Today, a 2007 Silverado came in with a complaint of "Service Charging System" message comes on at idle at times. It did not come on for me after a 10 minute idle period. Charge rate was 13.85 volts after the long idle period. Scanning the PCM, P0622 was set as history. I unplugged the two wire connector and probed the F terminal with a test light to B+. The F terminal signal on the scanner went from 0% to 97%.

This would seem to indicate an alternator problem. How best to easily confirm it? I graphed the L terminal and F terminal scan data and saw this...

F and L terminal

The L terminal command is fine but the feedback from the F terminal is all over the place. It should never reach 0% with a running engine. I replaced the alternator and rescanned the terminals.

F and L terminals

Looks a bit better now, doesn't it?

James from Florida

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