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By Dave Hobbs. No matter how complex ‘smart’ charging systems appear at first glance, it still comes down to a battery, an alternator and some modules monitoring and controlling voltage regulation. We’re sure you’ve got the smarts to handle that.

Nearly every time I look at an alternator it conjures up fond memories of growing up in a family-owned auto electric shop. In the 1980s, Chrysler pioneered voltage regulation with an engine control computer on its first EFI systems.

Chrysler still uses a similar method of voltage regulation today. Quite a few years later, Ford followed with a powertrain control module (PCM) connection to the alternator. Not to be outdone, GM and others joined the ranks of OEMs using some form of strategy to tie engine management and voltage regulation together.

Although the image of a full field test with a pocket screwdriver making sparks in the D hole of a Delcotron is outdated, simple tests for outsmarting today’s socalled smart charging systems are out there for the tech who’s willing to learn what makes these latest systems tick.

To keep this smart charging system visit manageable, we’ll focus on GM passenger cars and light-duty trucks. Before we venture too far out into the deep water on the latest charging systems, let’s do a bit of review. Not a bad idea considering that many techs reading this have never seen the inside of an alternator, as the art of rebuilding has becomes a thing of the past for most shops.

The primary missions for any alternator are to maintain a charge on the battery and keep up with the vehicle’s electrical accessory demand. To accomplish these tasks, a rotating electromagnetic field called a rotor induces an electrical charge into three windings that are 120° out-of-phase. These windings (the stator) create alternating current that in turn is rectified into direct current by a set of diodes in the back of the alternator. Pretty basic stuff, right?

The job of the voltage regulator has always been to control the amount of electrical energy (field current) in the rotor via electrical power supplied to the brushes on the rotor’s slip rings. The greater the current supplied to the slip rings, the more voltage (and current) the alternator puts out. As electrical loads such as the headlights, blower motor or rear defogger are added, the regulator senses a reduced voltage and applies more power to the rotor, allowing the alternator to keep up with the electrical demand. When the regulator fails, it usually causes one of two problems: Either the rotor does not receive power and there is no charging output, or the regulator gives full power to the rotor and the alternator puts out too much voltage, boiling the battery dry and burning out light bulb filaments.

Then and Now

To understand where we are now, it’s important to know where we’ve been. Historically speaking, the description just given is pretty much the way it’s been since voltage regulators moved off the firewall and into the back of the alternator in the 1970s. These fundamentals have changed very little until the last five years for GM. One change with voltage regulation did take place in the mid-’80s. It was found that as engines became smaller and developed less torque, the sudden reaction to an increased load on the accessory drive belt sometimes caused a rough idle/vibration, or even engine stalling. Power steering pressure switches and a/c compressor request/control circuits all helped with idle quality, but there was nothing to tip off the engine management computer that an increased load was occurring from the alternator’s varying field current.

Every tech has heard an engine bog down when the alternator’s output ramped up in response to increased electrical load. Faraday’s law of induction tells us why this occurs, but that doesn’t help the PCM with engine management. Engineers came up with the solution and the four-terminal voltage regulator was born in the late ’80s. This change caused confusion that has lasted until the present day, due to the variations of systems using one or more of those four-terminal regulators. The letters P, L, F and S were stamped on these regulators to identify the terminals.

Unlike SI (series integral) alternators with the older two-wire regulators, the newer design CS series alternators eliminated unmanaged load dump (fast field ramp-up) that caused the rough idle and stalling issues when the electrical loads were suddenly turned on. These CS series machines gradually ramped up their fields with a 400Hz pulse-width modulated (PWM) output from the regulator to the rotor’s brushes. No more stalling and no more vibration when the accessories were all turned on.

Of the four terminals, the P terminal was seldom used with the exception of some diesel applications. The P stands for “Phase” and is the output of one leg of the three stator windings, giving a tach signal to a diesel engine controller. The L stands for “Lamp” and goes exactly to where you might think it would—to the charge lamp indicator. The F stands for “Field” and provides a square wave to the PCM, allowing it to know the field strength (torque demand) of the alternator. On some models the third terminal is marked I for “Ignition” and acts as a backup for regulator turn-on voltage should the bulb in the cluster fail to provide voltage to the L terminal. The S stands for “Sense” and is connected to a location in the wiring harness where accessory current draw is high, such as a fusible link at the starter or a bussed electrical center. While many vehicles used only one wire (the L terminal), there were quite a few that populated the regulator’s connector with two or three circuits, and this led to a major source of confusion that still exists today! Which of these circuits is really needed when performing simple diagnostics? The answer is, in almost all cases, the L terminal.

Test Light or Lab Scope?

That’s enough theory; now on to something practical! If you’re working on a GM product, whether it’s a 1989 or 2004 model, you can determine with a simple test whether a charge problem is due to a defective alternator or a problem with one of those four wires going to the regulator. With a voltmeter connected to the battery, simply disconnect the voltage regulator, then start the engine. Next, connect the alligator clip of a conventional test light (not the newer LED style) to a nearby source of battery positive. Finally, touch the test light probe to the L terminal of the voltage regulator, as shown in the photo at left. Your test light will do the job of the cluster’s charge light.

When the alternator is not charging, that L terminal will be grounded (the test light will glow), and when it is charging, the light will go off. The reduced voltage provided by the test light will provide the necessary excitation signal for the regulator to come to life. Note: If the regulator connector is in a difficult location, use a short jumper lead to do the job. If the alternator “sings” and your voltmeter jumps up to show charging voltage, the alternator is likely good and you must move your focus to the wiring leading to the regulator connection. If it doesn’t start charging, the alternator is definitely bad and it’s time to sell a new or reman unit to the customer.

I said the alternator is “likely good” if you touched the test light probe to the L terminal and it began to charge. I left some wiggle room here because not all four-wire GM voltage regulators are created equal. Some aftermarket alternator rebuilders try a little too hard to reduce their part numbers to a leaner inventory, creating alternators that charge but may not charge properly. Without going too deep into that, I’ll just say that GM had good reasons to use one, two, three or all four of those regulator terminals, so the one-size-fits all reman part may give you grief.

During the late ’90s, we started seeing the L terminal on the CS series four-wire regulators connected to the PCM, as well as the F terminal. This led many techs to assume that the PCM controlled the field current, as it does with Chrysler. It does not. It does provide a signal to the L terminal for exciting the regulator and does receive a ground on that circuit when the engine is stopped or the alternator has failed, just as before when the circuit was connected to the charge lamp in the cluster. The difference now is this PCM connection to the regulator’s L terminal can inhibit sending the excitation signal to that circuit, which would prevent the alternator from charging.

Why would it do that? It does so to delay the introduction of field current into the rotor during engine start-up. Remember Faraday’s law? Improved starts occur when an engine doesn’t have to deal with an alternator load on its drive belt. After the engine starts, the PCM uses the circuit leading to the regulator’s L terminal as an input. If it sees a ground on that circuit, such as when the alternator senses a problem internally, the PCM then sends a serial bus message to the cluster requesting the charging indicator to come on. Most scan tools can read the PCM PIDs for the L terminal status and the percent of duty cycle the PCM sees coming from the F terminal. You can also scope the F and L terminals to watch the regulator ground the voltage pulled up by the PCM on the L terminal and observe the 400Hz signal on the F terminal displaying the duty cycle that the rotor winding is receiving (see Fig. 1).

Now We’re Getting Complicated

If the CS series of GM alternators weren’t smart enough for you, we have the latest systems called Regulated Voltage Control (RVC) showing up on the scene in the mid 2000s. Why a new system? Batteries are sensitive to temperature. The lower the battery’s temperature, the lower its chemical and electrical reactivity. This means the battery puts out less voltage in cold temperatures and needs more voltage to charge. Conversely, a battery puts out more voltage and needs less voltage from the alternator in warm weather.

While Chrysler did indeed use an external temperature sensor to sense battery temperature for its PCM-controlled voltage regulation, GM always used a thermistor built into the voltage regulator to sense underhood temperatures for charge rate compensation. These days, with battery locations spread out everywhere from the engine compartment to the trunk to the back seat, a better way of detecting and compensating for battery temperature had to be developed to replace the temperature sensing that traditionally occurred within the voltage regulator. Keeping batteries charged with just the right voltage and maintaining greater than 80% state of charge (SOC) helps them last longer.

Shedding Some of the Load

Helping the battery achieve the SOC goal has been the job of another smart charge feature called load shedding that has actually been around for well over a decade. The alternator’s pulley turns slower when the engine is idling, causing less output. When idling with a heavy electrical accessory load applied, the alternator may not be able to keep up, and the voltage to the battery drops below 12 volts. If this continues for very long, the battery can drop below the desired 80% SOC. Load shedding has been a function of the body control module (BCM) in both the older CS series alternators (four-terminal regulator) and the newer Regulated Voltage Control (two-terminal regulator) systems.

Load shedding basically is a function of the BCM or another module on the serial bus (Power Mode Master) watching battery voltage and doing something about it if it gets too low. The process involves first requesting the PCM to boost idle speed to raise the alternator’s output enough to keep up with the heavy accessory demand and still trickle a charge into the battery. If the first idle boost doesn’t do the trick, subsequent idle boosts are commanded before a more aggressive action is enabled—reducing power to certain power-consuming accessories, such as the rear window defogger grid.

Regulated Voltage Control Explained

Regulated Voltage Control uses information from the battery current sensor, calculated battery temperature and system voltage to determine what the perfect voltage to supply the battery with is. In addition to nurturing the battery into longer life by maintaining proper state of charge, RVC allows the charging voltage to drop below the 13.8 to 14.8 volts to which we’re all accustomed. If the battery can maintain an 80% SOC with 13 volts at a particular temperature, why would we want to charge it at 14.8 volts? Higher voltage comes at a price of increased drive belt load. Every bit counts with today’s fuel economy goals. Additionally, light bulb life is increased with a slightly lower charge voltage. Depending on the model and year, the newer GM RVC charging systems may include up to nine different charging rates strategies, among them:

Battery Sulfation Mode. No sense hammering a battery that has given up the ghost!
Start-Up Mode. Gets the battery back up to greater than an 80% SOC quicker.
Fuel Economy Mode. Alternators that put out less voltage help engines use less fuel.
Headlamp Mode. Okay, it’s time for a little more voltage now that the lights are on.

Referring to the RVC graph in Fig. 2, you can see how much thought has gone into tailoring the alternator’s charge output to suit various scenarios. An interesting sidebar note would be the ramp-up in field current that the RVC systems sometimes introduce on deceleration. When you’re cruising in Fuel Economy Mode (lower charge voltage), you can charge harder during a brief period of deceleration without sacrificing fuel economy or overcharging the battery. In fact, when there’s a greater alternator load on the engine during deceleration, that load contributes to engine braking—something that’s good for brake pad life as well. Think of this as a mild form of what hybrid vehicles do with regenerative braking.

Speaking of hybrid vehicles, DC-to-DC converters (high voltage in, conventional 14 volts out) come online whenever hybrid systems are powered up. This may occur with the key on and the engine off. The DC-to-DC converter receives serial bus messages from a BCM or PCM on the vehicle responsible for the equivalent of RVC functions. This allows a hybrid vehicle’s DC-to-DC converter to tailor a perfect charge rate into the vehicle’s 12-volt battery.

RVC Operation & Diagnostics

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.

The RVC systems use a Hall effect type current sensor that sends a PWM signal at 128Hz to the BCM. This sensor stays powered up long after the ignition switch is turned off. The reason is that this sensor also reports to the BCM any news of excessive parasitic current draw. This is also a scan tool PID—nice to know when diagnosing a battery rundown problem on a RVC-equipped GM vehicle. And speaking of PIDs, since the BCM is the module in charge of logic for this system, you’ll find up to 20 PIDs under a heading titled Charging Info when searching for charging system PIDs within the BCM. Although the BCM is the “brains” for the non-stand-alone RVC system, it’s the PCM that actually carries out the commands to control the L terminal with a PWM output to control the alternator’s charge rate. To find output PIDs, go into the Powertrain section on your scan tool. You may encounter output PIDs in the section of the PCM titled Electrical/Theft. As with earlier non-RVC, four-terminal CS series alternators, the L terminal can be commanded on and off by going into the PCM and selecting output control.

On RVC vehicles, you’ll see the charge voltage fluctuate while driving or even while running the engine in the bay. The changes will be subtle, as the rate of change varies from 10 to 20mV per minute. Charging rates as high as 15 volts are observable on some vehicles, depending on current demand and battery SOC, along with values as low as 12.9 volts. The PWM signal from the PCM to the L terminal on these twowire regulators varies from 10% to 90%, with the latter being the higher voltage command (15.5 volts). If this circuit goes open, the default charge rate from the alternator is 13.8 volts.

One way to outsmart the smart charging system is to disconnect the current sensor or SARVC module. On some systems you’ll see 15.5 volts with the engine running and a light electrical load applied. This should tell you if you’re dealing with a good, old-fashioned alternator problem or a vehicle wiring or module problem. Naturally, DTCs will be set that will have to be cleared. Don’t forget to look in the Body Controls section of your scan tool when clearing codes, in addition to the PCM.

But no matter how complex these systems seem, it all really comes down to a battery, an alternator and some modules monitoring and controlling voltage regulation. By knowing how these new systems work, you can keep outsmarting the latest smart charging systems that show up in your bays!

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5 replies to this post
  1. Great article. Very informative. Well thought out and well executed.
    Sometimes I think my job is boring because I spend so much time relearning old things and constantly learning new… and the general public doesn’t care to be told how their car works… they just want it fixed. But, me as the tech/shop owner, I have to know how it works in order to be able to make the repairs.
    But, that’s the thing about this industry… if you are not willing to learn the changes … you’re going to get run over by someone that does.

    Great article…

  2. As always Dave, an extremely well researched and brilliantly compiled technical article that has answered a number of questions I had regarding ‘Smart Charging Systems’. One area that has been grey for some time now is what the purpose of the F terminal was: One OEM technical bloke told me it was a signal TO the Regulator from the PCM; another told me it was a signal FROM the Regulator to the PCM! Your introduction to this article makes a very valid point with this type of technology – it still comes down the basics that we have seen under hood for years now. Keep up the great work and best regards from Australia.

  3. Great article, Dave. I was a service manager for a Dodge dealership in the seventies. Since then I have worked on Mack Trucks, Caterpillar equipment and now Schwing and Putzmeister concrete pumps. My mother-in-law’s 2002 Chrysler has a parisitic voltage draw. The battery goes dead after about three days of no use. I traced it to the BCM but not knowing how to test it, I told her to “take it to the experts.” The dealer has had the car twice, the last time for several weeks with several mechanics working on it without being able to find the problem. Your article was helpful to me to understand what the new components are and the purpose for them. I will try to keep searching for information on testing the BCM or information on the voltage draw.

  4. Hi!! great post! is there a way to emulate this function? i mean i can`t purchase the alternator in my country,

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