There are 3 wires on your alternator 1 very big red wire that goes to the back of the alternator with a rubber grommet and a nut holding it on and 2 other wires going into the plug that you confirmed are orange and black . To test you either need a volt meter or a test light attached to a ground. With a volt meter attach it to the positive and negative cable and read voltage . With the car off it should be around 12 volts, now start the car and see what the volts are and if they are not between 12.5-14.5 it's not charging. To test with a volt meter attach 1 end to ground and probe touch probe to big red wire and should be at battery voltage, un-plug connector and with key on the orange wire should have battery voltage. The black wire goes into PCM leave that alone.To test wires without a volt meter and not harming car using a test light. with car off connect the wire to a ground and touch the probe of the tester to the big red wire where it bolts to alternator , does the light come on ? un-plug the 2 wire connector an with key on touch the probe to the orange wire, does the light come on ? That black wire goes to the PCM and is internally, grounded don't do anything to that wire.

 

This problem sounds VERY familiar. I believe a local guy with an '07 Monte SS had this issue. I am pulling from vague memory... I believe on one of the battery cables is a device that is used to control that.

I looked in the GM Shop Book. I believe the attached PDF refers to what my memory is thinking. A Battery Current Sensor. This was introduced to the '06/07 LS4 (never part of the 2005 and earlier).

It's been a long day for me, I am sure there are steps on how to properly diagnose if this part has failed, but in a quick look, I cannot find them. I hope this helps at least provide something else to check that might lead to the solution.

 

Over the past weekend, I got to talk to the 2007 owner I new had a problem like this. He said his symptoms matched exactly to this issue. He mentioned you could unplug part of this current sensor and the charging system would charge the battery again.

 

I know this sensor(the plastic bracket that's supposed to hold it in place is one of the many things that's dry-rotted in my engine bay). I can't get it unplugged now, but if I could, I still don't understand how to unplug only part of it. Any ideas? If I unplug it entirely, might that work?

 

Unfortunately, I have not messed with that sensor. And most/any that I know about it is from a guy that had it fail on his Monte.

I would assume if you disconnect any part of the path to break the connection with the sensor and rest of the car, you would be good. Looking at the PDF I shared, I am guessing connection #2 is easy to reach.

Hopefully that sensor is the solve to your problem and disconnecting it in the short term gets things functional and replacing it gets this 100% resolved.

 

Mystery solved.

I had a mobile mechanic look at it, and the first or second thing he noticed is that the gauge of the disconnected positive battery cable at the battery is different from the gauge of the end at the starter. Turns out there's a splice somewhere in the wire loom which went bad last year, and the cable I removed from the starter and left hanging is actually the cable that goes to the alternator. (The disconnected cable was still hot, which I'm still trying to figure out.) When I reconnected it, problem solved.

I still need to chop out the rest of the old cable, and then start addressing the rest of the issues on the car, but it sure feels great to have it moving again.

 

Now I have a new mystery. The idiot light is coming back on for as long as two minutes at a time, and for as short as ten seconds. The car still runs and drives, and I get none of the other scary error messages I was getting.

Is it possible something's loose? It does seem to happen more at highway speeds.

 

What idiot light is coming on? I still feel like you could have a ground issue, and vibrations from driving might trigger the movement of the wire.

 

The little battery light. The warning comes on the DIC the first time, but not any of the subsequent times.

 

Not knowing/understanding enough about that Battery Current Sensor GM added, I am still wondering if that is related. I would recommend researching that sensor and seeing if there is any diagnostics that can be done to determine if that is the failure point.