Was able to follow the SI troubleshooting steps today for the P0622 and the results are pointing to a faulty Alternator.

But decided to check the SOC of the battery and found it to be low, so low that it failed a battery test. Its on the charger over night and will perform another battery test to see if it will pass. Its a 760 CCA, AGM battery that was at 12.2 volts (30% charge) and ended up at 460 CCA under load with a load tester, I'll see how it does in the morning after being fully charged.

Neal

 

To the OP,
the charging system can have up to 9 charging modes, 1 being startup mode which will at startup force the charging system to ignore the inputs form the BCM and PCM and force 14.8 volts for a given time to recover the SOC of the battery after startup, this may be what your seeing with your truck.

These videos shed some light on the strategies of GM to make the RVC system work;
in the videos they explain the 9 different modes that can be programed into the charging system

Neal

 

this one includes the 9 modes of charging;

Neal

 

GM RVC Voltage Drop (F)

 

 

Battery could be going bad. Battery could be taking charge when cold and tripping bad cell once it’s hot. Similarly with the alternator.

Did you try to swap the batteries and see if the problem resolved? Before doing anything else? The trick with diagnosis is to try the simplest stuff first. The stuff that doesn’t cost you any money.

 

GM RVC Regulators Explained:
The GM regulated voltage control system can vary from 11.5 –15.5 volts. As with other systems in use today, the GM system has a fuel economy mode, which is designed to lower the charging system voltage by reducing the alternator field strength. (Technicians often diagnose this mode as a faulty charging system, only to find out that the vehicle operates the same even with new parts installed)
Simply applying maximum load to the vehicle’s electrical system will cause the PCM to respond, raising charging system voltage. GM also designed this system to identify battery sulfation. In response, the PCM will command the charging system’s voltages to increase to as high as 15.5 volts, even when there’s no electrical load. Sulfation mode will typically last less than five minutes at a time, so, as the timer expires, the system will return to normal charging operation.
The BCM is the brains of the operation, but the PCM is the module that actually controls charging system operation. The PCM controls the signal to terminal L of the alternator to control system output. Terminal L is designed to control the charging system. Terminal L feeds the signal to a regulator inside the alternator. The PCM sends five volts to alternator terminal L. When the system requires an increase in output, the PCM will change the circuit duty cycle, which will cause the regulator to change the voltage set point. The duty cycle can range from 10% to as high as 90%, with the higher duty cycle creating a higher voltage charging rate. If an open circuit occurs, the system will default to a charging voltage of 13.2–13.8V.
Terminal F —> is a duty cycle signal that the PCM monitors. The duty cycle percentage represents the operation of the alternator field. The PCM monitors the duty cycle to determine the load the alternator is placing on the engine. Then use this input for idle speed control and the alternator voltage set point.

 

if you have aftermarket audio with some high wattage amps it can also cause the alternator to bump up the charge rate.

 

If you remove the gray(GRY) wire from the back of the alternator, the pcm loses control.

 

after testing the circuit (per the SI) and everything checked out good. I load tested the battery and it checked out good so I pulled the battery and the alternator and had them tested at the local Advance Auto, the alternator failed.

So now will install a new alternator and see how that goes.

Haven't heard back from the OP so not sure if any of this has helped him with his truck issue.

Hopefully he will post back with the fix on his truck,

Neal