2004 Chevy Classic Charging light on at idle, goes away when revving

On a drive home from work, the battery light suddenly came on. The alternator belt was replaced not too long ago, I'm figuring no more than one or two years ago tops. That was the first thing I checked. Still there and it looked good. I got 250,000 miles on the clock. I also replaced the battery I'm wanting to say during early 2020.
I checked the voltage off the battery at idle and indeed it was 11.6-11.7 volts. I had someone rev the engine up past 2000rpm and the light went out while revved and it still measured 11.7 or maybe 0.2 volts lower if anything, if the belt was slipping I'd expect to at least get a little bit of output when revving it, nothing though. The battery cables are on tight and there is no corrosion that I can see. I could barely see the ground connection on the alternator but I couldn't reach it so I took my meter and checked for resistance between a nearby ground and the case of the alternator. Zero resistance. Where do I go from here?

EDIT: Just did two tests. I did a voltage drop test by connecting the positive lead to the output of the alternator and negative lead to battery positive. It read a few milivolts at most. Seems the connections are good?
The other test I did involved connecting the positive lead to the alternator output and negative lead to a nearby ground. Car on or off, load or no load it read the same as battery voltage. Does this mean the alternator is bad or is there something else I could be missing? Perhaps I did a test wrong?

No tests done wrong that I can see. :blink: I can suggest a couple other quick checks just to get that 110% certainty, but you're pretty much done.

Here's a diagram for reference:



You already know that the L-terminal circuit is working correctly because the battery light comes and goes. But, you can still test this circuit by turning the key on and disconnecting the alternator field connector. The red wire should show 5V on your multimeter.

The F-terminal is a feedback circuit and won't cause the alternator not to charge. Testing this circuit is best done with a scan tool that can read OEM data.

The only other thing I'd suggest is voltage drop checking the case to B- while it's not charging. One lead on the case, the other on B-. Should easily be less than 300mV. If that passes, it's alternator time.

So just to be sure I'm understanding correctly the "Generator turn on signal" is going to the PCM and not coming from it correct?
I did the voltage drop test between battery negative and case ground and it read a few milivolts at most.
I don't have a scantool nor a scope so I have no way of knowing what the PCM is telling it to do, which judging from the schematic if I understand it correctly doesn't tell it to do anything anyway.

Stinky wrote:

So just to be sure I'm understanding correctly the "Generator turn on signal" is going to the PCM and not coming from it correct?

It's kinda a dual function circuit. :silly: The turn on signal actually does come from the PCM. No pulse width fanciness - the PCM just puts 5V on that red wire to tell the alternator to charge. The regulator in the alternator does the rest.

When the regulator detects a failure, it grounds that same 5V, pulling it to 0V. The PCM sees this, sets a fault code and tells the instrument cluster to turn the battery light on.

That's why we can infer that the L-terminal (turn on signal) is likely OK. It's not shorted to ground, otherwise the battery light would be on all the time. It's not open, otherwise the battery light wouldn't come and go as you raise the engine speed. That just leaves a multimeter check.

I did the voltage drop test between battery negative and case ground and it read a few milivolts at most.

Love it.

I don't have a scantool nor a scope so I have no way of knowing what the PCM is telling it to do, which judging from the schematic if I understand it correctly doesn't tell it to do anything anyway.

No worries, the scan tool isn't crucial in this case. Your multimeter will do everything we need.

Good to know So this is the only thing left to be 110% certain, it's a bit hard to get to in it's location but I'll see if I can get it unplugged for testing. (I was struggling yesterday getting at the output on the alternator). Even if I can't test it it's still a pretty good move to go ahead and replace alternator correct?

EDIT: I disconnected the electrical connector from the alternator and probed the red wire as suggested with the key in the on position. It read exactly 4.99 volts.

This good? www.rockauto.com/en/moreinfo.php?pk=1797...1642&pt=2412&jsn=912

Stinky wrote:

EDIT: I disconnected the electrical connector from the alternator and probed the red wire as suggested with the key in the on position. It read exactly 4.99 volts.

Perfect.

This good? www.rockauto.com/en/moreinfo.php?pk=1797...1642&pt=2412&jsn=912

Looks OK? Of the options available on RockAuto, it's a toss up between Delco and Bosch. I've had good luck with both.