Battery light comes on at idle, I have tried EVERYTHING
 

So my battery light comes on at idle, goes away upon acceleration above 1,000 RPM's. What I have tried:

• Brand new battery
• Tested alternator at auto repair shop, tested good
• Tested continuity between ECM/PCM Connector B Pin 41 to alternator 4-pin connector pin 2, tested good
• Checked every single fuse, both inside under the steering column and under the hood
• Checked all wiring, impedance values indicate that all wiring coming from alternator, to the battery, and all grounds have good connections.

I'm absolutely stumped. Is there anything else I can try? Thank you very much.

Re: Battery light comes on at idle, I have tried EVERYTHING
 

Has the alternator been replaced yet?
Is the belt tight?

Does the engine have an undersize crank pulley?

Re: Battery light comes on at idle, I have tried EVERYTHING
 

The alternator hasn't been replaced yet
The belt is tight.
The crank pulley is the stock one the car comes with.

Re: Battery light comes on at idle, I have tried EVERYTHING
 

Make sure the Battery Posts & +battery Cable & - Negative Terminals are absolutely
Clean and free of Any Cracks or corrosion reason I mention this is on my wifes vehicle here
(+battery cable end) had a small crack in it when I took it off to clean and Put back it started the engine.
but I seen the battery Light on.?

after I shut it off and went to start it again. it wouldn't start so my guess or hunch
was the + Battery terminal since it was green and corroded on a closer look I
found a very small crack around the terminal I put a new +terminal and Problem was fixed.

Re: Battery light comes on at idle, I have tried EVERYTHING
 

with the car running.... does it die if you disconnect the battery?

Re: Battery light comes on at idle, I have tried EVERYTHING
 

I haven't tried that. Is that safe? (I'm honestly not much of a car guy)

Most of the time the voltage from the alternator positive post to battery negative is around 14 volts, but out of nowhere when idling, with no load at all except daytime running lights and normal volume radio, the voltage will drop down to 12.5 or so. At the same time, the DC clamp meter on the wire coming from the alternator will drop down from 20-ish amps to like, 4-5. This triggers my battery light. And then a few minutes later it kicks back up.

Yet the alt is good apparently, according to Mountain View Tire.

I really appreciate the help, I've been troubleshooting this for a week now, and I've got AAA on speed dial for when my car inevitably breaks down on the way to work one morning.

Re: Battery light comes on at idle, I have tried EVERYTHING
 

im old school... i start the engine...turn the lights on and disconnect the battery... if it dies i assume the alternator bad and preplace.. its really kinda crude..but works.. your issue might be something els.. no check engine light?

Re: Battery light comes on at idle, I have tried EVERYTHING
 

No check engine light, just the battery light coming on when the voltage dips.

Re: Battery light comes on at idle, I have tried EVERYTHING
 

I did notice the tensioner pulley has a bit of "bounce" to it, maybe a couple millimeters, while the car is idling. There's no pronounced noise, but maybe there's slippage I can't see? Is the tensioner pulley supposed to have that kind of play in it?

Re: Battery light comes on at idle, I have tried EVERYTHING
 

Let engine idle and make sure the battery warning light is on.
Remove the 4 wire plug from the alternator.
See if that light turned off.

If that turned the light off, the alternator is at fault. Replace it.



Do NOT do this!
You can ruin expensive electronics in a damn hurry!
The battery does the job of a huge capacitor, and if it's disconnected from the system there is nothing to absorb voltage spikes.

This was ok back in the 60s when the only electronics in the car was the radio. NOT NOW.

What year is your car? Trim level? Engine size?

Re: Battery light comes on at idle, I have tried EVERYTHING
 

That's an awesome tip, I'll try that with the 4-pin connector.

The car is a 2008 Civic Si

Re: Battery light comes on at idle, I have tried EVERYTHING
 

Yes, disconnecting the alternator 4-pin connector does indeed turn the battery light off, and plugging it back in brings it back. I wonder why the alt tested good at the mechanic. Oh well, I guess that's that.

Re: Battery light comes on at idle, I have tried EVERYTHING
 

it sounds like one of the alternator diodes are weak giving You the eratic voltage reading.

Re: Battery light comes on at idle, I have tried EVERYTHING
 

Would that sneak past an alternator test at an auto repair shop?

Re: Battery light comes on at idle, I have tried EVERYTHING
 

Maybe because it wasn't acting up when they tested it, or the person only checked if it was charging at all, or maybe they don't know what they are doing, or maybe they didn't know what your actual problem is. The alternator may not have a specified amp rating visible on it so they have no idea what to test it at.

If the complaint is the warning light comes on at idle, then it doesn't matter if it's charging (but if it fails the charging test, that's a reason the light comes on).
Did you ask anyone to diagnose why the idiot light comes on? Did anyone notice and diagnose that? (besides me, from the other side of the freekin planet, over the internet)

The voltage regulator (inside the alternator) is responsible for turning the warning light on and off. It can charge fine but still have a faulty voltage regulator that can't control the circuit for the warning light correctly.

Several other items can also take control of the warning light (the alternator does not control the light directly), but you eliminated that possibility when you disconnected the alternator, proving it was the one keeping the light on.

EDIT: NOTE aftermarket alternators frequently have the exact same problem, right out of the box! I hate installing them because so many are crap!

HTH

Re: Battery light comes on at idle, I have tried EVERYTHING
 

I really appreciate all the help. The shop just told me that the alt was good and to check my wiring. I didn't pay for them to troubleshoot the battery light.

I feel like all I'm left with is to swap in a known good alt and see if the issue goes away. It's pretty much the only thing I haven't done.

Re: Battery light comes on at idle, I have tried EVERYTHING
 

Yeah, it was the alt. Swapped it for a known good alt and all problems went away. How the bad one passed the test at the auto shop is a mystery to me.