I have a 2000 slt with 5.9L. Here is the best description of the symptoms. A couple of weeks ago while driving the check gauges light came on and I noticed the voltage gauge at zero. I turned the car off and restarted it, then everything was fine for a while. Next I noticed that this would only happen when the a/c was on. I took it to auto zone and had them test the battery and the alternator however when they checked it the I couldn't get the darn thing to act up ( it did it about 2miles after I left and they were closed). They said the alternator was fine but the battery was bad. We replaced the battery and now it seems to do it without the a/c on. Usually it has been at slower rpm or speeds like city driving but today it did it on the interstate. Also when the voltage drops below 14 or so, you can hear a whining or squeaking under the hood at first I thought it was the a/c but today it did it with the a/c off. Could this be the alternator?? and auto zone couldn't find it since it is an intermittant problem?? Help!!

 

Yes. It sounds like the voltage regulator inside the alternator. You need to replace the alternator. Get a brand new one not a rebuild.

 

The noise has me leaning towards the alt as well. Can you isolate the noise when it drops below 14 volts.

Also probably not related but see below link. Not the same problems but some similarities.

https://dodgeforum.com/m_871981/tm.htm

 

Voltage regulator in the alt is definitely the culprit, mine was overcharging and autozone couldn't find a problem until I revved the engine to 2k rpms. It showed fine idling.

 

Ihad a similar problem with out the noise. AllI had to do was change the battery terminals!!! The previous owner had changed alternator and battery, the symptoms started 10 miles after I bought her, figure they just gave up and sold the car to me. Fixed it the next day and no issue since.

 

why dont you check the condition of the belt. sounds like the belt is slipping when the a/c adds an extra load. If its slipping it will squeal and it will also not charge the battery.