Charging system not working under load while the A/C is on

2009 CHEVROLET SUBURBAN
230,000 MILES • 5.3L • V8 • 2WD • AUTOMATIC
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JEREMYH141520
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If I turn off the A/C it will charge fine. If I am cruising under 1,500 RPM's it charges fine with the A/C on. The engine ends up shutting off when you come to a stop. I am assuming because of lack of power. But the battery is fine. It never has a problem starting. Any help would be appreciated.

Thank you.

Jeremy
Sep 1, 2020 at 6:13 AM
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ASEMASTER6371
  • ✓ASE Certified Mechanic
  • 52,796 POSTS
  • ASE Certified Mechanic
Good morning,

What is the voltage at the battery with the engine running and the A/C off?

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Then check it with the A/C on and tell me the reading.

It may be the alternator.

https://www.2carpros.com/articles/how-to-replace-an-alternator

The stalling may be a different issue.

https://www.2carpros.com/articles/stall-at-idle

Roy
Apr 19, 2021 at 10:44 AM
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JEREMYH141520
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The alternator is new. I changed it a few days ago.
Apr 19, 2021 at 10:44 AM
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ASEMASTER6371
  • ✓ASE Certified Mechanic
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  • ASE Certified Mechanic
Okay, good. I still need the voltage numbers I requested.

Roy
Apr 19, 2021 at 10:44 AM
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JEREMYH141520
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14.65 with the A/C off and 14.55 with it on. But now the dash says to service battery charging system. That just started when I turned on the vehicle to check the voltage.
Apr 19, 2021 at 10:44 AM
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ASEMASTER6371
  • ✓ASE Certified Mechanic
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  • ASE Certified Mechanic
Okay, 14.55 is good. All that means is that the alternator is adding more amps to the battery from the load and when that happens the voltage drops a little.

Do you have a scan tool or code reader? See if there is any codes set.

Roy
Apr 19, 2021 at 10:44 AM
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JEREMYH141520
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P0121 is the only stored code.
Apr 19, 2021 at 10:44 AM
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ASEMASTER6371
  • ✓ASE Certified Mechanic
  • 52,796 POSTS
  • ASE Certified Mechanic
Okay, the BCM and ECM work together with the charging system.

I believe the old alternator was your issue. I attached the info on the system.

According to this information, the light should go out on its own.

Roy

SERVICE BATTERY CHARGING SYSTEM

The BCM and the ECM will send a GMLAN message to the DIC for the "service battery charging system" message to be displayed. It is commanded ON when a charging system DTC is a current DTC. The message is turned "off" when the conditions for clearing the DTC have been met.
Apr 19, 2021 at 10:44 AM
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JEREMYH141520
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  • 5 POSTS
Thank you for your time and help.
Apr 19, 2021 at 10:44 AM
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ASEMASTER6371
  • ✓ASE Certified Mechanic
  • 52,796 POSTS
  • ASE Certified Mechanic
You are welcome.

Always glad to help.

Roy
Apr 19, 2021 at 10:44 AM
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