Battery light stays on - checked everything
 

The red battery light on the dash stays on while driving. Engine starts and runs fine. No other electrical issues. I know this means charging system malfunction so here is what I have done so far:

-replaced battery (it tested a little low but was starting engine ok).
- tested alternator - charging normal.
- Checked battery cables and chassis grounds, all good.
- checked condition of wiring coming from alternator, sleeve still in tact with no tears all the way to the engine bay fuse box.
- checked fuses and relays of charging system circuits, all good.
- checked for obd codes, none.

I’m out of ideas. Does anyone have any suggestions?

It’s a 2004 V8 4wd limited

alt - s fuse in engine fuse box
its 7.5 amp

check that. and use a multimeter to verify you can indeed getting a charge from the alternator on battery terminals.

13.5 to 14+ when the engine is on and running.

Thanks for the reply riceandpho:

Alt-S fuse is good. While I had the fuse out I measured the current going through that circuit with the key in the on position engine off, and with the engine running. Measured zero current for both. Seems like there should be some current? Any idea what this means?

Also measured the battery voltage: 12.8V with engine off and 13.96V while engine running.

Also checked the fuse labeled Gauge (7.5A) in the cabin fuse box. It goes to the battery light. Fuse was good and current measured .7-.8A with key in on position and .45A with engine running.

Attached is the circuit diagram from my Haynes manual. Any more ideas are much appreciated.

“While I had the fuse out I measured the current going through that circuit with the key in the on position engine off, and with the engine running. Measured zero current for both. So I guess the fuse might as well be blown right? Any idea what this means?”

If you remove the fuse, you break the circuit. You will get 0 volts with the fuse removed.

Either your alternator is going bad, the battery (how old is the battery?), or a poor ground.

With “ 13.96V while engine running.” that indicates it is the alternator.

Derek V

"If you remove the fuse, you break the circuit. You will get 0 volts with the fuse removed."
With the fuse out if you connect the leads of a multimeter to where the fuse would go and set it to measure current that completes the circuit as if the fuse was in and the multimeter measures the current that would go through the fuse if the fuse was in.

Battery is new (2 days old)

"With “ 13.96V while engine running.” that indicates it is the alternator."
Every website I have checked says 13.96V while running is normal (alternator is good). Here are a couple sources:

How to Test Your Auto Battery | Battery Testing & Maintenance | Autobatteries.com

What should the battery voltage be when car is running? - 2010 Toyota Corolla

i think its either a connection is loose at the plug at the alternator.
broken wire or a broken pin.

@GLP
Sounds silly but the diodes in the system can cause some problems like you discribed. Disconnect both battery cables and hold them together for a few minutes.
Reconnect everything and start it up.

- tested alternator - charging normal.

should be.

- replaced alternator - red battery light is gone. :)

You are 100% right about using a meter to complete the circuit when testing current. Sorry.

It can’t be (well, 99.999999%+) the battery.

Either a connection or the alternator is slowly failing. Check the small wires as well.

Found this;

After firing the engine up, you should get over 14.2 volts. If you got 14.2 or more volts, the alternator is charging fine. If it’s still showing in the 12’s, it’s time to replace the alternator. If it’s producing a voltage level in the 13’s, go ahead and take it into a parts store and have it tested. It’s probably bad.
Now, assuming that the alternator was producing over 14 volts, go ahead and turn on the headlights, air conditioner, radio, and any other accessory you can think of, and run the same test all over again. If the voltage dropped back into the 12’s, it’s a very strong indication that your 4Runner’s alternator is bad. It’s still worth having it tested, but be prepared to replace it.

So maybe try again with as much of a load on the alternator as possible and see what you get for voltage.

Derek V

I would check to see if you're getting approx 13-14 volts on the gray wire coming from the alternator to the junction block. That wire should provide opposing voltage to the charge light. It has battery voltage coming from the fuse. If it has battery voltage coming from the alternator, the opposing voltage will not allow the current to flow and thus the lamp will be off.

Thank you, this is exactly what I was trying to look up, what that grey wire should normally read. I'll check the gray wire voltage.


To others that replied, thanks for the input. I have driven it about 2 hours since Sunday when I measured the battery voltages. So after 2 hours driving here are the battery voltages today:

12.76V engine off
13.88V engine idling, no other loads
13.77V engine idling, with other loads (headlights high beams, stereo, seat heaters, interior lights)
13.93V 2000 rpm, no other loads

So I am pretty convinced the alternator is charging normally, plus AAA battery tech checked it and said the same. Checking the alternator grey wire voltage next.

Do you have any tips on how to measure voltage at the gray wire on the alternator? It seems like the only way is to unplug the 3-wire connector at the alternator then measure voltage on the alternator pin. But I'm not sure if running the engine with the 3 wires unplugged from the alternator is ok?

in my experience, when i get the battery light on my car, replacing the alternator fixes it. i've had the battery light to turn on twice already on my 4runner and replaced the alternator for the 2nd time. light is still off as of this time. could be the problem on your car is the voltage regulator in the alternator.

Same problem !!!!!
 

I posted about the same condition. I replaced battery with a new one, 12.7 on batt, replaced alternator, 14.1-14.2 after starting , 13.7-14.0 idling. Checked every fuse, engine bay and dash....all good. Starts fine, runs fine, just was on after sitting for a week. Where are the ground wires? I know about the one on the fender by batt. Any others? Sorry to hijack thread, but this is maddening... rsbhunter

When the battery light comes on it does not indicate that the battery is bad but tells you the charging system is not supplying enough power to the battery.
With the engine running at 2000rpm and no accessories on, the voltage at the battery should read about 13.9 to 15volts. Toyota does not specify and plus or minus range:-( which everything has. Your reading is 13.93v so the alt is great. Forget about the dashboard icon being lit. Here in VA one can not fail a vehicle inspection for a battery light lit.