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2019 Forester Touring
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122 Posts
Discussion Starter #1
Have new 19 Touring less than 2000 miles. Due to a bad experience with a previous car, I had a voltmeter that I could put in the power outlet and have been monitoring voltage. Most of the time it has been spot on where it should be holding steady around 14.2-14.3 volts. However, on a two hour trip yesterday my voltage was fluctuating wildly often down to around 12 volts and a few times as high as 15 volts. To make sure it was not the meter, after noticing this I then also connected a supplemental gauge set I have into the OBD port. Same results. Today, everything is back to normal on short trips. This erratic behavior was the same thing that happened on my prior car (not Subaru) that led eventually to lower voltage and the alternator not charging the battery. Problem there was oil getting on voltage regulator built into alternator. Repeated problems. Anybody else have experience with fluctuating voltage on 19 Forester?
 

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2019 Forester Touring
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122 Posts
Discussion Starter #2
This afternoon on return trip the voltage was dropping again. My wife noticed voltage returned to over 14 volts when turn signal was operated or when the headlights (Not DRL) were turned on. Once home I did some Googling and found that Subaru intentionally reduces alternator output under certain circumstances, putting it back to normal when certain systems are operated. The major possible problem with the system is if you add some extra accessory that the system does not know about it may not increase output when needed.
 

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2017 base model manual 6 speed
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1,259 Posts
Hi @nds,
I have a ‘17 and I mentioned a situation similar to yours to the dealer in my first oil change and they did a software upgrade on the alternator. Not issues that I noticed on mine. As you said, the reason of the alternator cutting off, is to help save gas, not working when there is no need for it.
Mention to the dealer about any software upgrade next time you visit them.
Good luck.


Sent from my iPhone using Tapatalk
 

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2015 Forester XT Touring
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653 Posts
Guess is just like the 14+ SJs your alternator is controlled by the ECU. It does not run the alternator all the time. Their is a trick you can do with the fuse box to let the alternator run all the time.
 

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2015 Subaru Forester Automatic
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1 Posts
Hello All, I purchased a 2015 SF-Limited (78K miles) from our local dealer just under 9 months ago, &, since that time, the car fails to start after a day or 2 of no use. It looks like the original battery had been replaced with one from NAPA. Anyways, the dealer stated the alternator bad. Has been replaced. Followed by new battery just yesterday. Honestly no idea if this has corrected the issue or just masked. I have an old simple tester stuck in the outlet in the center console, which the idiot lights (green) show alternator charging at idle, whiles any speed above idle switches to (yellow) indicating low charge. No idea if I need to continue to worry, &, or, look elsewhere within the car for issues. The original battery having been replaced in such a short time by who knows prior to me purchasing the car, probably not a good sign. I'm certainly open to any help, ideas, what, when, &, where to look. Thank You Kindly. Scott :smile2:
 

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2019 Forester Limited
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791 Posts
Hello All, I purchased a 2015 SF-Limited (78K miles) from our local dealer just under 9 months ago, &, since that time, the car fails to start after a day or 2 of no use. It looks like the original battery had been replaced with one from NAPA. Anyways, the dealer stated the alternator bad. Has been replaced. Followed by new battery just yesterday. Honestly no idea if this has corrected the issue or just masked. I have an old simple tester stuck in the outlet in the center console, which the idiot lights (green) show alternator charging at idle, whiles any speed above idle switches to (yellow) indicating low charge. No idea if I need to continue to worry, &, or, look elsewhere within the car for issues. The original battery having been replaced in such a short time by who knows prior to me purchasing the car, probably not a good sign. I'm certainly open to any help, ideas, what, when, &, where to look. Thank You Kindly. Scott :smile2:
Check your no-load/open circuit battery voltage after it rests for several hours after driving long enough to have a fully charged battery. Measure with the negative lead disconnected. The resting voltage of a fully charged battery should be around 12.8 volts. An 80% state of charge would read around 12.6 volts. If the voltage is good, your charge circuit is likely fine.

If you find your battery slow to crank (or voltage dropping for no apparent reason) after sitting a few days, your problem may be a parasitic draw elsewhere in the system.

Subaru has a TSB on how to measure "dark current" or parasitic draws on the battery. TSB 07-85-14

"This bulletin will provide the proper procedure for measuring Dark Current (parasitic battery draw)
along with the specification for maximum allowable limits. Customer concerns of batteries going
dead over a period of time should be diagnosed using this procedure once any obvious contributing
factors have been eliminated."​
 

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2014 FXT CVT
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36 Posts
Guess is just like the 14+ SJs your alternator is controlled by the ECU. It does not run the alternator all the time. Their is a trick you can do with the fuse box to let the alternator run all the time.

Hello Aggie113,

the same Problem is here 2014 FXT / Voltage jump from 12,5 to 14,4 and back all the time.

Can you explain what is the trick to have a constant voltage?

Regards Roland
 

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2014 Touring with Eyesigh CVT
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610 Posts
Hello Aggie113,



the same Problem is here 2014 FXT / Voltage jump from 12,5 to 14,4 and back all the time.



Can you explain what is the trick to have a constant voltage?



Regards Roland
It is not a Problem but engineered by design to save fuel by only using the alternator for the purpose to charge the battery. The battery on new vehicles are now the work horse and used as the main power source to handle the normal power requirements. This is the main reason I tender my batteries on all my vehicles on a monthly basis.
 

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2018 Forester XT Limited CVT
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653 Posts
Holding the voltage constant isn't the best thing for the battery or fuel economy. I don't know what Subaru does but a proper battery charger will be computerised and go through several steps and adjust the voltage and current depending on load and history. Algorithms will be different based on whether the battery is flooded lead acid, absorbed glass mat, etc. A battery charger, a good one anyway, is not just a dc power supply. Even if fuel economy is not considered, multi-stage battery charging is much preferable.
 
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