This is happened twice in 2 days. Yesterday I was driving down the highway at about 60. The battery light came on for about 4-6 seconds and then went off.
I then drove around making several client stops, most of the time at 60 or above (battery light never came one again) and stopped by my local garage near home and we hooked up their electrical system reader and the alternator was putting out about 15 volts and the battery showed it still needed charging and was hanging in the 12 volt area. So simple. I just went next door got a new battery, went home and had that done in about 30 minutes time. My old battery was about 3 years old (of course AGM) and that seems the normal life from previous experience in Texas heat. I did some drive after the new battery and no issue.

This morning I went for a quick trip. When I restarted the car after stopping at my destination, the battery light went off in normal fashion and then came on again for about 3-4 seconds and went away. Came home and checked battery voltage and it is at 12.8.

My assumption is that the alternator/voltage regulator is stating to short out and eventually I will just get the red battery light more frequently.

Has anyone seen an alternator start to fail in this way. I had replaced the alternator ( the original) about 3 years ago as well, but it seemed to fail all at once and when the battery light came on it stayed on and the electrical read clearly indicated it was bad. As I understand it, the alternator or battery problems are the only thing that would cause the battery light to come on for whatever period of time.

Thanks

Tom in Dallas/Plano
2005 S type 3.0, 117K

 

Check the battery voltage at the battery since it reads lower at the OBD port.

If the alternator is failing, remove it and have a local automotive electrical shop overhaul it. I personally have not had success with the 'remanufactured' units sold through the parts houses.

 

Two things to do before scaring yourself:

1) Make sure the battery is fully charged. New ones are not always fully charged at the time of sale. Your charging system may be okay, but is having a hard time topping off the battery due to driving conditions, such as short trips and high electric loads.

2) Inspect the serpentine belt. Replace if any sign of glazing. I'd even try a little belt dressing and see if that clears up the problem. Any doubt, just change the belt. They are not expensive. Check the tension, too. It's possible the belt has been slipping when the alternator is fully loaded.

 

I was checking at the battery. That is now new. The replacement alt. I put in 4 years ago was re-manufactured and has worked fine for 4 years. Did not have the battery light issue that some units have as they use the Denso one to re-manufacture. Also I have a warranty and the relacement will not cost me anything.

It is not practical for me to be without a car for a week or so to have someone many miles from me to redo the unit.
It would be cheaper, with paying for Uber, to buy a Jag new one, but since I don't own stock in the company I refuse to be cheated out of money.

Does the intermittent battery light coming on for a few seconds, now that I have a new battery, seem to indicate a failing alternator or voltage regulator to you?

Thanks Doug

Tom

 

Yesterday before I replaced the battery- even after about 1.5 hours of charging at highway speeds, it still only got to 12v. So a new one was in about 30 minutes after I got home and the new one showed 12.5 before I put it in. I had not driven it very much before I got the quick battery light this morning, but when I got home the battery measured 12.8 which is better that the 12.6 I was expecting.

I am planning on replacing the belt with the alternator- even though the belt is only 2 years old.

Thanks

Tom

 

This morning I measured the battery before starting and it showed 12.45 volts. Right where it should be. I then started the car and about 15 seconds later the battery light came on and so I went to measure the battery and it was showing 11.90 volts. I then went back and revved the car and the battery light went off and it showed 15 volts at the battery.

On the surface one might think the belt is bad, but does not appear so. It is like some internal electronics in the alternator is going out or perhaps something that should be turning inside is not getting started at idle. Very strange. However I think once the alt and belt are changed today- then it will be fine.

If anyone has any additional suggestions I am all ears.


Tom

 

Based on your descriptions, it seems the battery is suspect.

If the charge lamp is coming on, it usually indicates a failing alternator due to open or shorted rectifier/diode pack.

 

The battery is brand new as of 2 days ago. Makes sense on the alternator- I have a warranty of the alternator and so the exchange will cost me nothing except for about a hundred to have it put on. He is in walking distance and has a lift and I can let someone else get dirty for the day- ha!

Thanks Doug!

TBB

 

New alternator and problem solved. Interesting that the old alt was always showing, when on, voltage at the battery of 15. Now with the new one the voltage stays in the 14.3-14.6 range, which seems to be where it should be. New belt that was probably not needed, but just considered that a maintenance item.

Tom

 

Charge the battery overnight at 2 to 5 amps to make sure it's fully charged.

Glad to see the alternator replacement took care of the issue.

 

Great job Tom. 👍 Good info for people down the road.