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Just curious if there’s anyone out here that deals with Subarus on regular basis. My wife has a 2012 Subaru Legacy– the basic one– not GT. I’ve noticed many times when I drive it that if you have the fan (climate control) on Max, and say rear defrost on, it sounds like the fan slows down when the car idles and speeds up significantly when gas is applied. Even seems like the headlights dim a bit at idle and come back up at speed. The car is stock and has 30k miles on it. This has been happening since it was new. Are Subaru’s known for using weak alternators or do I have a problem? Dealer said it was ok.
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you could check the alternator output.
http://www.ericthecarguy.com/faq/solving-automotive-electrical-problems?start=2#ChecktheBattery
It is very likely that with a lot of electrical accessories running, your alternator isn’t able to keep up at idle, so voltage drops to the 12V battery voltage. Once the engine revs up a bit, voltage jumps back up to the 14.2V or so that the alternator charges at, accounting for your change in fan speed, and headlight brightness.
Tomh, that makes perfect sense, but the car is stock. That’s why I’m wondering if anyone is aware of perhaps 2012 Subarus and alternators that can’t keep up with the basic accessories.
The fan on high, and the rear defroster together do have a significant draw. You could check the condition of the drive belt, battery connections, battery (load test) and check the alternator to be certain that is is operating properly. I’m not entirely sure that there is anything wrong with it though.
At idle the alternator only puts out about half its max rating. This sounds lousy, but it’s much much better than the old, pre-1964 generators which would put out absolutely nothing at idle.
So if your load is more than half the alternator max rating, the difference is going to come out of the battery. If the battery is getting old or the cables or connections to it are corroded, the battery voltage will droop a bunch. How much is too much? A little droop is normal, maybe up to half a volt or so. More than that is tolerable, but you will notice the lights and fan sagging a bit.
The problem still could be in the alternator, if it never puts out enough to charge the battery fully, even a good battery will exhibit these symptoms. So it’s still not clear what is the prob.
I would first check the connections. Most are corroded and can benefit from a wire brushing of the terminals and inside of the connectors.
Then run a battery test. First charge the battery up fully with a guaranteed source of charge, the alternator doesn’t qualify as it’s an unknown right now. So charge it up with a battery charger overnight. In the morning remove the charger and turn on the headlights. A good battery will run the headlights nice and bright for at least 15 minutes to an hour. A weak battery will start dimming the lights after 10 minutes or even less. If the lights start getting yellow in under 20 minutes I would consider the battery weak and look for a replacement.
If the battery checks out okay, then the dimming is not caused by the battery, but may be a problem with the alternator. It might have a loose belt, so it never spins quite fast enough, or it may be never be putting out enough to charge the battery fully, or it may be set to cut off charging at too low a voltage, or maybe it’s temperature compensator isn’t working. Complicated isn’t it?
Good luck figuring this out. The above hints are a start and should help.
The alternator should put out enough to keep the vehicle running and your accessories working at idle. I wouldn’t look to the alternator first however. If you have a low battery, the alternator will have a harder timing keeping up with the rest of the system because it’s trying to make up for a weak battery. With electrical problems, you always want to start by checking the battery. More info can be found in this article on how to test your battery and alternator.
http://www.ericthecarguy.com/faq/solving-automotive-electrical-problems
Keep us posted on what you find.
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