When it comes to any modern automotive system, you can accurately say some things never change. You can also say with equal accuracy some things are not only always changing – they are changing faster than most of us are able to keep up with! If you’ve found yourself struggling to keep up with the changes in 12-volt charging systems on late model vehicles, this article will update you on what has changed and remind you of some things that never change!
Late model non-GM RVC alternators that don’t charge the battery correctly fall into 3 failure categories;
- Faulty alternator (won’t charge)
- Faulty control circuits / PCM
- Wrong alternator for application
To test for which failure category applies (1 or 2) simply disconnect the alternator regulator plug and start the engine with a voltmeter on the battery. Now take an old fashioned bulb type 12-volt test light and connect the test light’s alligator clip to a 12-volt power source. Then touch the tip of the test light to the “L” terminal of the alternator. Since you are re-creating with your test light what a cluster’s bulb would do to trigger the alternator into turning on, you will hear the alternator start singing (they all do that when charging) and see the voltmeter jump up to charging voltage if the alternator is good.
Terminal 1 = GEN COM or Generator Command (Output from PCM to alternator to control the charging rate.
Terminal 2 = GEN MON or Generator Monitor (Input to PCM to tell it how hard the alternator is working. GEN COM when monitored with a DVOM with frequency / duty cycle measuring capabilities or a DSO will typically be ½ charging voltage with a PWM variable duty cycle running around 132 Hz.
Keeping up with electrical changes in charging systems has been a challenge to me dating clear back to my days in the 1970s learning how to use a Sun VAT 28 (Volt-Amp-Tester) at Hobbs Auto Electric in Kokomo, Ind. A Sun VAT what? Oh – that sounds old! But since we can’t possibly know where we are going until we make a note of where we’ve been, here’s a short history of charging systems so we can move to the present before blasting off into the future.
Generators to alternators – My father’s story
In the early 1960s, my father Ray Hobbs was an outside sales rep for a parts store in Kokomo. An interest in two-way radios and a father-in-law (my favorite grandpa) who was an electrician gave dad a boost in learning electrical fundamentals. This in turn inspired him to learn more about a brand-new automotive component called the alternator. Chrysler had just invented the new charging machine, which featured a revolving field (stator) in lieu of the old generator’s fixed field winding (field coils). The fixed windings that became excited when exposed to the rotor’s changing magnetic flux featured three separate coils. Since the “excited” coils didn’t change polarity back and forth to keep their output in a Direct Current (DC) state, as in the case of the conventional DC generator, this new charging machine produced Alternating Current (AC). Because a vehicle’s electrical system has no use for AC, diodes were installed to rectify the AC current back into DC current. In those early days, some of the heavy-duty charging system applications placed diode rectifiers outside the alternator. Most light-duty truck and automotive applications, however, placed the diodes internal to the alternator.
During this period of the 1960s, electrical innovations were beginning to ramp up as the era of space travel and computers moved into the technology scene and automotive technicians (mechanics back then) had a difficult time keeping up with those changes. Dad was a good outside parts salesman but eventually became frustrated with trying to instruct his mechanic customers how to properly use test equipment and not return “No Trouble Found” alternators and voltage regulators to the store. Did I say some things never change? His frustration gave way to an entrepreneurial spirit when he called it quits with the parts business and started his own repair shop. That auto electric business would be the center of my family’s life for the next 40 years!
It’s always been about voltage regulation
Another advance in automotive electronics that would affect the charging system was the Engine Control Module. Sometimes referred to as F.R.E.D. (Frustrating Ridiculous Electronic Device) the ECM became mandatory on all the OBD I iron in the early 80s. By the mid 80s, Chrysler was back in the spotlight with another charging system innovation: the ECM controlled charging system. Their 1985 four-cylinder equipped cars began featuring Electronic Fuel Injection (EFI) using an ECM split into two parts — a Logic Module for calculations and a Power Module to do the actual outputs. In the case of the field wires connected to the alternator rotor’s brushes, the Logic Module made the charging rate decision while the Power Module carried out the solid state drive of either a low-side ground driver (earlier models) or switched high-side power drivers (after 2002) to the alternator’s field (brushes) to control the charging voltage.
For more than a decade after that, most other vehicle manufacturers continued on the path General Motors had taken since 1972 with a two-pin autonomous solid-state voltage regulator built into the alternator. Ford and others would eventually move their voltage regulators into the alternator. The two-pin GM voltage regulators only needed a voltage signal from the cluster to come to life and begin charging. The lower the voltage sensed internal to the alternator, the more field current the regulator would provide the two brushes and the more current the alternator would produce. Maximum current output was limited to the size of the fixed stator and rotating rotor windings, along with the speed of the pulley of course. Bigger stators, bigger rotors and hand throttles, along with smaller pulleys, were the tricks to keeping up with high demand applications such as emergency vehicles.
In the early 80s, GM began an experiment called the “X” car many of us remember as Chevy Citations and Buick Skylarks. These FWD transverse mounted four-cylinder engines were quite underpowered and prone to stalling and even strange tail pipe vibration as the two-pin GM “Delcotron” alternators were immediately turned on to a full-field condition in order to take care of a sudden increase of 12-volt accessory demand. Not quite ready to trust “F.R.E.D.” to handle voltage regulation, the GM/Delco Remy CS series alternators were born in order to gently ramp up the field current when a sudden current increase was demanded. As the alternator sensed internal voltage at the output post or external voltage at the new four-pin regulator’s new “S” terminal (tied to a strategic spot in the wiring harness), the smarter voltage regulator would ramp up the PWM of a duty cycle running at 400 Hz. Since this was all internal to the alternator, unless you were a rebuilder or a geek, you would have never known.
There was also a regulator terminal marked “P” (for Phase) that in applications of non-electronic diesels (GM 6.2 liter V8) became the tachometer input from one leg of the stator. A third voltage regulator terminal was in many cases the only terminal with a wire going to it (besides the heavy gauge output circuit) and was originally marked “L” for Lamp. This circuit was very descriptive – it went to the ground side of the cluster’s charge indicator bulb to be grounded when the engine was in key-on-engine-off state or when the regulator sensed a no charge/malfunction state. The voltage dropped across the light bulb in the dash also acted as a current limited input to the regulator telling it “key is on, ready to charge when you’re ready to charge.” A fourth terminal (“I” for Ignition) to those CS series machines was an option on some vehicles to supplement the cluster’s connection to the “L” terminal when the ignition was turned on.
OBD II & Smart Charge — Does F.R.E.D. really charge the battery?
1996 took a big turn when OBD II gave us all a new challenge. About that time, Ford began following Chrysler‘s technology of having the alternator controlled by the PCM. Not all late model Ford alternators are PCM controlled. Terminals for the voltage regulator in a Ford that is not controlled by the PCM are marked “I” (for indicator), which is hot with the ignition key in the RUN position. Voltage is applied through the warning indicator circuit to the voltage regulator. This turns on the voltage regulator, allowing current to flow from the battery sense “A” terminal circuit to the generator field coil. Another regulator terminal is marked “S” (for stator) and supplies rectifier charge information. You’ll measure this as half B+ voltage if the regulator is signaling the cluster to turn off the charge indicator. The last terminal is marked “A” (for absolute) and is a dedicated battery voltage sense line measuring system B+ voltage.
Ford’s later PCM-controlled charging systems use alternator connector circuits descriptively marked “GEN COM” and “GEN MON” for Generator Commanded PWM (PCM Output) and Generator Monitor carrying the generator load info and error condition to the PCM (PCM input). If you use a DVOM with a frequency counter or a DSO, you’ll find Ford’s GEN COM to be around 132 Hz with a variable duty cycle that changes with the PCM’s command for the charge rate. A third terminal may be used (A for Absolute) and is the dedicated voltage sense line. The PCM uses input info from the TPS, ECT, IAT and VSS sensors to set the charging rate.
On later OBD II GM vehicles, we saw the same CS series alternator we used in the late 80s, but now the PCM was connected to the terminals marked “L” (for Lamp) and “F” for field for about the first decade of OBD II. Many technicians did then and do now assume those GM four-wire CS series alternator regulator circuits are controlled by the PCM, as in the case of Chrysler and Ford. They are just correct enough in that assumption to get into trouble. In the case of most OBD II GM alternators using four-wire regulator connections, unless the vehicle is equipped with RVC (Regulated Voltage Control), the PCM is not really in control of the charging rate. More on RVC systems later.
The PCM’s circuit marked “L” titled “Generator Turn On Signal” in many schematics only triggers the regulator to turn on. The reason the PCM now controls this signal (as opposed to the cluster’s lamp circuit) is so that it can inhibit the charging output of the alternator until the engine is fully running. Why load a cranking/low RPM engine coming to life with a hard-to-turn alternator? Once the engine is running, the alternator gets its current limited turn-on voltage from the PCM. The PCM does not control this circuit to vary the charging rate as in the case of OBD II and later Fords. The second circuit connected between the GM PCM and the four-terminal CS series alternator’s voltage regulator is marked “F” for field and titled “Generator Field Duty Cycle Signal.” This is not a PCM output, but rather a PCM input for engine load from the alternator’s work.
GM RVC Smart Charge
One very basic fact with charging systems is that the alternator needs to tailor the charge rate to the vehicle’s current demand and battery temperature. Colder batteries need charging voltages closer to 15 volts to take a charge while warmer batteries need lower voltages closer to 13 volts to not overheat. Chrysler’s tradition has been to use a battery temp sensor in the battery tray on some models, while other OEMs have relied on either the voltage regulator’s built-in thermistor or the PCM’s ECT/IAT or even OAT (HVAC’s ambient air temp) along with some engineer-provided thermal modeling software values to tailor the charge rate in order to keep a battery happy and healthy. Another basic fact with charging systems is that they are woefully underpowered in some cases. For several years now, some light-duty truck and police car applications have been heading off heavy-use-induced problems with larger alternators (or even a second alternator) so the professional rebuilder/auto electric specialty shop doesn’t have to “hop things up” to keep the battery happy. That’s wonderful, but what about the luxury cars with every gadget known to the business? For years, various OEMs including GM have been utilizing “Load Shedding” to turn off/limit power to heavy current accessories (rear defoggers and heated seats, for example) whenever the 12-volt battery begins to show signs of getting too low during times of heavy demand and low RPMs.
Having the BCM to simply “keep an eye on voltage” is only good to an extent. Since in the mid 2000s, GM’s smartest charging system dubbed “RVC” (Regulated Voltage Control) uses either a four-wire or newer two-wire Bosch alternator regulator connector and a current sensor. The current sensor looks a lot like an inductive amp clamp’s jaws permanently installed around the battery cable. Typically mounted close to the battery, these sensors show up in two versions: stand-alone (SARVC) and non-stand-alone (RVC) models. The stand alone has its own logic functions. Acting in concert with the PCM and BCM via the CAN bus, this current sensor with multiple wires actually controls the field wires connected to the voltage regulator in the alternator. These are used on mid to late 2000 GM LD trucks and SUVs. Since the BCM (big player in the accessory world) is involved in some of the charging system “thinking,” it also logs DTCs from the SARVC module. This means you must look at BCM codes along with the usual PCM DTCs when diagnosing charging system problems on these models.
Non-stand-alone RVC models (mid 2000 GM passenger cars and later 2000 LD trucks and SUVs) use a three-wire sensor, which is basically a current-sensing Hall sensor. The sensor will typically stay powered up courtesy of the BCM several minutes after the key is turned off to watch for battery drains. Both the sensor and the PCM’s output to the RVC alternator put out 5 volts at 128 Hz with a variable duty cycle.
The real magic of GM’s RVC as opposed to other PCM-controlled smart charge systems is the use of the current sensor to supplement the “intel” for charging. If the vehicle knows just exactly what the system and the battery needs to be healthy and happy it can work harder when it needs to (slightly over 15 volts) and take it easy when it can. This taking-it-easy approach to charging voltage control allows for lower outputs from the alternator, which of course means the alternator is easier to turn. That spells a fuel economy gain. Even if the gain is very small, add it to several more changes on the vehicle to help with economy and you have some noticeable gains. Besides increasing MPG, the smart charge systems can float back to just over the 12.6-volt mark to maintain battery state of charge, thus increasing the battery’s life. Light bulb life and switch life are increased as well when the voltage levels are slightly lower.
Back to basics – When the alternator won’t charge
The best alternator and fancy computer controls can’t get past a faulty battery so a thorough battery state of health test with a modern capacitance-type battery tester (think Midtronics) followed with a good old back up carbon pile load test (whenever in doubt) are always first orders of business. Next would be checking for excessive voltage drops between the battery post and alternator output terminal while the engine is running with a heavy load. Double checking for poor grounds, of course, is the proverbial fatherly advice that must always be remembered in the real world of electrical problems. Finally there is the test for diode leakage from the alternator’s rectifier bridge.
Way beyond basics – Hybrid 12-volt charging systems
HEVs and EVs are not a disappearing fad as some have presumed. Since every one of them have a 12-volt battery for 90 percent of the vehicle’s electrical functions regardless of how many volts their high-voltage batteries have, it is important to not overlook the profound differences in these charging systems. First off, all HEVs use a DC-DC solid-state converter to keep up with the vehicle’s 12-volt electrical demand and 12-volt battery charging. Using the higher level voltage (anywhere from 42 volts to 360 volts depending on the model of HEV you are working on) the charging voltage for the 12-volt system is derived from this higher voltage source via a step down transformer. Even on GM BAS systems (Belt Alternator Starter) that had only 42 volts of high-voltage power (blue cables) in mid 2000 models and now 130 volts (orange cables) in 2013 and 2014, the device that looks an awful lot like an alternator is really a high-voltage three-phase AC motor/generator. Basic testing is identical to conventional mechanical alternators: monitor charging voltage at the 12-volt battery both unloaded and loaded (accessory turn on or with a carbon pile) and look for DTCs related to the DC-DC converter, much like you would an alternator setting a DTC. Keep in mind DC-DC converters are solid state – they only need a good high-voltage battery pack as their muscle for charging, not engine RPMs. On many HEVs the gas engine doesn’t even have to be running for the DC-DC converter to put out 14.5 volts. Since there is not a conventional diode bridge for rectifying AC, HEV DC-DC converters make the perfect DC voltage for all conditions without any ripple voltage.
Even though those alternators dad taught me to rebuild still have diodes today, some experts are saying within the next few years diodes in conventional mechanical alternators on non-hybrids will be replaced with solid-state transistor controls like IGBTs (Insulated Gate Bi-Polar Transistors) now used in HEV/EV inverters. From Delcotrons to DC-DC converters – my father (who’s now 86) and I both agree: some things never change — and some things are always changing!