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2003 Forester 2.5L
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122 Posts
Discussion Starter · #1 ·
Anyone have an issue with your voltage in the car?

When I drive around at a steady 2k RPM, I'm usually around 13.5v. However when I pull up to a stop light, I drop to 12.8-12.7. And If I let it idle for a while, i drop to 12.4-12.3.

Any thoughts?

2.5L non-turbo.
 

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Premium Member
2019 Crosstrek 2018 Forester XT
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14,948 Posts
If those are valid numbers there is possibly something wrong with the charging system or the battery is needing replacing or possibly the terminals need cleaned.
 

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2003 Forester XS auto
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640 Posts
This is the due to the "smart" alternator control circuit, the reason for the third pin on the alternator control plug.

With no load running, e.g head lights off, demist coil in rear window off, blower motor off, this was one of Subarus attempts at reducing fuel consumption through less drag on the motor.


Turn on a load and you should see the alternator turn back on and "normal" voltage again. I almost replaced an alternator before I learnt about this.


Sent from AutoGuide.com App
 

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2007 Forester Sports XT 4EAT
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Probably a bad alternator, but it wouldn't hurt to check that your battery cables are clean and tight. A bad battery could be a possibility, but I would be pretty confident in saying the alternator needs to be replaced.

Those voltage levels at idle wouldn't be due to a "smart" charge system. At idle with those voltages, you are running purely on battery power, the alternator is not doing anything. And 13.5V is about the absolute minimum to begin recharging a lead acid battery, but barely and slowly.
 

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2003 Forester 2.5L
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122 Posts
Discussion Starter · #10 ·
This is the due to the "smart" alternator control circuit, the reason for the third pin on the alternator control plug.

With no load running, e.g head lights off, demist coil in rear window off, blower motor off, this was one of Subarus attempts at reducing fuel consumption through less drag on the motor.


Turn on a load and you should see the alternator turn back on and "normal" voltage again. I almost replaced an alternator before I learnt about this.


Sent from AutoGuide.com App
I do put a load on it (giggidy). FOG on, heat all the way up, only thing that fixes it is pushing on the gas.
 

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2003 Forester 2.5L
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122 Posts
Discussion Starter · #11 ·

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10 XT 4EAT
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893 Posts
Doing a voltage regulator mod won't help if the alternator is damaged, which is likely the case here. Get it tested or just get a new one and try it.
 

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2007 Forester Sports XT 4EAT
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2003 Forester 2.5L
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122 Posts
Discussion Starter · #16 ·
Is the alternator belt at the proper tension? At low RPM it could be slipping.
No belt slip.

UPDATE: I idled for a while and noticed my voltage dipped to 11.8v which means my alt isn't even running at idle and it's all on battery power. But when I go, my voltage bumps up again. It's weird.
 

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03 Forester 2.5X 4 Speed Auto
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674 Posts
I've had 2 stock alts in my car
Both of which do that

Its normal.

XS power battery and a volt meter. I'm assuming you have a stereo? You'll be watching and noticing things like this that most owners would never know existed.

Depending on budget and equipment, DC power inc. Makes some really really nice alts
 
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