Battery light on dashboard came on?

IBRENDABBY

MEMBER
2000 LEXUS RX 300
V6
2WD
AUTOMATIC

Battery lights on my lexus dashboard and everything started dimming after 15 minutes or so. This happened before and it was caused by the alternator going out. Instead of buying a new alternator, My brother had open the old alternator up and changed out the alternator brush? That got it back up and running better than before for about a good 2 week. Now the same thing happens, battery light came on again, etc. I do have after market audio system in my Lexus too. Can that be the cause of my alternator failing? Or is it best if I just have my brother open up the alternator again? Or just buy a new alternator?

Wednesday, December 2nd, 2015 AT 3:23 PM

3 Replies

CARADIODOC

MECHANIC
34,180 POSTS

You didn't bother to list the mileage so I can't make any generalizations to that, but brushes don't wear out very often. When they do, the symptom always starts out as intermittent failure to charge the battery, and it gets progressively worse over weeks and months. Chrysler alternators and some older GM and Ford generators are the only ones where you can test for worn brushes. On all others you can only go by the symptom.

Start by measuring the battery voltage when the no-charge is occurring. It should be between 13.75 and 14.75 volts. If that is okay, there are four computers in your charging system and any one of them could cause the "Battery" light to turn on.

More likely you'll find battery voltage staying near 12.6 volts or less. First, measure the voltage on the large white wire at the generator. That should always have the same voltage as you find at the battery. If it is different, check the 140 amp fuse and be sure the connections are clean and tight. You can find bad connections there by measuring the voltages on the studs while the engine is running. Look for anyplace the voltage is different.

Next, at the generator, check that you have full battery voltage on the white / blue wire all the time, and on the black / red wire when the ignition switch is on. If either of those are missing, suspect a blown fuse.

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Wednesday, December 2nd, 2015 AT 4:01 PM

IBRENDABBY

MEMBER
2 POSTS

Thanks for the fast respond. So I tried starting up my Lexus bout 30 mins ago and sadly it won't start up. So im guessing it's the alternator failing, And was running off the battery only and drained it.

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Wednesday, December 2nd, 2015 AT 5:11 PM

CARADIODOC

MECHANIC
34,180 POSTS

First charge the battery at a slow rate for an hour, then see if the engine cranks normally. If it starts, measure the battery voltage while it's running.

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Wednesday, December 2nd, 2015 AT 5:23 PM