I had a service charging system message so the dealer replaced the alt. It came back so I had a new red top optima battery installed. It still gave the message so I had the charging system double checked and it was fine so I had my tuner turn off the L terminal code so the message will not display again. Usually it did this at stop lights and the voltage at the time of the message was 13ish

Now I watch my voltmeter more and noticed this:

approx 100 to 105 degrees outside
air cond on full blast
A6 in gear headlights on or off
volts go from 13ish to 12-12.4 at idle
off idle they climb immediately to 13+
cooling fan were on full.
turn off the air and it goes up a little

Is this normal? If I was held to it I would swear it never dipped like this before.

 

Im pretty sure this is normal/common problem.

I had the same problem 3wks after buying my car. Took it in, and they replaced battery. 3 days later the code is popping again. This time they replace my alternator and all wires in between. Buuuut, the corvette tech there said he had ran a relay wire(no idea where to), and my volts never went below 13 with a/c full blast, hids/fogs on and radio going and in florida heat. a6 tranny as well

Had my motor torn out an put everything but the kitchen sink in it. Now when i sit in traffic with lights off, a/c off, radio off, same heat my volts go from 13.6-14.2 down to 11.4.

Already looking into buying a alternator thats more heat resistant, but gonna take it back to the dealer and ask the same tech about the relay wire he put in.

 

so are you saying that this voltage drop at idle happened after the tech placed a relay wire OR after you modded the engine?

I am wondering if the drop I see is what the battery has and that I am not charging at all while idling.

 

The voltage drop happened when it was completely stock....i mean booone stock. When i went in the second time for the alternator/wires, he mentioned hes seen this problem on quite a few C6s/C5s, and every time he does the same "trick", and voltage is never a problem.

I think when the motor was taken out modded then put back in, shop wasnt aware of this "trick", so im back to the code popping.

I shouldnt have a problem getting it fixed again, i know theyll pull the your too modded excuse, but this was a ongoing problem when it was stock. will see when i take it in, otherwise im gonna penny up for the alternator and sell the new stock one.

 

two different problems in my case but maybe one cause--not sure. My message was coming on with the volt meter reading 13 volts so the voltage was not triggering the message.

I read that the alt needs 5v from the ecm to charge at idle. Maybe the tech was getting this from somewhere else?

But I guess you are saying that the volts should not dip to 12-12.2 at idle....correct?

 

No, mines dipping down into low 11s atm. The car should be around 13.1 at minimum.

Once my car sits overnight, the next morning the first 30min my cars voltage wont move to anything under 13. Theres been a couple times though were, i can be driving out on the hwy and soon as i let off the gas *chime* service charging system... ill look down at the volt meter and im at 13.6, so idk wtf is going on.

 

please keep me posted. We both have had the same message regardless of the volts in the "normal" range and both of us are having dipps in the volts at idle BUT mine are not as severe in drop as yours at idle.

The dealership rec a new ECM to me. Maybe we both need this?

BTW has the car left you stranded? How many miles have you been driving with the voltage drops?

 

My volts dip at idle too with the AC on.

Do you know what specific code your tuner turned off? I have HP Tuners but don't know what code to turn off. That loud as# chime is distracting and I'm tired of it dinging every so often.

 

If its chrome I want one.

 

I had a Service charging system message and watched the volts drop from 11.8 down to 11.1 I took it in and they called to say they were replacing the ECM and some BS about if the car had hit the rev limiter the winding could have a problem? They call me back the next day and say they are getting the same code so they were replacing the alt. Got the car back yesterday with no code or message and volts read fine. The tech told me I may still get the message but the voltage is fine for which I replied I would bring the car back again.

 

Joe_G the code is P0621.

Smitter, are you saying that the new ECM did not cure the problem and that a new alt did? If so this is different than my situation since my new alt did nothing to help with getting the SCS message and it seems now the idle voltage drops.

TLewis4095, I would buy one in a heartbeat to if it would fix this problem. Are you saying that heat is killing the alts and the ECM has nothing to do with this?

 

well after more testing this a.m it seems the volts drop the most when the cooling fans are running AND the AC is on(they go to 12). Otherwise things seem fine. Maybe this is normal and I just noticed it.

 

Tlewis,

thanks for the help. However, I am skeptical since my message popped up and the voltage was 13+ so it doesnt seem that the alt was the source of the problem.

In this thread LS2Powa states earlier he is driving down the freeway and lets off the gas and gets the message and volts are fine too.

I am not trying to hammer you TLewis just want to get an explanation of why.

 

Here is how you test your charging system. For that code it is either the alternator, ECM, sense circuit or turn on circuit problem. These checks will show which one. That code triggers if the system voltage drops below 11 or above 16. I wouldn't rely on the DIC voltage readings, go by what these tests show.

Get the motor nice and warm, take a nice long drive.

Ignition OFF, disconnect the harness connector at the alternator.

Ignition OFF, test for B+ (>12V) between the alternator sense circuit terminal D and ground.

If less than Battery Voltage, test the alternator sense circuit ( term D) for a short to ground, an open/high resistance.

Ignition ON, test for less than 1 volt between the alternator turn on signal terminal B and ground.

If greater than 1 volt, test the alternator turn on signal circuit (term B)for a short to voltage. If the circuit tests normal, replace the ECM.

Engine running, test for greater than 3.5 volts between the alternator turn on signal (term B) and ground.

If less than 3.5 volts, test the alternator turn on signal circuit (term B) for a short to ground, an open/high resistance. If the circuit tests normal, replace the ECM.

If the circuit tests normal, replace the alternator.





These tests check the circuits and ECM functions that are involved with that code and setting the Service Charging System message.
If these all check out, it's an alternator output problem.

 

As long as you don't have any excessive aftermarket accessories installed, IE stereo equipment, you should not have to add any kind of high output alt. . Otherwise, we would be changing alts. left and right. The voltage reading will change up and down much more so then in the past. The ECM is controlling alt. output, and at times will just about turn off the alt. .

There is a bulletin out please read:
Service Charging System Message on DIC and DTC P0621


Models: 2005-2008 Chevrolet Corvette
2004-2008 XLR and XLR-V Series




--------------------------------------------------------------------------------

This PI was superceded to add information about aftermarket spark plugs. Please discard PIC4536A.


--------------------------------------------------------------------------------

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:
Some customers may comment that the Service Charging System message displays on the DIC. Using the Tech2, DTC P0621 is stored. This may be a result of aftermarket spark plugs or an open in circuit number 2540.

Recommendation/Instructions:
1. Check for aftermarket spark plugs. Plugs that are designed with a "Pulse" technology may interfere with this circuit and cause the Service Charging System message to illuminate on the IP and/or P0621 DTC store in the ECM. If aftermarket plugs are installed, replace plugs with OEM parts. This is not considered a warranty repair and all cost associated with this repair will be the customers responsibility.

2. Circuit 2540 is supplied voltage from the Horn fuse, position #2 in the UBEC. Apply the horn to see if it works. If the horn does not work, suspect an open fuse. This fuse may open if the horn is shorted due to water intrusion. If the horn is functional, check the integrity of circuit 2540 between the UBEC and the generator. Refer to SI document 1481125 for the schematic. Repair circuit 2540 as needed and re-evaluate the customers concern.

 

I'll bet! Are the A/C compressors are a close #2. Seriously, we haven't needed to replace the Gen. all that often. Your heat in FL. I am sure has something to do with it. I wonder if the Chrome would be such a good Idea??

 

based on all the facts and that the car messaged literally 10 minuted after the new alt was placed, I am going to have the ECM replaced then go from there.

The HO alt from revextreme is a nice thing though. I am considering it also. Figured it cant hurt.

 

Thanks knkali, sure enough in HP Tuners P0621 is "Generator L-Terminal" and the stock tune has an error reported on second occurence of the error, with no MIL (check engine light). I can set it to "no error reported" and see what happens.

Anyone else try this? Did it work? Any downside?

 

I had the code disabled and it works, but our cars are probably telling us something. I noticed that my car was not charging at idle so watch your volt guage esp when there is a large load on the charging system. IOW while at idle in gear with the ac on and the cooling fans running full. It migh take a minute or two but test your system to see if your volts start to drop. Mine went to 12 v then would slowly go down from there.

 

Good call. I notice that when I first start the car, if I don't rev it a little, the volts will be 12 or less, and if I don't rev it backing out of the garage, it'll ding by the time I hit the street. I think you have to rev it a little to get the charging system working. And at idle with AC full the volts do get a little low sometimes.

What I didn't like is the LOUD DING service charging system while driving down the road at 45 mph...I'd check the volts instantly and all would be fine - I ran with the volts showing on the DIC and they always read fine...

Someone told me to replace my battery - it's 3 years old now and my C5's died in 3 years so maybe that's the best plan.