Hi,

I noticed that my battery light turns on pretty much randomly. I tried to reproduce it and I found that it the light comes on more often when the car is rolling down a hill or in idle. But not always, so it is very hard for me to measure the voltage.

Battery is 100% full. 12.7v when the car is off. When the engine is running, I measure 13.9volts and with air conditioning, radio, fog lights etc. I measure 13.7 volts, without the battery light coming on at all. It takes some time to drive until the issue starts occuring again.

How are the chances fixing the issue by replacing the alternator, since it also carries the voltage regulator? What alternator can you recommend? I'm looking at the PA Performance from AM, but it's around $260. I also don't want one that is too cheap. Any recommendations?

 

I've been driving around for a couple of miles and can confirm that the light turns on deferred when slowing down and turns off when accelerating. I also noticed variations of the air conditioning when slowing down/stopping and accelerating.

I came to a full stop with the engine still running, air conditioning, low beams and radio on. The light turned on again after a couple of seconds. I measured 14.2V while the light was still on. I turned the headlights on in addition. The voltage dropped to 13.9V. Isn't it in normal range? If yes, what else than the alternator could be broken?

 

The voltage regulator inside the alternator seems to be fine but you could have a problem with one of the diodes. They're easy enough to test if you have a multi-meter, and there are instructional videos on Youtube about how to do it.
If you do need to replace the alternator, get a 2010 OEM version and it'll probably be the last one you'll ever need.

 

Thanks for the good information.

I did a complete test drive today with an OBD2 scan tool monitoring the system voltage. The voltage stays around 13V when slowing down or driving down a hill. If I rev to 1800rpm the voltage goes up to 13.9V-14.2V. If I turn on air conditioning, the voltage drops to 12.3V-12.9V. So I believe there's clearly something wrong with the alternator.

I did a check on carfax and the alternator already got replaced 40k miles ago. I believe they put on some cheap replacement/rebuild. Even if it is the diodes, I'm not sure if the bushings could fail as well at some point in time.

So I think I'd go for a redesigned alternator instead, such as the SR or PA one and do the connectors as well. I'll also save my time for fixing the alternator.

 

Sounds like the voltage regulator is indeed in the early stages of failure.

If you go with another aftermarket alternator, don't be surprised if you find yourself replacing it again in 3-4 years' time. They're no better than the original 05-09 OEM alternators and a 2010 OEM alternator is the best replacement.

 

Double check the smaller bolt on the positive terminal. The one that has the cable that runs to the alternator. Seen some come loose, yes, yours truly.