P0621.. Help please. I have installed 2 new batteries and 2 new reman alts. Still not charging and the battery light is still on. Can anyone lead me to finding the issue here? This is on a 2013 Chevy Express duramax Ambulance. Thanks
I am also having there very same issue with one of our 2006 Chevy Express 6.6L ..
I'm working on a 2009 Express duramax ambulance as well, with dual alt setup. P0621.. but charging voltage is 13.3V - battery light on - Key on, engine off and key on, engine Running only 2.9 volts running to the L (lamp) circuit. Tested wiring from alternator connectors to ECM with good readings (0.4 ohms). Going to finish the diag Monday morning will keep you posted
Brian did you end up finding your problem? I noticed your post is from a year ago now...
Primary (passenger side) alternator was not pulling to ground on signal circuit (orange wire) key ON engine OFF. Replaced alternator and re-test - good now
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