Regulated Voltage Control


Original poster
Nov 20, 2011
Found this while looking for something else,

"To be clear, GM’s RVC systems use alternators with field currents that truly are controlled by logic external to the alternator. You’ll see two things right off the bat on RVC vehicles tipping you to the presence of this latest technology: First, you’ll see a two-wire voltage regulator connection instead of the familiar four-wire connection. Second, you’ll notice a funny-looking sensor or module wrapped around either the negative or positive battery cable. It may remind you of an inductive current probe, because that’s what it is.There are two types of these systems—stand-alone RVC (SARVC) and RVC. The former was used for a few years (mid-2000s) on light-duty trucks and SUVs. The current sensor in the SARVC system is built into a full-fledged electronic module complete with a Class 2 serial bus circuit to receive and transmit information. You probably won’t find the data PIDs for this system in the PCM. The SARVC module is also known as a generator battery control module (GBCM) and is located within a list of body modules on the scan tool. Not all aftermarket scan tools can read this module, but they will display PIDs like Battery Voltage, Battery State of Charge, Regulated Voltage Control Current and Generator L Terminal Signal as a percentage".

I suspect, the Stand Alone system is being used in the 360/370 trucks. This explains the short charging delay we have noticed when the trucks are first started on a cold morning.



Nov 20, 2011
Milwaukee, Wisconsin
Nice find. :thumbsup:

Now if we just knew a GMTN member that was an Electrical Engineer so we knew for sure. :wootwoot:


Aug 26, 2012

service manual describes the generator. control is internal. (at least for my 2003)

there is a wire which must be set to turn it on, (and can be used to turn it off, like when just getting started, by the control system.)

and a wire which lets the generator tell the computer what the output is (% load if over 5%) (so it can give some gas to the engine( not really, but does increase power) when loads are high.

search for DTC P0563 and DTC P0621 and ok, DTC P0622 (in the service manual)


Dec 4, 2011
I am assuming that the PCM turns off the alternator when it is cold so that it can properly ascertain the state of charge. If it did this test right after the starter operated then the battery would be artificially low in amps.


Aug 26, 2012
a better way to think of it is to not turn on the generator until the engine is known to be operating steady. it is not a test of the state of charge, but a test of "is the engine running well enough to turn the generator" kind of like when you wait for the engine to come up to speed in your snowblower before turning on the auger and going to work clearing snow.

it is true that this results in operating with less charge than would be available with the generator turned on, but if the motor sputtered out and you had to turn on the starter again, this would further reduce the charge available. you loose a lot less energy not turning on the generator for a few seconds than you spend cranking the engine.

since we really do not have the source code, we are guessing on the exact details of what is checked and what measured conditions allow the generator to be turned on. (what I am saying is it could be that the same program checks for parameters for every start, or there could be a dedicated subroutine which kicks in for very cold starts) I suspect there are other conditions which would have the PCM turn off the generator.

(let me know if you have the source code:smile:)


Aug 26, 2012
the generator F terminal on my 03 is used to output the PW signal generated by a dedicated controller/circuit internal to the generator module.

on the trucks which use RVC, this signal is generated external to the generator and used to control generator output.


Lifetime VIP Donor
Nov 19, 2011
Portland, OR
Details of what I've been posting about are in my service manual. Especially subtle, that I never ever read a member post about it happening to them, is the load-shedding algorithm meant to reduce optional system loads on a battery whose voltage is dropping, and/or whose alternator is running flat out (high field control PWM duty cycle). Entertainment, HVAC, and lighting is shed in that order to keep the engine running as long as possible to allow the driver to limp to a place of safety. Our system is documented to use the field duty cycle signal as an output from the alternator and an input to the PCM. The on/off control signal is from the PCM to the alternator.


Aug 26, 2012
forgot to say thanks for the original post.

had fun with the search (in the service manual and online), and it seems alternator/generator applications for cars and trucks are way more complicated than they used to be.



Dec 4, 2011
Ottawa, ON
Revival! Found this article which explains the SARVC very well and has a picture.

Introduction to Battery Control Modules


The module is usually located near the battery on the negative / ground line.

The module is usually located near the battery on the negative / ground line.
Basic concept and Benefits

Traditional charging systems use an internal temperature sensor inside the generator to establish generator voltage set points. When the generator is cold, it raises the voltage output set point. When the generator is 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 the battery on short trips with low vehicle speeds.
Regulated Voltage Control (RVC) is a new dynamic control of the vehicle’s system voltage. It regulates the generator’s output voltage, based mainly on estimated battery temperature and battery state-of- charge. The main benefits of this system are:
• Improved fuel Economy • Extended battery life
• Extended lamp life
• Extended switch life
There are two types of RVC systems used today Integrated RVC and
Stand-alone RVC (SARVC).
Integrated systems use a battery current sensor to provide a body controller (BCM) the amount the battery is charging or discharging. Accurate voltage measurements are taken through the battery positive voltage and ignition 1-voltage circuits. The BCM then communicates information over serial data circuits for the ECM/PCM to directly control the generator.
SARVC systems 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.
Both types of system have two types of corrective actions to insure the battery stays at 80% state of charge. These include up to three levels of load shed and up to three levels of idle boost operation. Refer to the (GM) service manual Description and Operation for load shed and idle boost.
We will discuss the basic operation of the RVC system, the three different generations of systems used today on GM vehicles, components used on each separate system and general diagnostic information section.
Charge Mode
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 (state of charge) 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 and 15.5 volts (depending upon the mode of operation the system is presently in) at a rate of 8 mV to 50 mV per second.
fuel economy mode
The control module enters Fuel Economy Mode when the following conditions are met:
• The calculated ambient air temperature is above 0°C (32°F).
• The calculated battery current is less than 15 amperes and greater
than -8 amperes.
• The battery state of charge is greater than 80%. • The generator field duty cycle is less than 99%.
Its targeted generator output voltage is 13.0 volts. The control module will exit this mode once the criteria are met for Charge Mode.
System Operation
Not all systems will enter all modes of operation; refer to the applicable (GM) service manual for exact Description and Operation.
The purpose of the RVC system is to maintain the battery state-of-charge at 80% or above and support vehicle loads.
The six modes of operation include: Charge Mode
Fuel Economy Mode Voltage Reduction Mode Start Up Mode Windshield Deice Mode 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 through the generator field duty cycle signal circuit. The signal is a 5 volt PWM (pulse width modulated) signal of 128Hz with a duty cycle of 0-100%. Normal duty cycle
is between 5-95%. The ranges between 0-5% and 95-100% are for diagnostic purposes. The following table shows the commanded duty cycle and output voltage of the generator:

Duty Cycle
Generator Voltage set point
The generator provides a feedback signal of the generator load through the generator field duty cycle signal circuit to the control module. The signal is a 5 volt PWM signal of 128 Hz, with a duty cycle if 0-100%. Normal cycle is between 5-99%. The ranges between 0-5% and 100% are for diagnostic purposes.
Voltage Reduction Mode
The control module will enter Voltage Reduction Mode when the following conditions are met:
• The calculated ambient air temperature is above 0°C (32°F).
• The calculated battery current is less than 2 amperes and greater
than -7 amperes.
• The generator field duty cycle is less than 99%.
Its targeted generator output voltage is 12.9 volts. The control module will exit this mode once the criteria are met for Charge Mode.
start up mode
After the engine has started, the control module sets a targeted generator output voltage of 14.5 volts for 30 seconds.
Battery sulfation mode
The control module enters this mode when the battery voltage is less than 13.2 volts for 45 minutes. Once in this mode, the generator battery control module will set a targeted output voltage between 13.9 to 15.5 volts for 5 minutes. The control module will then determine which mode to enter depending on voltage requirements.
RVC Mode
The control module bases the charging voltage on battery state of charge (SOC). Battery SOC is estimated during a key off event every 8 hours, after 3 voltage measurements every 24 hours thereafter, and then monitored constantly while the ignition is on. These measurements of voltage 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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