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Discussion Starter #1
A few weeks ago, my brake light and battery light came on. I suspected my alternator, so had it tested and that is what it was. I replaced it with a new alternator and it has done fine. A few days ago, the brake light and battery light came on again. Then, if I turn the car off and back on, they went off. This has happened 2 times. Any ideas what this could be?
 

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NF Mod/Nissan Master Tech
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If you are using aftermarket parts, it could be the "new" alternator. Assuming your battery connections are tight, check the charging system with a voltmeter. It should read 13.2-15.5 volts, both at idle and at 3000 RPM, and both with loads "on" and loads "off." {"Loads" meaning headlights, heater fan, radio, etc.} Most late model Nissans tend to charge around 14.7 volts +/- at idle with loads off.
 

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Thank you for your reply! I went to the parts store yesterday and they tested the alternator and they said it tested fine. But when they tested the battery it was only charged at 50 percent, buy tha actual battery tested fine other than the charge. The battery light and brake service light were not on in my car, but my service engine light was on. When he tested that it mentioned something with my cadilic system...I think I'm saying that right. It mentioned a cadilic converter. So, I cant understand how my alternator tested fine but my battery is only 50 percent charged. I know nothing about cars...but my understanding is that your alternator charges your battery.
 

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NF Mod/Nissan Master Tech
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Your alternator does charge your battery, but batteries do go bad over time (about 4 years, +/-, for many automotive batteries) and lose their capacity to maintain a charge. Just like cell phone batteries; in the beginning, we charge them and they hold their charge a pretty long time. But, if you keep your cell phone for a few years, you'll find yourself having the battery run down quicker and quicker and require more frequent charging to keep it from dying out on you. In order for a charging system to work properly, the battery has to be good. Now, whether the battery in your car is the problem or not, I cannot say as I did not diagnose it personally. What I can tell you is that if your battery is older than three years, there is a possibility that it is starting to weaken and may even be due for replacement, as the guy at the parts store suggested.
As far as the check engine light and stored code, whenever a vehicle has a charging system issue, it may cause a false triggering of one or more codes. So, it makes sense to address the charging system issue, first, then erase the codes and see if they come back when you drive the vehicle. If they do, then it's time to look into those codes, assuming there isn't an obvious drivability issue, like an engine misfire, for example.
 

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Discussion Starter #5
ok thank you! The guy at the part store acted like the battery tested fine, other than that it was only charged at 50 percent. I am not sure what all they test, so maybe that in itself shows me that I am possibly needing a new battery. The way I understood what he said, it was a problem the battery wasn't charging, but that it wasn't because of the battery needing to be replaced, if that makes any sense at all. Like, that there had to be another issue causing it not to charge. I appreciate all your help!
 

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NF Mod/Nissan Master Tech
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ok thank you! The guy at the part store acted like the battery tested fine, other than that it was only charged at 50 percent. I am not sure what all they test, so maybe that in itself shows me that I am possibly needing a new battery. The way I understood what he said, it was a problem the battery wasn't charging, but that it wasn't because of the battery needing to be replaced, if that makes any sense at all. Like, that there had to be another issue causing it not to charge. I appreciate all your help!
In order to properly test the battery, it should have been slow charged for at least 8-hours. You might also want to check for parasitic draw in excess of 50-millivolts. Anything more than that while the car is turned off can drain the battery down overnight or during a long sit.
 
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