P0621 dtc

Hi. My client's van is a 2008 Chevrolet express 1500. V6 4.3 liters. His complaint: battery light is on at high RPMs. He replaced the alternator twice, he still has the problem. I used my scan tool and got the P0621 dtc ONLY when the engine is at high rpms, at idle no dtc and the charge system works perfectly. I already checked for a short to ground or a short to positive right at the L wire of the alternator regulator, no issues. Checked voltage at the L wire with KOEO. 0 VOLTS, with KOER at idle 5 volts, at high rpms 1.3 volts. I cut the alternator regulator L wire and put it away, then I ran a wire directly from the pcm regulator output to the alt, my problem is still there. My question: Is the pcm the cause of the failure or is there something else I haven't done so far? Wiring diagram attached. Thanks for your help.

how high of rpm?? w.0.t? try wiggling the alternator connector?

Thanks for including the diagram, much appreciated.

The way I understand it, this code will set if the PCM sees high voltage on the L circuit (5V) KOEO, or low voltage KOER. The latter sound like your case.

Honestly, it sounds like the replacement alternator is suspect. Aside from rechecking voltage drops at the B+ cable and case ground at high RPM, and belt tension (possible slipping belt), I'd go with an OE alternator.

The only other test I can think of would be to disconnect the alternator and monitor the L terminal at high RPM. If it doesn't pull down, then the issue was in the alternator.

Thanks a lot for your message. I already checked the wiring integrity, the harness and connector are ok. At 3500 rpm the voltage drops from 5 to 1 volt

Thank you very much for your help. This dtc was set with the original alternator then the owner told me that he replaced it with a new one that fits perfectly but I don't know if it is a genuine chevy part. A will follow your advice and install another OE alternator. Voltage drop checks were done, test OK. In my opinion the failure is inside the pcm but I am not certain. Thanks again.

charless01 wrote: Thank you very much for your help. This dtc was set with the original alternator then the owner told me that he replaced it with a new one that fits perfectly but I don't know if it is a genuine chevy part. A will follow your advice and install another OE alternator. Voltage drop checks were done, test OK. In my opinion the failure is inside the pcm but I am not certain. Thanks again.

You might be right! :lol: I'm eager to see the result of this one, as I've never heard of an L-terminal problem like this one before.

I figured that raising the engine speed with the alternator disconnected might be a good test of the PCM, just to see if something is feeding back and causing a circuit issue. It's an open circuit check, though.

Thought about a belt slip problem because I've seen belts spin on pulleys and not make noise. :huh: The regulator in the alternator will pick up on this and try to ground the L-terminal.

Tomorrow I'll recheck the belt operation and do the test with the connector unplugged. I will let you know as soon as I have any repair progress.

Repair update: I took the alternator to a test bench. It failed at high rpms, . Tomorrow I will be installing a new OE alternator. I don't know what happened with the last alternator, it looks new but doesn't work at high rpm. Thanks again for your help

Aftermarket parts quality. Let me know how the new-new alternator works out!