I have an 06' Trailblazer that I recently purchased. I am troubleshooting a problem with the voltage fluctuating. The gauge will fluctuate from 14+ to about 12v. This problem ONLY happens when the car reaches normal operating temperature. I have tested the voltage from the alternator while the fluctuation is happening and I get an erratic reading of approx 12-13 volts. After some research I thought it was the voltage regulator so I replaced the alternator, no luck. I am now at the point where I have nothing to troubleshoot. I am new here so let me know if any further information is needed.

 

Not so sure these vehicles use a voltage regulator, but rather a module. Voltage regulator would be the first suspect in my mind. It also could be (very) dirty winding in the alternator. Since you replaced the alternator, I'd suspect the BCM.

 

You might check/clean the plug in connection at the alternator