There are a lot of times when you should just leave well enough alone. This sounds like one of them to AAA Car Doctor John Paul:
Q: I purchased a used 2007 Chevy Silverado with a 4.8 liter V-8 engine and I have be watching the charging system voltage. The voltage has been varying between 13.4 volts and 15.2 volts, so thinking it was alternator I replaced it with a remanufactured unit but the problem is still there.
The auto parts store tells me the replacement alternator is performing fine. I have worked on vehicles for years and generally the alternator voltage is consistent and doesn’t vary.
What else might be wrong, voltage regulator, computer wiring or something else?
A: This varying voltage reading is perfectly normal for an alternator that’s part of a “smart charging” system.
In the old days, alternators put out 13.4 volts and stayed there. That was fine when we didn’t care about fuel economy. Now, manufacturers are trying to squeeze every tenth of a mile per gallon out of every vehicle, including light trucks like yours. There’s no sense in putting out 13.4 volts when the system doesn’t really need it.
The voltmeter fluctuating between 12 and 15.5 volts is due to the regulated voltage control (RVC) system. This system reduces the targeted output of the generator to 12.6-13.1 volts when in “fuel economy mode” to improve fuel economy.
An alternator like this can have up to nine charging modes, with voltage ranging anywhere from 11.5 to 15.5 volts. Those modes include:
Battery Sulfation Mode
Fuel Economy Mode
According to General Motors the generator may exit “fuel economy mode” if additional voltage is required, during times of high electrical demand. This will cause the voltmeter to fluctuate between 12 and 15 volts as opposed to non-regulated systems that you are more familiar with that usually maintain a more consistent reading of 14 volts.
Again, according to General Motors this fluctuation with the RVC system is normal system operation and no repairs should be attempted.
Sometimes all of us have to pay to learn new things.
If you’re looking for a full description on GM’s Regulated Voltage Control system you can read this piece in MOTOR magazine.
John Paul is public affairs manager for AAA Southern New England. A certified mechanic, Paul tests dozens of new cars each year and also hosts a radio show on AM 950 and 550.