6 September 2015

RVC ReduxThe July Trouble Shooter column on charging sys-tem problems on a 2007 Chevy Silverado generatedsome interest among MOTOR readers, and prompt-ed the following insights from Mohammad Samii,owner of Sammy’s Auto Electric Service, Inc., inChampaign, IL. Samii has written the “Auto ElectricCorner” column for the Automotive Parts Re-builders Association’s APRA Global Connection formore than 20 years, and shares his knowledge as atraining instructor. Here are his comments:

I’d like to add some information to your discussionof a charging problem on a GM regulator voltagecontrolled (RVC) charging system. In an RVC sys-tem, the PCM constantly supplies a 128Hz signal of5V amplitude to the L terminal. The PCM changesthe pulse width of this signal when the system goesinto a different mode of operation (there are sevenor eight different modes). Changing the pulse widthcauses the voltage regulator to change its voltage set-ting, bringing the generator’s charging rate close towhat the PCM has ordered. When an RVC systemis being tested in a shop, you would rarely see amode change, and the average voltage on the L ter-minal measured by a DMM would be around 3.5V.

Activation of an RVC generatordoes not require a test light con-nected between the B+ and the Lterminal. A self-exciting feature isbuilt into the system and the gener-ator will work (albeit in defaultmode) when the engine is revvedup, even if there is no connector at-tached to the generator except theoutput wire. This is one of the fea-tures we must verify on the testbench when an RVC generator isrebuilt and being tested.The RVC generator’s default

mode has another feature. If ananomaly is detected in the system,the generator will disregard the Lterminal input and will turn itselfon in a default mode with a 13.8V

setting on the regulator. This feature is designed toensure the continued operation of the generator andcharging system if there is a failure in any of the asso-ciated modules that participate and control the RVCcharging system. Of course, the dashboard warninglight would be on if the generator is running in de-fault mode, to indicate a problem that needs to beaddressed 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 de-fault, the charging voltage stayed around 13.8V andthe charging system warning light started to comeon.

Thanks very much for writing, Mohammad. Yournote indicated to me that there’s quite a bit more tobe said about the RVC charging system than whatwas included in the July column. So perhaps it’s bestto begin at the beginning, with a description of theRVC system, its purpose and how it’s supposed towork.Conventional charging systems use an internal

temperature sensor to establish generator voltage setpoints. When the generator is cold, it raises the volt-age output set point. When it’s hot, it lowers the volt-age output set point. This type of system tends toovercharge the battery on long trips at highwayspeeds and undercharge it on short trips with lowvehicle speeds. Regulated voltage control (RVC)regulates the generator’s output voltage, based main-ly on estimated battery temperature and batterystate-of-charge (SOC). The main benefits of this sys-tem are: improved fuel economy, extended batterylife, extended lamp life and extended switch life.The RVC system allows voltage changes, up or

down, based on battery state-of-charge, the vehicle’spresent electrical needs and other factors, so thevoltmeter reading may fluctuate. Conventional sys-tems usually maintain a consistent reading of around14V. The RVC system voltage may range between12 and 14V. This is normal system operation, butmay be perceived as a problem by vehicle ownerswho are accustomed to seeing a relatively consistentvoltmeter reading.

KarlSeyfert

As Paul Harvey used to say, ‘And now, the rest of the story.’ An

industry veteran shares his insights regarding the charging

system problems discussed in the July Trouble Shooter.

kseyfert@motor.com

Trouble Shooter

The four-terminal harness connectorshown in the July 2015 Trouble Shooterwas for a nonregulator voltage con-trolled (RVC) charging system. The har-ness connector for an RVC generatorshown here has just two terminals—Land F. The L terminal has very little todo with the charging system warninglight on the instrument cluster.

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The control module enters Fuel Econ-omy Mode when the following conditionsare met:•The calculated ambient air tempera-

ture 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 willexit this mode once the criteria are met forCharge Mode.The control module will enter Voltage

Reduction Mode when the following con-ditions are met:•The calculated ambient air tempera-

ture 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 willexit this mode once the criteria are met forCharge Mode.After the engine has started, the control

module sets a targeted generator outputvoltage of 14.5V for 30 seconds (Start UpMode).The control module enters Battery Sul-

fation Mode when the battery voltage isless than 13.2V for 45 minutes. Once inthis mode, the generator battery controlmodule will set a targeted output voltagebetween 13.9 and 15.5V for five minutes.The control module will then determinewhich mode to enter depending on volt-age requirements.In RVC Mode, the control module

bases the charging voltage on batterySOC, which is estimated during a key-offevent every eight hours, after three voltagemeasurements every 24 hours thereafter,and then monitored constantly while theignition is on. These voltage measure-ments are then compared to estimatedbattery temperature, as battery tempera-ture vs. battery voltage directly corre-sponds to battery SOC. While the engineis running, the system uses both the bat-tery voltage and estimated battery temper-ature to determine the battery current inand out of the battery. The control modulethen regulates the charging voltage tokeep the battery above an 80% SOC.

8 September 2015

Trouble Shooter

There are two types of RVC systems inuse—integrated RVC and stand-aloneRVC (SARVC). Integrated systems use abattery current sensor to inform a bodycontrol module (BCM) how much thebattery is being charged or discharged.SARVC systems (found mostly on

trucks) do not use the BCM for operation.They have a generator battery controlmodule mounted to the negative batterycable, to interpret battery current and volt-age and battery temperature inputs. Thebattery current sensor is internal to themodule. This module also directly controlsthe generator L terminal duty cycle in-stead of the ECM/PCM. The purpose of the RVC system is to

maintain the battery state-of-charge at80% or above and support vehicle loads.To accomplish this, it has several differentmodes of operation. Six common modesof operation are Charge Mode, FuelEconomy Mode, Voltage ReductionMode, Start Up Mode, Windshield De-Ice Mode and Battery Sulfation Mode.The PCM/ECM (generator battery

control module on full-size trucks) controlsthe generator through the generator L ter-minal control circuit. It monitors the gen-erator performance though the generatorfield duty cycle signal circuit. The signal isa 5V PWM (pulse width modulated) signalof 128Hz with a duty cycle of 0% to 100%.Normal duty cycle is between 5% and95%. The ranges between 0% to 5% and95% to 100% are for diagnostic purposes. The control module enters Charge

Mode whenever one of the following con-ditions is met:•Under WOT conditions and when the

fuel rate (sent by the ECM/PCM) isgreater than 21 g/S and the throttle posi-tion is greater than 90%.•The headlamps are on, low or high

beam.•The wipers are on for more than 8 sec-

onds.•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 voltageslowly to a level between 13.4 to 15.5V(depending upon the mode of operationthe system is presently in) at a rate of 8mVto 50mV per second.

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