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Alt voltage fluctuating

Mossyoakglock

By Mossyoakglock
in Ask A GM Technician

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Mossyoakglock Senior Enthusiast

Mossyoakglock

Posted
Posted

Is it normal for voltage to fluctuate between 12.5 and 15.5 volts while either driving or idling? I confirmed with a multimeter and advance tested as well.

 

I first noticed the gauge was way below 14 and on the same trip was way above 14. It would be right on 14 for a while while driving too.

 

Battery is a week old and tested good.

Zane Senior Enthusiast

Zane

Posted
Posted

Yes! It's a well documented feature that the alternator changes its output based on the need. This is something I noticed and had to search about too for our 2011 :cheers:

 

From a TSB:

Voltmeter Fluctuation Condition
Some customers may comment that the voltmeter is fluctuating between 12 and 14 volts on their full size pickup or utility vehicle. Starting with the 2005 model year, light duty full size pickups and utilities are equipped with a 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. The generator may exit "Fuel Economy Mode" if additional voltage is required. This will cause the voltmeter to fluctuate between 12 and 14 volts as opposed to non-regulated systems that usually maintain a more consistent reading of 14 volts. This fluctuation with the RVC system is normal system operation and NO repairs should be attempted.

See this thread for more details: http://www.gm-trucks.com/forums/topic/125031-voltage-fluctuation/

Mossyoakglock Senior Enthusiast

Mossyoakglock

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  • Author
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Thanks. It doesn't say anything about going up to 15 - 15.5 volts though, which is what my truck is doing the majority of the time.

 

Nevermind. Read the whole TSB. My goes up to 15.5 but I didn't have the lights on or the truck in Tow/Haul mode. It's weird because it has never done this since I bought it new until I replaced the battery last week. I didn't get any DIC messages or anything about the charging system though.

Mossyoakglock Senior Enthusiast

Mossyoakglock

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If for whatever reason the alternator is actually bad, I'm not sure which alt I need. Mine is stamped D44 with part number 15905872 145 A. When I search on Rockauto the alt for the KG3 RPO is now stamped R37 and the 160 amp is D44.

 

Did they change the D44/D37 designation and I need the D37 one now? I found that the pulleys are different between the two as well.

Mossyoakglock Senior Enthusiast

Mossyoakglock

Posted
  • Author
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I'm trying to find the specs on both because I think the D44 160 amp has a different size pulley and bigger case compared to the D37 145 amp. Not sure if it will work.

 

Did the 37 replace the 44 145 amp?

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

Take your VIN and old alternator into the place you are going to get replacement alt from. Ask for the highest amperage alternator that will fit where the old alt came from. Different size pulley will change alt rpm in relation to engine rpm. Larger pulley, slower rpm.

Generally you won't see rapid voltage changes unless battery is completely charged. The battery is used as a buffer or conditioner for voltage spikes. Sort of like a shock absorber. According to the printed books I have from trade school back in late 70's, a 12 volt auto battery is actually a 13.2volt battery(6 cells, each cell 2.2v). Charging voltage must be higher than 13.2v to charge the battery. Once it gets past 15.3, it is heading into overcharging land. I think the newer sealed batteries handle the 15.3+ volts better than older removable cap lead acid batteries did.

 

Looking online now, I see they have either changed the data, or typical battery construction has changed and now each cell makes 2.1v, making a 6 cell battery be 12.6v. Perhaps AGM batteries are producing 2.1v/cell.

 

I would not be surprised that the volt meter behaved differently after changing battery, One of the main functions of the battery is to smooth out the entire charging system. If battery is fully charged, you would see more rapid change of voltmeter, as compared to discharged battery.

 

Generally if you are actually getting voltage that high for any length of time, you will burn out bulbs. In my experience, front park lights, headlights, and signal bulbs are the first to burn out due to overcharging. Also, if overcharging, you will likely hear a whining noise in stereo that is related to engine speed, higher speed, higher frequency of whine.

Mossyoakglock Senior Enthusiast

Mossyoakglock

Posted
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I'm not getting any symptoms that you mentioned. I'm not getting any charging system warnings on the truck so I assume its working properly. I'll just keep an eye on it for a week.

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