With the engine running the gauge should measure the output of your alternator. The low voltage may be indicative of issues with your alternator. If you have one of those that shows the voltage without the engine running, then check your gauge for a bad resistor on the dash panel. I would definitely start measuring the output there, and make sure that all the ground is properly connected. Just came across this. Seems to be an issue with GM
From GM a notice.> This bulletin is being revised to add the 2009 model year. Please discard Corporate Bulletin Number 02-06-03-008C (Section 06 -- Engine). ________________________________________ Any vehicle may have a low voltage display (if equipped with gauges), lights that dim at stop lights, slow cranking, no start, low generator output at idle or dim lights at idle when electrical loads are heavy at idle or under slow driving or infrequent usage conditions. These characteristics may be more noticeable with customer added electrical accessories, or with a discharged battery. These are normal operating characteristics of a vehicle electrical system and no repairs should be attempted unless a proven fault has been diagnosed. During normal driving conditions, when engine speed is above 1000 RPM, the generator is designed to do two things: • Supply the current necessary to operate the vehicle's originally equipped electrical devices (loads). • Recharge/maintain the battery's state of charge. The following factors may affect generator and battery performance: • Non-usage of the vehicle for extended periods of time. The vehicle's computers, clocks and the like will cause the battery state of charge to drop (For example; 30 days in a parking lot and the vehicle may not start because of a dead battery or a vehicle which is driven only a short distance once a week may end up with a discharged battery to the point where the vehicle may not start). This would be considered abnormal usage of the vehicle and the normally expected result for the vehicle battery, generator and electrical systems. • At idle, vehicle electrical loads may exceed the low speed current (amperage) output of the generator and when this happens the shortfall comes from the battery. This will result in a drop in the electrical system voltage as the battery delivers the additional electrical current to meet the demand. This is equivalent to the brown outs experienced by homes and businesses when the electrical demand is more than the supply. See Figure 1. • Extended periods of engine idling, with high electrical loads, may result in a discharged battery. Attempting to recharge a battery by letting the engine run at idle may not be beneficial unless all electrical loads are turned "OFF". • Increased internal generator temperatures from extended idling can also contribute to lower electrical system voltage. As the generator's internal temperature rises, the generator's output capability is reduced due to increased electrical resistance. The following are some typical examples of electrical loads: System Amperage Load Rear Window Defogger 25 Electric AIR Pump 25 Heated Seats 5 Amps per seat Headlamps (high) 20 Blower Motor (High) 20 Headlamps (Low) 15 Brake Lights 6 Windshield Wipers 6 Ignition 6 Figure 1 Depending on the vehicle application, generator current (amperage) output at engine idle speeds of 600-700 RPM can be as low as 35 percent of the full rated output. With enough electrical loads "ON", it is easy to exceed the generator current (amperage) output when the engine is at an idle of 600-700 RPM. This is a normal condition. The battery supplements for short periods of time. Items that affect the vehicle's electrical system current and voltage at idle are the number of electrical loads being used, including add-on accessories, and extended idle times. When the vehicle speed is above approximately 24 km/h (15 mph), the engine/generator RPM is high enough and the generator current (amperage) output is sufficient to supply the current (amperage) requirements of the vehicle as originally equipped and recharge the battery. Dimming lights at idle may be considered normal for two reasons: 1. As the engine/generator speed changes, so will the current (amperage) output of the generator. As a vehicle slows, engine/generator RPM slows and the current (amperage) output of the generator may not be sufficient to supply the loads, the vehicle system voltage will drop and the lights will dim. Dimming of the lights is an indication that current is being pulled from the battery. If the battery is in a low state-of-charge (discharged condition), the driver will notice a more pronounced dimming than a vehicle with a fully charged battery. 2. When high current loads (blower, rear defogger, headlamps, cooling fan, heated seats, power seats, electric "AIR" pump, or power windows) are operating or cycled "ON", the generator's voltage regulator can delay the rise in output. This effect, usually at lower engine speeds, can take up to ten seconds to ramp up the generator output. This is done to avoid loading the engine severely. To increase current (amperage) output, additional torque is consumed by the generator. The engine computer (ECM/PCM) will ramp up engine/generator speed in small steps so engine speed variations are not noticeable to the driver. For diagnosis of the battery and/or the generator, refer to the appropriate Service Information (SI) or Corporate Bulletin Number 05-06-03-002C. Hope this helps,
This is common on large GM Vehicles: 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 new 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.
My Yukon does the same thing, after a bit it comes back up to just over half way, I like it that way at least I know the voltage is getting regulated like it should be.
Apparently it is to preserve the system over a longer time. This is what GM refers to as the preservation fluctuation
it helps prolong the charging system, dont always have to run 14 volts
Turning on the headlamps sometimes will raise the indicated voltage. Have the alternator tested to be sure it is good.
This was very help because I encountered this last night on my newly purchased Yukon.
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