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Old 11-17-2014, 06:00 PM
 
Location: Caverns measureless to man...
6,704 posts, read 4,163,830 times
Reputation: 14935

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I have a 2008 Subaru Forester, with just under 130K miles at the moment. It's been very well-maintained and taken care of, but lately we've had a couple of electrical issues, and I want to make sure I'm on the right track with my thinking.

I noticed in early October that the battery was starting to sound weak when starting, and made a note to start shopping around before winter. Static charge tests with 2 meters showed between 12 and 13 volts, but a load test showed that the battery was in much worse shape than I thought. So I replaced it last week, before it got cold. I got an Interstate Group 35 battery with 640 CCA, and a reserve capacity of 100. Should be more than ample for a few Kentucky winters.

I was out of town for a week with a different vehicle, and the Subaru sat for 6 days - during which the night time temps dropped into the teens. When I tried to start it tonight (with temperatures in the mid-20s), it sounded alarmingly weak. But with a brand new battery, only a week in double digit temps shouldn't drain it too much, so that didn't sound right at all.

I immediately suspected that perhaps the alternator wasn't fully charging the new battery. So, I went for a 30 mile drive on the highway, brought it home, and shut it off. I started it again immediately, and it still sounded weak. I then hooked up a Diehard 3-amp trickle charger. It showed 95% capacity. I left the trickle charger on for 15 or 20 minutes, then checked again. It now showed 98% capacity.

Now, I don't know how accurate that meter is on the trickle charger, but when I disconnected it and started the car, it turned over and fired up noticeably better than it had before the 15 or 20 minutes on the trickle charger. I then drove the car again, and pulled up in front of a brick wall while the car was running. I then turned the blower motor on and off, and observed that the headlights dimmed and brightened quite noticeably.

In addition, the battery I just replaced was a Diehard Gold that was only 3 years old. The car was in Florida most of that time, so it lived in a pretty hot climate, but still... I'd have expected a Diehard Gold to last a little longer.

To me, this all points to the alternator, but I am not an experienced mechanic. I usually just diagnose things by observing clues and trying to reason out how they're all connected. I have replaced an alternator in the past, and am comfortable doing it again if necessary, but before I do that I thought I'd ask for opinions and expertise. Is there anything else I should be trying here?

Thanks in advance for your help! Rep and cookies for everyone who answers....
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Old 11-17-2014, 06:24 PM
 
Location: North Port, Florida
774 posts, read 1,825,751 times
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Get the car running and disconnect the positive battery terminal.

If the car stalls, you alternator is not charging.

Mikey
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Old 11-17-2014, 06:51 PM
 
Location: Broomfield, Colorado
656 posts, read 927,348 times
Reputation: 849
Or just test the voltage at the alternator, check your belts, maybe have your battery load tested, rather than risk damaging the ECM by disconnecting and reconnecting cables while the car is running.
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Old 11-17-2014, 06:56 PM
 
Location: Inland Empire, Calif
2,887 posts, read 4,604,807 times
Reputation: 2766
Quote:
Originally Posted by mikey2 View Post
get the car running and disconnect the positive battery terminal.

If the car stalls, you alternator is not charging.

Mikey
dumb....!!
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Old 11-17-2014, 07:53 PM
 
1,690 posts, read 2,009,467 times
Reputation: 2592
Quote:
Originally Posted by mikey2 View Post
Get the car running and disconnect the positive battery terminal.

If the car stalls, you alternator is not charging.

Mikey
It may have been charging before the battery was disconnected. Disconnecting the battery on a modern car with the car running is a great way to zap electronic components -- including the charging system.


Don in Austin
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Old 11-17-2014, 08:15 PM
 
Location: Denver
3,138 posts, read 6,901,436 times
Reputation: 2907
Quote:
Originally Posted by mikey2 View Post
Get the car running and disconnect the positive battery terminal.

If the car stalls, you alternator is not charging.

Mikey
Do not do this.

You need to get a volt meter and report back to us.

Multi meters are cheap....under $10.

You need to find out what the voltage is when the car is idling. When the car has everything on (head lights, defroster, blower fan, brake lights) and idling. And then put it up to 2000 rpm and see what voltage is then.

It also could be damaged / broken / weak battery cables, cable from the alternator to the battery, cable from the starter to the battery, or weak grounds.
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Old 11-17-2014, 08:20 PM
 
Location: Poway, CA
2,698 posts, read 9,519,350 times
Reputation: 2205
Another option is to run by any large parts retailer. They'll check your system free of charge. Takes maybe 2 minutes.

Mike
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Old 11-17-2014, 08:43 PM
 
2,888 posts, read 4,475,732 times
Reputation: 1852
Quote:
Originally Posted by whiteboyslo View Post
another option is to run by any large parts retailer. They'll check your system free of charge. Takes maybe 2 minutes.

Mike
x2
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Old 11-17-2014, 10:18 PM
 
Location: Verde Valley, Az
405 posts, read 1,056,596 times
Reputation: 510
Quote:
Originally Posted by mikey2 View Post
Get the car running and disconnect the positive battery terminal.

If the car stalls, you alternator is not charging.

Mikey
Do not do this! It might have been ok in the 60's, but not with a modern car and all its electronics. Its a good way to damage your charging system or other electronics.
Do what others have suggested and take it to Auto Zone or a similar place for a charging system check. Better yet, get yourself a cheap volt meter (harbor freight) and check the battery voltage yourself. It isn't rocket science (yet) Check voltage with the engine off and again with it running. You should see at least a .5 voltage increase from engine off to on, and then running-- 13.5-14ish at idle is a good number.

Curly
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Old 11-18-2014, 03:02 AM
 
10,868 posts, read 41,128,193 times
Reputation: 14009
It's not unusual at this mileage for the alternator to be failing.

You may even see correct charging system voltage, but if diodes are bad, the current output will be below rated specs.

Do not disconnect the battery when the engine is running. To do so risks voltage spikes that can damage the electronics.

The problem with failing diodes is that not only can the current output be low, but you may be getting an A/C component into the charging current ... not shown by a voltmeter, but detectable with an o'scope. Few shops test for this anymore, but it's hard on electronics and the battery. As well, there may be a parasitic draw upon the battery when the engine is not running, hence the sound of low voltage cranking to start the engine.

As others have posted, best to get the alternator/charging system tested with the proper equipment for voltage and current output.

Don't forget to inspect and clean the battery cables and system grounds. Your subie has a number of grounds on the firewall and left inner fender that are all critical to proper electrical system function. They may look clean on the outside, but what counts is their conductivity and that can be compromised by corrosion at the terminal. Best to take these apart and wire brush them clean, then reconnect. You don't want more than .2v drop at any of these, and few shops will test for this anymore. You can test it yourself with a voltmeter, but it's generally worthwhile to simply clean up the connections to begin with. An '08 is more than enough years in a humid climate to have corrosion at critical points in the system.
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