Possible failing alternator???

[firstdue]

Ok, I had the opportunity to run the G8 today on an hour drive to Norwalk for the Pontiac Nationals. I have the Atari gauges and to tell the truth I really don't watch them to closely. I did notice on the trip there that my battery voltage was above 14. Upon startup to leave I noticed it was at 12.9 and stayed at 12.9 for about the 1st 15 minutes of my drive....then it climbed to 14.7 for about 5-10 minutes and then dropped again to 12.9. When I got home and parked it...I restarted in engineering mode and the battery was at 88.7 and jumped to 90% after a couple minutes at 14.7V. I know the alternator is 9 years old...but I'm only at 8000 miles.

Is the voltage climbing and dropping normal? Anything else I can watch for?

I don't want to get stranded somewhere.

Thanks!!

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[G8eraid]

The G8 alternator is PWM controlled, so the fluctuations are normal. If engineering mode is showing 80+% charge that's also a good sign. My alternator just went out recently, started making a high pitched whining sound which got louder when ac was on or rpm increased. Engineering showed voltage at idle between 12.0-13v fluctuating very fast(unsteady) Battery saver came on eventually along w/alt light...

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[WhatNext]

When actively charging the battery, I've seen it as high as 15.1 V but dials itself back to 12 something after the battery is charged up.

[firstdue]

Thanks guys!!

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[Napalm]

Question did you pull the car off a maintenance charger before you started up? or say recently.

As said above alternator varies the voltage and current output via a field command voltage as commanded by the Body control module (I believe or the ECU). Anyway if your battery is in good nick and the intitial charge is floating above 12 - then the output will be fairly low as it assumes the battery doesn't need much.

Later in your drive the battery dipped so it kicked up the output. I've seen mine go as high as 15 something for a few minutes when my car has sat undriven for a few days. Similar thing start up will be around 13 and change - then it ramps up for a bit before drifting back to 14 ish and then down in the 13s.

AND oddly enough - even now with weekly commute when I get to friday's drive home it might show as low as 12.5. And I just replaced my alternator because the rectifer went wonky. New one is doing the same thing. again - commanded by the computer.

[firstdue]

I usually do have it on a maintenance charger, but its probably been off of it for 1-2 months.

Thanks all for the info!!

[LDM]

The charging system has different modes depending on the conditions it sees. From a document Crazy Paul posted a few years back:

Charging System Operation Modes
There are four main EPM charging modes that are available.

• Idle Boost Mode
• Load Shed Mode
• Regulated Voltage Control (RVC) Mode
• Fuel Economy Mode

Idle Boost Mode:
At idle, if the battery voltage is lower than a particular threshold and if a high current draw is detected, the EPM will command an increase in idle speed to support the vehicle load and to avoid further discharge of the battery. There are 3 levels of Idle Boost depending on the severity of the situation.

Load Shed Mode:
Load Shed Mode is requested when the vehicle loads are still drawing too much current from the battery even when Idle Boost Mode is active. EPM would send out a request message via GMLAN communication to the various controllers that control loads capable of being shed. Similarly, there are 3 levels of Load Shedding depending on the load levels.

Regulated Voltage Control (RVC) Mode:
In RVC Mode, the EPM system determines the optimum charging voltage, based on estimates of its SOC and battery electrolyte temperature. On starting the engine, the BCM sets a higher alternator output voltage for a short period of time for the system to recover the charge lost during crank. The BCM will enter Headlamp Mode whenever the headlamps are ON (high or low beams). Voltage regulated to 13.9–14.5 volts. This voltage maximises globe life; conventional globes run best at a slightly lower voltage than required by calcium batteries.

Fuel Economy Mode:
The fuel economy mode is an algorithm that allows a fast battery voltage reduction after engine crank and during vehicle driving conditions. The BCM enters Fuel Economy Mode when ALL the following conditions are set:

• The estimated battery temperature is between 0°C (32°F) and 70°C (158°F)
• The calculated battery current is less than +15 amps and greater than −8 amps
• The battery state of charge (SOC) is greater than or equal to 80 percent.
• Vehicle speed is less than 145 kph.

Its targeted alternator output voltage is the open circuit voltage of the battery and can be between 12.5–13.1 volts. The BCM will exit this mode and enter normal RVC Mode when any of the conditions described above are present.

Sounds like you just went into Fuel Economy Mode. It freaked me out the first time I noticed it too.