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C6 Service Charging Error/Battery down when Idling
1) Bought my 2010 C6 4 years ago and I've always experienced problems with the battery, whether needing a terminal extension for the positive side so it could reach the battery or needing a new battery and alternator.
2) Few weeks ago the car sounded like it was hesitating to start; had battery checked at Autozone, it was bad. Bought the Optima Yellowtop Battery. Car ran fine for a week.
3) Started hesitating to start again & I noticed when idling, the battery would lose amps. Autozone checked battery and alternator; the check showed bad alternator. Bought the Duralast Gold alternator. Autozone also said the starter was good.
4) Now when I'm idling at a light or a drive through the battery starts to loose amps. It's never died, but gotten below 10 & I get the "service charging error".
~The Onstar fuse has been removed.
~The 2 grounds under the battery were corroded so I cleaned those. I included before & after pics.
It's STILL giving me the service charging error and battery goes down after idling. I'm at a loss here!
First thing is you don't lose "amps"...you mean voltage...do you have a DMM and know how to use it ??...you need to voltage drop the charging system which takes a few minutes...if you're not mechanically inclined seek out an automotive electrical shop in your area and what are those terminal pictures you posted ??...below is voltage drop values on the positive (.32 volts) and negative (.10 volts).
The pics are of the 2 ground points under the battery.
What is that gadget called to drop the voltage drop the charging system?
Originally Posted by C5 Diag
First thing is you don't lose "amps"...you mean voltage...do you have a DMM and know how to use it ??...you need to voltage drop the charging system which takes a few minutes...if you're not mechanically inclined seek out an automotive electrical shop in your area and what are those terminal pictures you posted ??...below is voltage drop values on the positive (.32 volts) and negative (.10 volts).
Your issue has nothing to do with ground points...it is a fancy multi meter I have and I'm thinking you may not have one...just read your profile and I'm thinking an electrical specialist may be your best bet in fixing this.
Hi, I do have a multi meter, just did the voltage drop test & the volts of the battery are 12.1 (red probe on positive terminal and black lead on negative terminal) , there is no voltage drop (both probes on negative terminal, reads 0).
The reason I checked the grounds under the battery is because I saw another post where that solved the same problem (cleaning the corroded grounds).
Originally Posted by C5 Diag
Your issue has nothing to do with ground points...it is a fancy multi meter I have and I'm thinking you may not have one...just read your profile and I'm thinking an electrical specialist may be your best bet in fixing this.
Last edited by dspanier; Jun 18, 2026 at 07:53 PM.
No, voltage drop is placing one lead on battery positive and other lead on the alternator B+ cable and reading the drop...checking negative side is placing one lead on battery negative and the other lead on the alternator CASE...should be less than 0.5 volts on either side...don't worry if DMM shows a negative sign...just read the number...those grounds have nothing to do with that...with car OFF a fully charged lead acid battery should read 12.66 volts...an AGM close to 12.8 to 13.0 volts...BTW the car must be RUNNING to do this voltage drop test and with some electrical loads on the car...I run the car around 2000 RPM.
I did the test again & included pics. Did battery black to alternator case 0, battery red to alternator wire 0, and battery black to alternator wire 12.1, and voltage of battery w/car running was 12.1.
The battery gage on dash was very low when I started the car. After running about 10 minutes it went up a little.
BTW, thank you for your help.
Last edited by dspanier; Jun 18, 2026 at 09:11 PM.
OK, first thing I noticed is 0 volts on the drop which is impossible...my connections are super clean and I have .10 and .32 voltage drop...the alternator case measurement must be done on a metal section on the case...yours looks like it's on plastic...2nd where on the alternator cable are you taking your measurement ??...on my picture take it on the stud and not on the nut...and is the car RUNNING when you're taking these measurements ??...is the 12.1 measurement with the car running ??...if so your alternator looks like it's bad...I never go by voltage gauge measurements...only with a voltmeter.
Yes, the car was running. Before I started it, battery measured 11.7 volts.
I redid what you said w/alt metal part of case & stud. For the heck of it, I also did the battery black to Alt stud since I was getting 0 again on the Alt metal part of case.
I think I definitely need to go back to Autozone. I just bought the battery & alternator within the past month, close to $900 for both.
Thank you again for all of your help. This is so frustrating.
Looks like your alternator is only putting out 4.2 volts...before I condemn an alternator I check the wiring connector and wiring but that may be over your head...your battery voltage should be in the 13's !!...your drop on the positive side is 6.6 volts and if that is paint or whatever on your alternator case you need to get to CLEAN metal !!...at the starter solenoid the battery positive cable and alternator cable are connected together there and if loose or corroded the alternator will not charge the battery...you got a couple things going on there...let an automotive electrical shop handle this !!
What alternator did you install?
I would fully charge battery, start engine and have assistant rev engine to 1500rpm and then measure voltage again at alternator terminal.
Some of the fancy upgraded alternators require higher rpm to put out charge.
The other thing is the starter solenoid cable needs to be properly torqued to get proper contact.
If I had to remove the starter I would replace with new starter/solenoid and be done with it.
Labor is expensive and an old starter/solenoid is a known weak spot for GM.
Catalytic converters 1 inch from starter has slowly cooked that electrical part.
One thing I noticed was when the car is running, the volts would go down, but when the car is tuned off the volts would go back up. The car finally stalled on me when I took it back to Autozone to check the battery & alternator again, but the battery was too low to check the alternator. Had my son jump me with his truck & he noticed the jumper cables got way too hot. I had Autozone recharge the battery & then their test said the alternator & starter are bad.
I'm going to get a starter tomorrow and alternator too.
Thank you!
QUOTE=Z51Madison;1609718333]What alternator did you install?
I would fully charge battery, start engine and have assistant rev engine to 1500rpm and then measure voltage again at alternator terminal.
Some of the fancy upgraded alternators require higher rpm to put out charge.
The other thing is the starter solenoid cable needs to be properly torqued to get proper contact.
If I had to remove the starter I would replace with new starter/solenoid and be done with it.
Labor is expensive and an old starter/solenoid is a known weak spot for GM.
Catalytic converters 1 inch from starter has slowly cooked that electrical part.[/QUOTE]
What alternator did you install?
I would fully charge battery, start engine and have assistant rev engine to 1500rpm and then measure voltage again at alternator terminal.
Some of the fancy upgraded alternators require higher rpm to put out charge.
The other thing is the starter solenoid cable needs to be properly torqued to get proper contact.
If I had to remove the starter I would replace with new starter/solenoid and be done with it.
Labor is expensive and an old starter/solenoid is a known weak spot for GM.
Catalytic converters 1 inch from starter has slowly cooked that electrical part.
To recap --> 1) You bought this car 4 years ago and have always experienced problems with the battery, 2) a few weeks ago you threw in a non standard battery, 3) a week or so later you threw an alternator at it. Now you're saying the new battery and new alternator are bad in addition to the starter being bad. No stock C6 ever needed a terminal extension for the positive side so it could reach the battery if the proper parts are being installed.
IMO you are just throwing money at a problem that is beyond fixing in the Autozone parking lot. Heed the advice of C5 Diag and seek out an automotive electrical shop and let them fix this car correctly.
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C6 Service Charging Error/Battery down when Idling
