Hi all...sorry for the long-winded post, but I figure more is better than not enough...

I have a '96 Base Coupe, 3.8L, all factory spec. Since I've owned the car I've always observed the ammeter reading a steady 14V with slight, .5V, fluctuations depending on load, but always normalizing again near the 14V mark, never registering above the 15V mark (all good). A few weeks ago, however, I began to notice fluctuations from 14V down to 11V or so (on the edge of the red zone), with my "Check Guages" light coming on to notify me of an "improperly low" reading. The fluctuations VARIABLY last anywhere from a couple of seconds to upwards of 5 minutes before normalizing again at 14V, sometimes taking place one right after the other, at other times with longer (2-15 minute) gaps in between fluctuations, and the fluctuations almost always beginn only after 15-20 minutes after engine start (heat related problem?). Accompanying the drop in voltage is a noticable dimming of my lights, a decrease in the heater/AC fan's rpm's, slowing of turn signals, etc...indicating to me the gauge is accurately reporting the condition. I have noticed it mostly happens when I have a larger load on the system (ex. when the headlights are on), but it does...on a RARE occasion...fluctuate when only the engine is running. During the low voltage condition I have shut all the lights off, the radio, heater/AC, etc., and the condition will typically persist for anywhere from a few seconds to several minutes until it normalizes...at which point it will not typically repeat the low voltage condition so long as I don't add back the electrical load(s). I did have a routine check of the battery during a tire check about a month ago, and the technician's test indicated I needed to replace my two year old battery as the amperage in the battery had dropped about 50 amps from when it was purchased (it's a 700 CCA rated battery), though the voltage was ok. I have checked the cables coming to and from the battery and alternator, and all seem to be corrosion free and in good condition, and the belt driving the alternator seems appropriately tight.

Final notes...my alternator (110 amp?) did fail about 5 years ago, and it was replaced with a NEW one, operating without incident. And for what it's worth....hey, ya never know....my "Low Oil" indicator light started coming on (sometimes on, sometimes not) about the same time all these fluctuations started (I can assure you there is NO low oil situation), and my "Security" indicator came on about a week ago, and remains on constantly, though my anti-theft system seems to work just fine.

Question - any suggestions what the problem might be? What observations should I make about the charging system that I didn't already indicate had been observed. What tests can I perform in my garage with my meter to test the alternator and/or battery, and what indications should I see? (Note: I am NOT electrically inclined, so precise points on the circuit where I would put my probes would be greatly appreciated).

Thanks for any help.

I just had a very similar problem with my Camaro. To make a long story short it turned out to be a short in my bat. When it was tested in the car it checked out ok, but when I pulled it out and to it to Autozone they checked it out and it failed. I replaced the bat and no more problem.
I hope this helps.

More than likely the alternator again. 'NEW' alternators are simply no better than anything else; I've noticed 90% of the time after replacing a alternator it goes bad soon. It seems rare when you find a good alternator that lasts more than 3 years.

Ok, I've replaced the batter (confirmed it was bad by my local mechanic buddy), but the problem still persists. I'm assuming it's the alternator, but they checked it also, and said it was charging just fine. Any ideas?

Also, how does one test the alternator using a multimeter?

Thanks.

Oh, and for what it's worth, the fluctuations typically swing VERY quickly up and more slowly downward towards the red, though not always (gotta love when nothing is consistent).

Is it possible that you alt belt is slipping

One tests the battery charging voltage with the meter across the posts with the engine running, black lead to ground and red lead to positive post.

With a load or no load, voltage should never go below 13.8 volts. You might try this measurement with lights and heater blower on and with them off to compare. If it fluctuates, then as mentioned, a loose belt or a bad alternator.

It could also be a poor connection between the alternator BAT post and battery post and the other route from alternator case to negative post of battery. Minimum reading for each would be 0.4 volts. This measurement would be from the stud to the post itself, not on the nut or clamp.

It could also be a intermittently touching fusible link to the positive side, located at the starter solenoid terminal. Follow the positive battery cable down to the solenoid. If the intermittent link is there, this could be providing a short time load onto the system thus indicating a low voltage for the time the link is working. A broken link is hard to see and the only real way to check their continuity is to disconnect both ends and measure continuity with the OHMS position of the meter. Sometimes, by grabbing and bending the length of the link, the smaller section of wire, one would feel a very limp wire, as the inside conductor would be broken and the insulation would bend like cooked spagetti.

What concerns me is the Low Oil light. If this is indicating low oil pressure, then I would say the engine idle speed is lower than specs and this event would contribute to low voltage as the alternator would not be spinning fast enough to charge/supply electrical current. If the light is indicating low oil level, then it is an effect of low voltage.

Do you notice any other heavy load circuit coming into effect when the voltage is low? Hear any relays clicking with nothing selected?

If there is aheavy load on the electrical system, the alt case will be hot to the touch, and should be warm during normal operation. If heavy load comes into the picture, the engine rpm will drop as the load is applied. This may happen fairly quickly for the ECM, or PCM, would correct the idle speed for the load applied, as in A/C on. You may not be able to detect it quickly.