Battery Light is on, but charging system voltage is acceptable

Joined
Aug 30, 2004
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2009 Honda Pilot, 203K.

Battery light is on.
There is an audible whining noise from the alternator.
There is also a noticeable burnt smell from under the engine bay.
Two different battery testers both say the ripple is ok; one tester said it was 364mV.
Charging voltage is 14.0v
Battery is new and tested perfect.

Tried grabbing a waveform using my scope (be easy on me, I am learning how to use it):
1646883518704.png

Some folks have advised me that a good waveform should look like the McDonalds Golden Arches, but I have yet to see a known-good one for comparison.

Any idea why the alternator light is on? It stays on under all conditions.

I recognize that the car needs a new alternator due to the noise, but I wonder what is triggering the alternator light since the charging system voltage is acceptable.
 
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Several parts of the alternator aren't being tested. The alternator could still be bad.

It could be that the alternator is putting out more amps than commanded. Most alternators default to "old school mode" when there's a regulator failure and they work like an old 1 wire alternator and produce max output all the time. This will trigger the battery light but show 14-14.7v at the battery terminals.

A normal modern alternator like yours is externally regulated and computer controlled and only charges as much as demanded under normal circumstances. If you have an amp clamp see what it reads for giggles.
 
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Honda also have load detector gizmo. This used to be somewhere in the fuse box at least for 2000 era Hondas and it did "multi-stage" charging. Depending upon the alternator load, it kept the voltage at "barely float", "just above that" and "14.xx". When the alternator and that gizmo do not talk nicely to each other the alternator light comes on.

But in your case, it being 2009, it probably has fancier and this contraption could be built in the alternator or the regulator. A dead or going diode seems to be the most plausible explanation given the mileage of the vehicle.
 

LvR

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SA
Alternators are three phase, so there should be 3 equal pulses of voltage. Something is wrong with one phase of yours.
This .................... that graph imo shows a missing phase - see every 3rd peak missing (negative) ................ guessing gaga diode pack
 

The Critic

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Alternators are three phase, so there should be 3 equal pulses of voltage. Something is wrong with one phase of yours.
This .................... that graph imo shows a missing phase - see every 3rd peak missing (negative) ................ guessing gaga diode pack
THANKS! That makes perfect sense now.
 
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Off topic, how does the AM radio sound in this thing?

I've heard they're supposed to have a buzz/ whine in this scenario but wonder how true that is.
 

The Critic

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Off topic, how does the AM radio sound in this thing?

I've heard they're supposed to have a buzz/ whine in this scenario but wonder how true that is.
Not sure, I didn't see the car for very long.

The failed alternator has been replaced with a Denso and is all is well, supposedly. I'll try to grab a waveform of the new one someday.
 
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Off topic, how does the AM radio sound in this thing?

I've heard they're supposed to have a buzz/ whine in this scenario but wonder how true that is.

Also, if you listen to AM radio, be REAL careful selecting LED bulbs for halogen housings. They are all poorly shielded and they are a total signal jammer for AM frequencies.
 
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Oct 8, 2005
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GA
Even though you've got the alternator sorted out, here's an article on alternator ripple from Motor that might be of interest to you.

Check out the 20+ year archive of other auto repair articles as well(plenty cover using an oscilloscope).
 
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2009 Honda Pilot, 203K.

Battery light is on.
There is an audible whining noise from the alternator.
There is also a noticeable burnt smell from under the engine bay.
Two different battery testers both say the ripple is ok; one tester said it was 364mV.
Charging voltage is 14.0v
Battery is new and tested perfect.

Tried grabbing a waveform using my scope (be easy on me, I am learning how to use it):
View attachment 92145
Some folks have advised me that a good waveform should look like the McDonalds Golden Arches, but I have yet to see a known-good one for comparison.

Any idea why the alternator light is on? It stays on under all conditions.

I recognize that the car needs a new alternator due to the noise, but I wonder what is triggering the alternator light since the charging system voltage is acceptable.
Don't be like me and get stuck on the highway and pay $170 for a tow. It's only a matter of time before that alternator kicks the bucket. The whining and battery light tells you everything you need to know.
 
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