of course they now say not to do that as it messes with the computer, so... If the car is running off of the battery and the battery dies, how is that any different from disconnecting it? I think that may be the whacky issues surrounding the way it ran (and didn't) at first.
I called the parts yard and he said they only check them off the vehicle if I wanted to bring it over... but he suggested the Auto Zone guy didn't have a good connection when he clipped to the side terminals (which I thought while he was doing the test) so I think I'm gonna give it a few days before I go through removing it and dealing with being a car short on the roster for the duration of time where I can rush over there after work and such.
Thanks
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I suspect the Autozone data too. Battery tests have to be done carefully and by a skilled person to be of any use. Too many times, 'testers' will tell you that the battery is okay, just needs a full recharge, when in fact the battery is toast.
Just because you measure 12.6 volts or so doesn't mean anything about the ability of that battery to provide the current you need to crank an engine.
I had a rear engined car once that gave me all kinds of hell. The battery would go down, the ECM would stop working, the alternator would frag itself....rather periodically. Finally found that the dealership had put the brackets together incorrectly after a warranty recall procedure and the belt would loosen gradually. As the belt began to slip, apparently the alternator got confused and tried to keep up but couldn't. When the battery got low enough, the thing just died. I couldnt see the problem with the brackets with the engine in place, but finally -when the Iron Duke cracked the block - I pulled the engine and things began to make sense.
An aftermarket ammeter, and even a voltmeter, can be helpful. At least you will get a hint in real time what is happening.
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Ralph D. wrote:

If its running and the battery "dies," the battery is still there acting as a big somewhat inert resistor/weak battery in the circuit. It can still damp out any rapid variaitons in voltage and stabilize the system. When you pull a terminal connection off with the engine running, you're basically running the alternator into an open circuit apart from the small loads that may or may not be running (ignition system, etc.) The alternator and voltage regulator need more damping in the system in order to produce a stable voltage, and without the battery there to serve as the damper the voltage may swing very high and low. It was actually bad to do that test even 20+ years before computers, because it can easily blow the diodes in alternators. It was reasonably safe to do when cars had generators and vacuum-tube radios, though.
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