Voltage drop while driving. Bad fans?

[DMCVegas]

Does it happen as soon as you hit the A/C switch? Or is it random once it turns on?

[Evildeli]

A simple way I test the alternator is start the car, and while it idles I pull the kill switch on the battery. If the car dies, it's the alt. If it keeps running, alt is good. Curiously, since you're mentioning the fans, are you noticing a change temp on the engine?

[David T]

This test is a bit too "quick and dirty" for me. You take a BIG chance of damaging a good alternator. You should NEVER run an alternator without a battery connected to it. A better quick way is to measure the voltage of the battery with the motor off. Now measure it with the motor running. It should be higher. If it is not you can suspect a problem with the alternator or the wiring. When you run the A/C you add a BIG load to the motor, cooling system, and electrical system. Any problems will show up at idle, the alternator isn't putting out it's max even if it is fine. The battery is supposed to help carry the load at idle till the motor goes faster and can put out more "juice" (like when you are stopped and waiting for a traffic light to change).
David Teitelbaum

A simple way I test the alternator is start the car, and while it idles I pull the kill switch on the battery. If the car dies, it's the alt. If it keeps running, alt is good. Curiously, since you're mentioning the fans, are you noticing a change temp on the engine?

[Ron]

+1 except the alternator should supply all the power consumed at all times while the engine is running. At idle the voltage should be at least 13.5V (up to 15V), more than enough to do the job.

...but this time didn't recover like before.

Either you are not watching the gauge close enough or there is a problem with the alternator. Any other problem (save a sudden LARGE draw on the system) would not result in the light coming on and a sudden EXTREME drop in voltage...The bulb has power anytime the key is on, but one connection on the internal voltage regulator grounds it to mak it come on -- When the engine is running, the alternator puts out and this connection loses its ground potential, so the light goes out. (If this connection is grounded in any other way (eg a short along the Brown/Yellow wire etc), the light will come on and the regulator will almost certainly blow, instantly -- If the connection is open (B/Y wire broke) the light will never come on.) The gauge works independently, so when a output problem occurs, the light should come on, but the voltage should start falling from ~12V, not 15.5 and certainly not anything as low as the 8V you had...but, assuming this be the case combined with the light working (at least intermittently) and regulator not blowing (they totally die rather than act flaky here), you only have to confirm good connections to the alternator's main terminal (12V) and case to ground (-) to reasonably eliminate everything but the alternator. (To be clear, a intermittent problem here should also result in the voltage dropping from ~12V).

I would pull the alternator and battery and have them load tested (as previously suggested). If no problem is found with them, I would carry a set of jumper cables and the next time this happens, jump straight off of the battery (-) to the alternator case and (+) to the alternator main cable. If the voltage does not come back up then, the alternator is bad (with the light on you should be able to eliminate the only other needed connection, the Brown/Yellow field wire, but, jumping it with 12V also will make the conclusion 100%, if the battery is sound.)

[Notifier]

Does it happen as soon as you hit the A/C switch? Or is it random once it turns on?

Happens about 20 minutes into running the AC, not immediately.

+1 except the alternator should supply all the power consumed at all times while the engine is running. At idle the voltage should be at least 13.5V (up to 15V), more than enough to do the job.

Either you are not watching the gauge close enough or there is a problem with the alternator. Any other problem (save a sudden LARGE draw on the system) would not result in the light coming on and a sudden EXTREME drop in voltage...The bulb has power anytime the key is on, but one connection on the internal voltage regulator grounds it to mak it come on -- When the engine is running, the alternator puts out and this connection loses its ground potential, so the light goes out. (If this connection is grounded in any other way (eg a short along the Brown/Yellow wire etc), the light will come on and the regulator will almost certainly blow, instantly -- If the connection is open (B/Y wire broke) the light will never come on.) The gauge works independently, so when a output problem occurs, the light should come on, but the voltage should start falling from ~12V, not 15.5 and certainly not anything as low as the 8V you had...but, assuming this be the case combined with the light working (at least intermittently) and regulator not blowing (they totally die rather than act flaky here), you only have to confirm good connections to the alternator's main terminal (12V) and case to ground (-) to reasonably eliminate everything but the alternator. (To be clear, a intermittent problem here should also result in the voltage dropping from ~12V).

I would pull the alternator and battery and have them load tested (as previously suggested). If no problem is found with them, I would carry a set of jumper cables and the next time this happens, jump straight off of the battery (-) to the alternator case and (+) to the alternator main cable. If the voltage does not come back up then, the alternator is bad (with the light on you should be able to eliminate the only other needed connection, the Brown/Yellow field wire, but, jumping it with 12V also will make the conclusion 100%, if the battery is sound.)

How accurate is the voltage gauge normally? Engine running with everything ok, it seems to read slightly above the 13V tick, engine off it reads between 13V & 10.5V. I can assume the "normal" voltage for the alternator running is the 13V+ reading. First time I noticed the problem I looked down and the gauge was almost to 8V and the engine was starting to stall. Second time it happened, the gauge started at around 10.5V and slowly dropped to 8V for the 22 mile drive. And believe me, Saturday I was focused on that gauge all day! Problem is I haven't been able to duplicate the problem while home. Or even on Sunday when I drove it around just to try to get it to happen again. For some reason, if it happens again I think if I turn off the engine, charge up the battery, and restart the car it will run fine off the alternator. But that's just a theory and unable to prove it yet. My next step is to double check all the connections, did a quick visual on the alternator but haven't had time to do a more detailed inspection. But more than likely will have the alternator tested just to prove if it's good/bad.

[Ron]

How accurate is the voltage gauge normally?...
Engine running with everything ok, it seems to read slightly above the 13V tick, engine off it reads between 13V & 10.5V.

They are not very accurate, but, judging with those numbers, it sounds like yours works better than most.
Should be ~12V with everything but the key off....check it with another meter just in case.

And believe me, Saturday I was focused on that gauge all day!
...But more than likely will have the alternator tested just to prove if it's good/bad.

Then I'd say the alternator is going out (or belt loose).
One thing, most of the testers at the auto parts stores can load test the alternator, but they aren't going to get it hot enough for a true test (heat is what breaks down the voltage regulator...diodes and windings as far as that goes), so, if it don't show bad and you don't eliminate it with a new one, don't forget the jumper cables and a short jumper wire...

[DMCVegas]

Then I'd say the alternator is going out (or belt loose).
One thing, most of the testers at the auto parts stores can load test the alternator, but they aren't going to get it hot enough for a true test (heat is what breaks down the voltage regulator...diodes and windings as far as that goes), so, if it don't show bad and you don't eliminate it with a new one, don't forget the jumper cables and a short jumper wire...

I'd concur with this. It sounds like the Alternator's output isn't able to keep up with the load of the A/C. You've got one of two scenarios here:

Start the car, it works fine because the alternator is able to crank out enough voltage to keep up with the demand of the car. Turn A/C on (compressor, switches, & fans) and the alternator can't keep up with the output of the electrical load. So the battery then is used to feed the demand. As the battery drains past the point where the residual charge can't keep up with the load and the voltage drops, the light comes on.

or

Same scenario as above, but instead of pulling the extra load from the battery, the alternator finally overheats and fails to output the correct voltage.

In either case as soon as you shut the A/C off, the alternator is able to power the car as normal so the light goes away. Probably also able to slowly replenish the battery so you're able to restart. I have a similar symptom with my car, but for an entirely different reason. I have one of Hervey's old D150 alternators (very tight tolerances so it whistles when it spins) and when I drive through flood water, the unit would get soaked and would die out. I'd drop into neutral and rev the engine for about 2-3 seconds to sling the water out, and the voltage would pop right back up. And no, that's not a complaint nor an inherent problem. Hervey's alternator works great, I'm happy with it, but I just don't crawl through puddles. I keep driving normal, and it's all on me.

For you though, I'd definitely pull that alternator and get it checked at a couple of different auto parts stores to ensure it's working properly.

[Ron]

Same scenario as above, but instead of pulling the extra load from the battery, the alternator finally overheats and fails to output the correct voltage.

This is my guess...bad regulator or open rectifier diode. The light does "work" and it says it's bad LOL