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Old 03-25-2007, 09:58 AM   #1
Overlord2kx
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Join Date: Jan 2007
Chapter/Region: South East
Location: West Palm Beach, FL
Vehicle:
2005 Impreza WRX
Silver

Default Is something wrong wtih my battery/alt?

Hello,

I searched around a little bit for this topic, but I didn't feel that any of the threads answered my question.

I bought a used 05 WRX a few weeks ago, with 7500 miles on it at the time. I am the third owner. The first owner left the car at his vacation home for about a year and a half, and according to carfax he sold it to the second owner with less than 4,000 miles on it several months ago. The second owner drove it for a few thousand miles, but decided he wanted an SUV instead, and now I own it. I noticed that while the car is idling, I can see the cluster lights (especially the odometer) dim noticably, and sometimes the headlights, but if I tap the gas, they brighten up and dont dim for another minute or so. I have a Alpine headunit (not sure of the model, but its one of the lower end ones), and a Kenwood 200 watt amp with 10" sub. Even with the stock headunit, Ive noticed the dimming problem, and I notice it slightly more with the upgraded audio.

I pulled out a voltmeter, and it seems to be like this:
Battery voltage after car sits overnight 12.6v
Voltage after car idles a bit somewhere between 12.9 - 13.9
Voltage with A/C, stereo, headlights, etc on - sometimes as low as 11.8, usually not higher than 13.2
voltage with everything off, engine above 2,000 rpm - 13.8 - 14.0, never higher than 14.1

Im not sure if there is even a problem, but its still mildly annoying at the least. The car still has the stock battery, which I am suspecting is the problem. Could it be the alternator? What should I check further to narrow down this problem?

Its not that big of a deal now, but I am just worried once I put a better system, some bigger amps, etc that I am going to run into a problem.

thanks!
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Old 03-25-2007, 10:16 AM   #2
Mulder
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15 WRX/05 GTO

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Do you have the HVAC set in defrost mode? If so the AC compressor will run periodically and when it engages you'll see momentary dimming. Set your HVAC to vent with the AC off and if that was the cause the dimming will stop.
The WRX alternator can be maxed out at idle with a lot of accessories running, so it is normal to see the voltage dip below 12 at idle under those conditions.
Your voltage readings don't really look abnormal, and if the battery reads 12.6 after sitting it's probably ok. However the stock batteries are notoriously weak so you might want to think about replacing it as a preventive measure, possibly with an Optima.
If you really want to ascertain the condition of the electrical system you can bring the car to a shop which has the testing equipment to check it thoroughly. It is possible to have bad diodes in the alternator, which will reduce its output and cause other problems, and still see normal voltage readings with a standard voltmeter. Only a scope can fully diagnose this type of problem.
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Old 03-25-2007, 10:27 AM   #3
Overlord2kx
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Silver

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Quote:
Originally Posted by Mulder View Post
Do you have the HVAC set in defrost mode? If so the AC compressor will run periodically and when it engages you'll see momentary dimming. Set your HVAC to vent with the AC off and if that was the cause the dimming will stop.
The WRX alternator can be maxed out at idle with a lot of accessories running, so it is normal to see the voltage dip below 12 at idle under those conditions.
Your voltage readings don't really look abnormal, and if the battery reads 12.6 after sitting it's probably ok. However the stock batteries are notoriously weak so you might want to think about replacing it as a preventive measure, possibly with an Optima.
If you really want to ascertain the condition of the electrical system you can bring the car to a shop which has the testing equipment to check it thoroughly. It is possible to have bad diodes in the alternator, which will reduce its output and cause other problems, and still see normal voltage readings with a standard voltmeter. Only a scope can fully diagnose this type of problem.
Thanks for the input. The HVAC was not set to defrost, but the A/C compressor was on. I'll probably get a red top or yellow top and see if that makes a difference.
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Old 03-26-2007, 12:08 PM   #4
Uncle Scotty
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...I'd take all the grounds apart and clean them and make sure that they are all making good contact......these cars are prone to poor grounding....also, clean the battery terminals
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Old 03-27-2007, 09:42 PM   #5
TheRedWRX
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2011 Subaru WRX
Dark Gray

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I would go ahead and get you an red top. I bought mine about a year ago when my battery died at the airport. I love it. I had some similar dimming problem (thought not was bad as you) and it seemed to help.

Recently, I did the grounding mod and Big 3 Mod (for stereo reasons) and that really helped me. (Grounding helped more than Big 3 - I did them separately). Hope this helps.
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