Battery Light On, Battery and Alternator good, no problems starting

The short story is everything tests good however the battery light is on and will not go off. I tried several alternators, one being new, and put in a new battery and a new belt.

I have a 2004 Toyota Matrix XR. Several days ago my battery light started blinking on. It would only occasionally flicker on then off and it was mostly off. A day after that it started to go on and off more regularly staying on for 5 to 10 seconds sometimes and being on around 50% of the time. I must also point out I have had zero problems starting the car and no problems with lights being dim or anything to make me think there’s an issue other than the battery light.

I took the car into Advance Auto and they tested the car with their meter. The test showed my batter was a little low and out of specification for the battery and it showed the alternator as being good. So I bought a new battery and put it in. This didn’t fix the battery light from coming on. I had Advance test the battery and it tested good.

So I figured it was the alternator and decided to put a new alternator in. They only had the remanufactured alternator in stock so I purchased one and put it in. This did not fix the battery light from coming on. The light at this point was almost solid on and just flickered off occasionally. I had advance test the remanufactured alternator and it tested good. It showed 13.9 volts and 13.89 volts under load. I felt it should be in the 14 volt range as the one I took out tested around 14.3 volts. They said they would let me exchange or return it. I decided to get a new alternator instead of remanufactured so I had them order it and I picked it up the next day.

They tested the new alternator before I got it and it looked good showing 14.3 volts. I put the new alternator in however it did not fix the battery light from coming on. At this point the light is completely on and not flickering off at all. My lights are bright and I’m not having any trouble starting. I drove the car around for half hour or so figuring that maybe the battery light is on because the battery was low and needed some charging however the batter light stayed on. Again, I had no trouble with dim lights and no signs of trouble except the battery light being on. I took the car out again after dark and drove around for about a half hour with the lights on and the windshield wipers running. No signs of low battery or any issues. I’m going to take the car into Advance and have them check it again and I’m pretty sure everything will pass.

I’m not sure what to do or what to check. Could this be a shorted or broken wire? Is there a blown fuse that could cause this? Could the Computer be bad or corrupted?

This seems fairly common with some Toyotas. Battery cables can be the problem. I assume the connection at the battery was cleaned and sprayed (anti corrosive) and the cables are in good condition. The other end, especially the ground (negative), can also corrode.

You can quickly test the negative cable by hooking jumper a jumper cable to the negative battery connection, the other end to a convenient ground. If the light goes out, it’s either the battery cable, or the engine grounding strap. If grounding to the engine doesn’t work, try grounding to a bolt on the body. (If the second works, it could be a corroded engine-to-chassis connection.)

Rust is a poor conductor and can add enough resistance to cause erratic electrical behavior.

Sounds like a copy of my friend’s problem with his Toyota. His problem also started with the battery light flickering and no CEL. See:
Calling for help one more time
There is an update as of now.
It might be the same problem.

Is there a wire from the alternator to ground? I had one break years ago on a Chevy Cavalier. The alternator may test out fine on the work bench but its connections to the car have to all be good for it to work there.

Just to point out, the car starts fine and tests fine. When advance is testing they connect to the terminals on the battery with the battery in place. There tester is saying the alternator is charging the battery with 14.3 volts. Before starting the car the battery showed 13.07 volts. The cranking starter test showed 12.05 volts.

Would everything look good if it were a bad ground? I did try connecting a jumper cable from the battery to the engine and the battery light was still on. I also tried it directly on the alternator and it didn’t change anything.

Your car’s computer system has many sensors and warning indicators. It is likely a computer system issue if anything else is fine. You need a mechanic that has a obd2 scanner to troubleshoot this problem.

I just went to Advance and here are the test results:

Good Battery
Voltage: 12.87V
Measured: 825 CCA
Rated: 785CCA (actually 640)
Temperature: 95 F

Starter Test: Results Cranking Normal
Time: 2.07S

Charging System: Results No Problems
No Load: 14.27V
Loaded: 14.06V
Ripple: 32mV

My question would be does the Alternator control the charging or does the computer. If the computer controls the charging would there be issues of it telling the alternator to always be charging and then overcharge the battery causing battery failure?

It appears everything is working except the battery light (computer knowing everythings working)

I would advise taking it to a qualified mechanic to diagnose this rather than the counter guy at an auto parts store. Sure, it will cost a few bucks,.but it will be cheaper than another new battery after this one does.


Several Asian imports have shown up at my door with similar problems and it appeared to be the integrated voltage regulator causing the problem. Unlike long ago when the idiot charge indicator light was directly involved with the charging system some of the current alternators have a sender circuit and a charging drop out circuit that can cause a lot of grief. Bench testers don’t check those circuits and rebuilders seem to reuse faulty regulators because they function perfectly in the charge mode. Somewhere out there in youtube there is a video of someone who explains how to disconnect and bypass the failing circuits and keep the charging system operating well without them.

One thing not mentioned by the others is to check that the serpentine belt is tight enough. At 15 years old the tensioner may have lost it’s ability to keep proper tension and the belt slightly slips but still turns.
I have replaced a few tensioners that barely put any tension on the belt at all.

If all that is ok, then I would look into cleaning, or maybe replacing both battery cables. The wire under the insulation may be rotted away enough that it can not carry the load.


Can anyone tell me if the Alternator (regulator) controls the charging or does the cars computer tell when the charging should happen? If I continue to use the car what will happen? I suspect the alternator might be always charging due to the computer thinking that somethings wrong. I read one article that said the alternator controls if the battery light is on.

To the previous quesions:
The tensioner seems to be fine. I don’t see any slipping. I put on a new serpentine belt
I cleaned both battery cables and terminals. The positive terminal had some corrosion on it before I cleaned it and put in the new battery.

I’m thinking about putting a new connector cluster on the cables that connect to the alternator. I could also try putting on new battery cables. I don’t see any power issues. This confuses me. No problem starting and no signs of low power. My other thoughts are a cable from the alternator to the computer has shorted or broke causing a disconnect of information getting to the computer.