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Running at 15 volts


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My 2012 Sierra needed to be jumped recently from my wife running heated seats when ignition was in aux position. Started being slow to start this past week in the cold. Noticed 15 volts on the voltmeter when it always use to be 14. Battery was original so I changed it out. Starts great now but still getting 15 volts. Just to confirm it's accuracy, I had a mechanic (guy who is really just a home mechanic but works on the trucks at our small trucking company) hook up a bolt meter to it and got 15 as well. He says must be alternator but i online I'm finding varying opinions. Don't want to waste money at the dealer to get it diagnosed if it's not an issue. Would rather spend that money getting a new alternator if needed.

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07 and up have "smart" charging systems on them. I would say that your original battery being weak caused the charging system to go into charge scheduling and bump the voltage up. If you want to, I would maybe do an alternator output test to see if its working more than it should, and possibly is faulty.

 

#07-06-03-009D: Information on Voltmeter Gauge Fluctuation Due to Regulated Voltage Control System and Concerns About Camper/Trailer Battery Charging - (Jan 16, 2015)

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.

Camper/Trailer Battery Charging Concerns

Some customers may comment that when towing or hauling a camper/trailer, the auxiliary battery for the camper/trailer will not stay charged. In most cases, this concern is blamed on the new RVC system. While the RVC system does reduce the generator's targeted output voltage to 12.6–13.1 volts when in “Fuel Economy Mode”, this feature is bypassed if the tow/haul feature is enabled. With the tow/haul feature enabled, the RVC system will stay in “Charge Mode” and the targeted generator output voltage will be 13.9–15.5 volts, depending on the battery state of charge and the estimated battery temperature. To keep the generator in the “Charge Mode”, use either of the following two methods.

  • The first method is to use the tow/haul mode when towing or hauling a camper or trailer.
  • The second method is to turn on the headlights, which will increase the generator's targeted output voltage to 13.9–14.5 volts.

 

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New battery not fully charged. Normal with today's maintenance free battery's.

My '11 Tahoe can sit in the garage for a week or two at a time so it's on a battery maintainer and since doing that I no longer have the high charge readings. Voltage drops down to just under 14v after driving a while now.

Work Chevy van does the same, will be high voltage all day from multi restarts and drops down on the ride home.

This is temperature related also, colder temp higher voltage. 45 deg and up you'll see voltage drop faster.

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