Strange alternator and battery light problem

Maintenance/Repairs
, ,
1

So let me preface this by saying that I bought the car used with 30k miles on it. At 35k miles I had to replace the alternator. I am sure the garage used aftermarket parts but it was a brand new alternator. I have had the car for about 6 years and it currently has about 108k miles on it. Ever since I replaced the alternator the battery light would randomly come on. I would tap the gas or breaks, the battery light would go off. I once let the car coast without touching the brakes or gas just to see if it would eventually turn off and it did, eventually. There is no rhyme or reason to when the battery light would come on. I can drive it for a week and have it happen 3 times a day, 3 times per trip, or a month without incident. I have found that it happens the most when I am coasting, never when accelerating, and sometimes when I break.

I have taken it into a shop now 4 times in the last 6 years. Every time its exactly the same: the battery is fully charged, the alternator is putting out correct voltage and amperage. Nothing seems to be wrong with the car itself. I will say that when the compressor is running my head lights tend to pulse. They will rapidly get dimmer and brighter in a cycle of about a 1/4 second. It is just enough to notice and I don’t think it has to do with the battery problem but it might so I mention it.

Is this even worth worrying about? What should I ask for next?

Thanks for the help,
~Ben

2

@Benedict, in spite of what the shop told you, it sounds like you DO have a charging problem.

How old is that battery?

If it’s more than 5 years old, replace it, regardless of how it tested.

With a marginal battery, the best alternator in the world can’t do its job properly.

Are all the connections clean and tight?

3

Check the system voltage as you rev up the engine to around 3000 RPM. I had replaced the alternator in my wife’s car with a new one, and a few months later, she had the same symptoms, battery light going on, but would go off when she slowed down. At idle, the voltages were correct and all seemed fine. On a test drive, I noticed the battery light going on whenever I revved it up. Checking voltages when revving, the voltage would jump to as high as 16v at about 2500 RPM. That is due to a bad regulator, and they replaced it under warranty.