RVC Redux
The July Trouble Shooter column on charging system problems on a 2007 Chevy Silverado generated some interest among Motor readers, and prompted the following insights from Mohammad Samii, owner of Sammy’s Auto Electric Service, Inc., in Champaign, IL. Samii has written the “Auto Electric Corner” column for the Automotive Parts Rebuilders Association’sAPRA Global Connectionfor more than 20 years, and shares his knowledge as a training instructor. Here are his comments:
I’d like to add some information to your discussion of a charging problem on a GM regulator voltage controlled (RVC) charging system. In an RVC system, the PCM constantly supplies a 128Hz signal of 5V amplitude to the L terminal. The PCM changes the pulse width of this signal when the system goes into a different mode of operation (there are seven or eight different modes). Changing the pulse width causes the voltage regulator to change its voltage setting, bringing the generator’s charging rate close to what the PCM has ordered. When an RVC system is being tested in a shop, you would rarely see a mode change, and the average voltage on the L terminal measured by a DMM would be around 3.5V.
Activation of an RVC generator does not require a test light connected between the B+ and the L terminal. A self-exciting feature is built into the system and the generator will work (albeit in default mode) when the engine is revved up, even if there is no connector attached to the generator except the output wire. This is one of the features we must verify on the test bench when an RVC generator is rebuilt and being tested.
The RVC generator’s default mode has another feature. If an anomaly is detected in the system, the generator will disregard the L terminal input and will turn itself on in a default mode with a 13.8V setting on the regulator. This feature is designed to ensure the continued operation of the generator and charging system if there is a failure in any of the associated modules that participate and control the RVC charging system. Of course, the dashboard warning light would be on if the generator is running in default mode, to indicate a problem that needs to be addressed as soon as possible.
The reader’s problem in your July column was most likely an example of this type of failure. Loss of “F” created a DTC P0622, the system went into default, the charging voltage stayed around 13.8V and the charging system warning light started to come on.
Thanks very much for writing, Mohammad. Your note indicated to me that there’s quite a bit more to be said about the RVC charging system than what was included in the July column. So perhaps it’s best to begin at the beginning, with a description of the RVC system, its purpose and how it’s supposed to work.
Conventional charging systems use an internal temperature sensor to establish generator voltage set points. When the generator is cold, it raises the voltage output set point. When it’s hot, it lowers the voltage output set point. This type of system tends to overcharge the battery on long trips at highway speeds and undercharge it on short trips with low vehicle speeds. Regulated voltage control (RVC) regulates the generator’s output voltage, based mainly on estimated battery temperature and battery state-of-charge (SOC). The main benefits of this system are: improved fuel economy, extended battery life, extended lamp life and extended switch life.
The RVC system allows voltage changes, up or down, based on battery state-of-charge, the vehicle’s present electrical needs and other factors, so the voltmeter reading may fluctuate. Conventional systems usually maintain a consistent reading of around 14V. The RVC system voltage may range between 12 and 14V. This is normal system operation, but may be perceived as a problem by vehicle owners who are accustomed to seeing a relatively consistent voltmeter reading.
There are two types of RVC systems in use—integrated RVC and stand-alone RVC (SARVC). Integrated systems use a battery current sensor to inform a body control module (BCM) how much the battery is being charged or discharged.
SARVC systems (found mostly on trucks) do not use the BCM for operation. They have a generator battery control module mounted to the negative battery cable, to interpret battery current and voltage and battery temperature inputs. The battery current sensor is internal to the module. This module also directly controls the generator L terminal duty cycle instead of the ECM/PCM.
The purpose of the RVC system is to maintain the battery state-of-charge at 80% or above and support vehicle loads. To accomplish this, it has several different modes of operation. Six common modes of operation are Charge Mode, Fuel Economy Mode, Voltage Reduction Mode, Start Up Mode, Windshield De-Ice Mode and Battery Sulfation Mode.
The PCM/ECM (generator battery control module on full-size trucks) controls the generator through the generator L terminal control circuit. It monitors the generator performance though the generator field duty cycle signal circuit. The signal is a 5V PWM (pulse width modulated) signal of 128Hz with a duty cycle of 0% to 100%. Normal duty cycle is between 5% and 95%. The ranges between 0% to 5% and 95% to 100% are for diagnostic purposes.
The control module enters Charge Mode whenever one of the following conditions is met:
- Under WOT conditions and when the fuel rate (sent by the ECM/PCM) is greater than 21 g/S and the throttle position is greater than 90%.
- The headlamps are on, low or high beam.
- The wipers are on for more than 8 seconds.
- The electric cooling fans are on high speed.
- The rear defogger is on.
- The battery SOC is less than 80%.
When one of these conditions is met, the control module ramps up the voltage slowly to a level between 13.4 to 15.5V (depending upon the mode of operation the system is presently in) at a rate of 8mV to 50mV per second.
The control module enters Fuel Economy Mode when the following conditions are met:
- The calculated ambient air temperature is above 32°F.
- The calculated battery current is less than 15A and greater than –8A.
- The battery SOC is greater than 80%.
- The generator field duty cycle is less than 99%.
This mode’s targeted generator output voltage is 13.0V. The control module will exit this mode once the criteria are met for Charge Mode.
The control module will enter Voltage Reduction Mode when the following conditions are met:
- The calculated ambient air temperature is above 32°F.
- The calculated battery current is less than 2A and greater than –7A.
- The generator field duty cycle is less than 99%.
This mode’s targeted generator output voltage is 12.9V. The control module will exit this mode once the criteria are met for Charge Mode.
After the engine has started, the control module sets a targeted generator output voltage of 14.5V for 30 seconds (Start Up Mode).
The control module enters Battery Sulfation Mode when the battery voltage is less than 13.2V for 45 minutes. Once in this mode, the generator battery control module will set a targeted output voltage between 13.9 and 15.5V for five minutes. The control module will then determine which mode to enter depending on voltage requirements.
In RVC Mode, the control module bases the charging voltage on battery SOC, which is estimated during a key-off event every eight hours, after three voltage measurements every 24 hours thereafter, and then monitored constantly while the ignition is on. These voltage measurements are then compared to estimated battery temperature, as battery temperature vs. battery voltage directly corresponds to battery SOC. While the engine is running, the system uses both the battery voltage and estimated battery temperature to determine the battery current in and out of the battery. The control module then regulates the charging voltage to keep the battery above an 80% SOC.
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