Battery light stays on but its charging

Hi everyone,

I have a 2012 Tacoma, 2.7L Access cab. My battery light came on a couple weeks back, on the original battery. I cleaned the battery contacts, ground connections, and checked the fuse. Put it back together and it stayed off for a week. Then it came back and stayed on. Starts and runs great all the time.

New battery two days ago gave me no change. I have a voltmeter in the cab and it is showing 14.2V at startup. On the highway, it is usually 13.9V. At idle with no AC or anything else running, the lowest I've seen it dip to was 13.4V, but it is usually higher than that.

I removed and cleaned the 4-pin plug on the alternator. Belt doesn't seem to be slipping. What could it be???

If the alt is going, I shouldn't be getting a charge. So if I have a good battery and it's charging, why is the light on?

Suggestions more than welcome!

 

How is the amperage output of the alternator?

 

I was reading another thread about the same problem awhile back. On his it turned out to be the wire from the alt that sends the voltage info was loose. There where also some links and schematics posted that you would probably find helpful. I don’t remember what the thread was called but I’m sure if you search battery light problems you will find it.

 

The alternator controls the battery warning light by grounding the L terminal (Grey wire), the only things that can turn the light on are a problem with the alternator, open or high resistance in one of the wires going to the alternator, a short to ground on that wire, or a problem in the instrument cluster.
Since you say it's charging but the light is on you can unplug the connector at the alternator and see if the light goes out, if it does the alternator is failing, if it doesn't you need to check that wire for a short to ground.

 

Great advice here, thank you! I will test with the connector unplugged. I can also rig up a means to test the alternator amperage. I'll report back

 

I mean if it maintains at least 13.2v with all loads turned on it's putting out plenty power but the light being on is concerning so you definitely want to be sure it's the alternator turning the light on and not a shorted wire.

 

So I popped the alt connector off and the truck fired up.

No more battery light.

Voltmeter showed 12.4V obviously as the alternator was not charging.

Plugged it back in and I’m back at 13.9V with a battery light.

I’m not sure if I’m understanding the circuit properly. Does the dash light receive power, then send it to the alternator, which normally leaves the circuit open and therefor no light? But when it fails it grounds the circuit causing the light to illuminate? So by unplugging the connector, and having no dash light, that means there isn’t a unwanted path to ground somewhere in the harness, otherwise the light would stay on. At least I think that’s what is happening here.

Not sure why I’m getting a good charge but still a dash light. I’ve got an amp meter at my dads place so I’ll check the output later this week.

 

You got it.

I've seen alternators have many different failure modes, sometimes they turn the light on but still charge, sometimes they turn the light on and don't charge, and sometimes they don't charge but don't turn the light on.

 

Well, put a new Denso reman alternator in and no change. Still have a battery light.

tried pinching and flexing the wire leading to the alt in case there was a break, no change.

I am stumped.

 

Interesting, is the alternator charging?
Did you have someone watch the light while you wiggled the wires and see if it flickers or goes out?

 

Yes, 13.9V, and I tried wiggling the wires while my buddy sat inside watching the light.
Also cleaned the engine and body ground again, as well as the main charging wire at the fuse box. I am stumped!

 

But you say the light goes out with the alternator unplugged?
Have you checked the pink, blue, and both white wires at the alternator for power? They should all have battery voltage.

 

I have a blue, white, pink and black wire at the four pin connector. The white has no voltage. The other three all show battery voltage. What does that mean?

 

Check your ALT-S (7.5A) fuse in your underhood fuse box, it's interesting I didn't think the alternator would charge with no power there.

 

It shouldn’t, I’ve seen a tech replace an alternator before thinking it was bad and it turned out to be the 7.5 amp alt/s fuse.

 

I checked the fuse already. The fuse is good. Alternator charges normally

 

That's what I was thinking, if it hadn't of been charging that would have been the first thing I would have suggested.

 

Tried a brand new fuse just in case my eyes tricked me. No change

 

Double check and make sure the white wire in the 4 pin connector has power or not with the truck running.

 

Check the wire for power at connector EA1 Pin 11.