DocSki808

Hello everyone,

I own a 2006 C6, which has been having some alternator issues for a while now and I am at a loss for what could be causing it. No issues while driving her other than battery drain (not charging). Headlights do not dim when turning vehicle on, nor do cabin lights. I posted here before, this is more of a comprehensive post after a while of troubleshooting, thank you all in advance for your help.

1st Issue:
Car is having issues starting and staying above 11v-11.5v (standard battery)
- Replaced Alternator from high output to an Original Duralast Alternator
- Replaced Starter
- Replaced Battery to d34 Yellow top (installed in backwards due to pos/neg being on opposite sides for this one.)

Everything worked great after this was fixed for roughly 4-5 months.

Next issue:
After fueling up one morning around 9 am the car was sitting at 13.5v as usual. Come lunch time around 11:30 when I started the car, it was instantly down to 11.5v.
- Put a charger on the battery rest of day into the night, removed around 10pm that night. When removed the battery was at 13.5v. Come morning, go to start her up and back to 11.5v.
- Put charger on her again, same issue, back down to 11.0-11.5. Driving her she will get up to maybe around 12.0 then drop back down to 11v-11.5v.


I am currently at a loss as to what could be causing the issue. There is no aftermarket sound system in the vehicle. All current work done on her is COMP CAMS, headers, the exhaust, internals and she has a deep loping idle.

Here is what else I have done to diagnose issue.

- Tried installing additional ground cables (4ga) off of the battery (2 additional, upgraded the old one) just in case battery wasn't properly grounded).
- cleaned off all grounds I could find, replaced battery cable terminals.
- removed the ONSTAR fuse.
- cleaned the throttle plate of all carbon deposite.
- had an alternator test done on it by auto store, battery is still good, alternator is charging at only 12.06v.

- conducted a voltage drop test with these results
- alternator housing to negative battery terminal = 0.00v
- alternator bracket to negative battery terminal = 0.00v
- alternator B+ bolt to positive battery terminal =3.73v
- alternator housing to positive battery terminal = 11.73v
- negative terminal to engine bolt = -0.01 to -0.03

With car off after car charger used to charge to 12v (generic car charger) -

Battery at 12.10v car off and holding down accessory button for 10 sec mode w/headlights on
- 12.19v headlights off

Car on
Headlights off - 12.01
Headlights on - 11.93
- stereo on - 11.93
- heater on - 11.89
- seat warmers - 11.84

All was measured using a multimeter, during this time DIC was reading
11.8 at car on
11.6 with all accessories on
11.3 at end of all testing (20-30 minutes running)

I did notice that part way through testing with all accessories on, car sounded like it was about to die or struggling to stay running, when I turned off accessories it returned to normal, then did it again a little later with no accessories running, I want to chalk this up to it just being low battery but not sure. It doesn't make sense that my car should be eating alternators this often like this, any ideas would be greatly appreciated.