headlights and interior light issues

I have read a couple of posts regarding interior and headlights dimming at idle and maybe one or two of them dimming while driving but I have not seen anything regarding both conditions at the same time. I have had the elec/charging system checked w/mechanic at idle with and w/o all systems turned on(used a diagnostic computer) checked for codes also. The results were neg for all. I was told that the battery was undersized for this vehicle so I bought a new one w/720CCA. The headlights and interior lights still dim momentarily (at idle and driving) then brighten again etc. The belt and alternator both look good as do the battery terminals and wires.I don't want to just change the alternator for the sake of changing it.How do you know if the alternator is on its way out if the problem comes and goes? Could problem be related to Light Control Module or Bad Ground. Where do I begin? Thanks very much for the help

Remove your alternator and take it to an auto parts store and they will check it for you. If it is good, then I don't have any answers other than it is pretty much not your Light Control Module. It would not likely cause those symptoms.

If you have a volt meter, turn on all of your accessories, and check the voltage at the battery. It should be about 13.5 volts I believe. (Long time since I did this) If it is is under that, you have an alternator problem. Even if you don't have a volt meter, with all the accessories on, you should be able to hear the alternator whine if it is producing current.

What year car are we talking about here? Do you have a volt meter in the IC, or just an idiot light?

Idiot light. Also, I Forgot to mention that when lights dimmed the battery idiot light came on briefly.Should I wait around with motor running w/volt meter attached to battery terminals until problem reappears then get a reading to compare to "normal" operating volts from alt?

Idiot light. Also, I Forgot to mention that when lights dimmed the battery idiot light came on briefly.Should I wait around with motor running w/volt meter attached to battery terminals until problem reappears then get a reading to compare to "normal" operating volts from alt?

No, you should refer back to my earlier post for testing your alternator. Once established as good, then move on to other tests. The reason I suggest taking it out of the car is because:

1. The testers they use put a high load on it to make certain of it's working condition, and

2. Taking it out of the car eliminates other things related to your car which could taint your results (like the wiring to and from the alternator). If you KNOW your alternator is bad after having it tested, then replacing it is a no-brainer. If you test it in the car you could have a bad wire or connection and misdiagnose it as a bad alternator and then unnecessarily buy a new one. (that said, I doubt a mechanic at a shop would take it out--he would probably test it in car, then replace it, then later tell you it was bad (even if it wasn't), while in the mean time fix the bad wire that he discovered after he replaced the alternator, and then send you on your way with a "fixed" automobile--am I too cynical?)

It is very easy to remove. First, take an 8mm socket or wrench and remove the negative battery cable. Next, take an 18mm wrench and put it on your serpentine belt pulley tensioner at about the two-o-clock position (if you put it at 3 o'clock you will find that there is not room for it to go back down without hitting things). Lift up until you can get the belt off the alternator pulley. Get a 13mm socket and a ratchet. Remove the two bolts holding the alternator in place. Note which is which (they may be different lengths.) Then grab a 10mm wrench and pick the alternator up and flip it toward you and remove the 10mm nut and wire from the back and also remove the wiring harness. Installation is the reverse.

Ok, Alternator removed and taken to 2 parts stores- alternator checks out as good. Checked battery terminals, alternator wiring and plugs and Battery ground- all appear to be tight and in good shape. Any suggestions where to proceed from here?
Thanks

Talked to my mechanic yesterday(another car in his shop) about this problem. He said GMS renowned for these type troubles and that a diode in alt is bad and may not show up during auto parts store alt testing. SOoo I took a leap of faith and replaced alt w/AC Delco reman- volt light still going on /off. Volts Alt to battery when eng running and hdlits on =13-14.5V.Volts Idiot light still coming on then/off/on etc every few min espec w/headlts on. Battery volts w/o car running =12.75 V. Today Mechanic suggested I change pigtail plug (thin single wire) on back of alternator as wire may be loose or plug going bad. What is the purpose of this wire and does this sound like a possibility? Could reman be bad also- with same problem? Sounds shaky to me. BTW tensioner spins freely w/o belt tension and alt engages (charges Batt) frequently w/eng running. Very Slight whine can be heard when actively charging. :shakehead

Rather than take the alternator out and a trip to the autostore for the check....just drive the car to the autostore (autozone) for the check. Some stores will do it on the car...for free. Same for the battery.

My cousins 98 LeSaber is doing this exact same thing. Battery and alternator have been replaced. Voltage still fluctuates whether at idle or higher rpm. We used an ohm meter to check for a good ground between engine and battery. With the engine not running it was at .6 ohm. with the engine running it goes up to 300 ohm. That leads me to believe there is some voltage being conducted through the ground path. Does anybody know how the ground system on this car is set up? We went as far as hooking up a separate ground from the engine block to the battery neg. terminal using one lead of a jumper cable. This had no affect on the problem. Any help would be appreciated.

My cousins 98 LeSaber is doing this exact same thing. Battery and alternator have been replaced. Voltage still fluctuates whether at idle or higher rpm. We used an ohm meter to check for a good ground between engine and battery. With the engine not running it was at .6 ohm. with the engine running it goes up to 300 ohm. That leads me to believe there is some voltage being conducted through the ground path. Does anybody know how the ground system on this car is set up? We went as far as hooking up a separate ground from the engine block to the battery neg. terminal using one lead of a jumper cable. This had no affect on the problem. Any help would be appreciated.

With an ohmmeter between a good engine ground and the negative terminal of the battery you should have close to 0.00 ohms. 300 ohms seems fishy to me. I'd start pulling fuses one at a time and retesting to eliminate circuits in order to find the potentially bad circuit, including any under hood fuses or under passenger side under dash fuses. Where ever they happen to be. With the proper circuit disengaged the resistance should go down to close to 0.00. If you get that to happen after removing a particular fuse, then you know what wiring or circuit to begin investigating.

To save time, perhaps you should also check the wiring for the alternator before doing any other testing. How? Use an ohmmeter and put one end on a wire in the harness near the alternator, then the other end where ever that wire goes, and preferably with the other end unhooked from whatever it goes to. Repeat for all the wires at the alternator (on a 92 it is a total of three I think, but for a 98 I don't know.)