Other Articles - July - 2018

High Output: A Look at Today’s Charging Systems

The longer I’m around, the more I realize the importance of the basics. How many times have you tackled a complex vehicle issue, only to find the problem was something really basic?

With each year’s seminars, I find that we likely have some information that deals with system powers and grounds. The more complex the system, the greater the need for a proper power supply and ground.

Vehicle charging systems have become heavily taxed with the advent of more and more controllers and vehicle options. If the charging system isn’t providing the voltage and amperage required, it’ll likely cause issues with the operation of various systems, including the transmission. Understanding today’s charging system technology is key, not only to diagnosis, but also successful vehicle repair.

So let’s look at a typical charging system used in today’s GM, Ford, Honda, and Chrysler applications. Regulated voltage control (RVC) systems used on today’s vehicles are quite different from the designs used just a few short years ago.

As with systems of the past, today’s charging systems still come down to some basic components: battery, alternator, and some method of controlling the system voltage and amperage; generally a module.

One of the differences with today’s systems is that they must do much more than in the past. The charging system now helps improve fuel economy as well as charge the battery and supply supplemental power to operate the vehicle’s electronic systems.

To perform its duties, today’s charging systems have several modes of operation:

  • Battery sulfation mode — increases the charging system output after a specific time (usually around 45 minutes) if the battery state of charge (SOC) is still low.
  • Start-up mode — after start, the system typically ramps up the output to at least 14.5V for a minimum of 30 seconds.
  • Fuel economy mode — the system lowers the voltage to 13 volts or less to reduce the load on the engine and improve belt life.
  • Headlamp mode — with increased demand, the system will increase voltage output.
  • Voltage reduction mode — reduces voltage output when the battery is at 80% or greater and the electrical demand is low.
  • Windshield de-ice mode — increases output when the de-icing grids or seat heaters are turned on.
  • Charge/normal mode — designed to maintain an 80% state of charge.
  • Deceleration mode — during deceleration, the charging system output may increase. This helps accomplish a couple goals: increase the charge rate without sacrificing fuel economy, and help the vehicle decelerate, similar to hybrid regenerative braking.


Today’s charging systems work together with the controllers on the vehicle. This may be with the PCM, BCM, ECM, or a standalone module, depending on the application. Information for system operation is carried over networks such as CAN and LIN, or it may be hardwired between the alternator and a specific module.

Some alternators have internal regulators, which are controlled typically by a PWM signal from a module, such as the PCM. Chrysler and others have the voltage regulator mounted inside a vehicle computer, such as the PCM.


GM and Honda are examples of charging systems that use a device to monitor the battery voltage and current flow.

GM uses a battery sense module on some applications, which is typically mounted to one of the battery cables. The sensor module uses a Hall Effect sensor, which provides a 128 HZ PWM signal to the BCM (Body Control Module). The sensor stays powered up even when the key is off, so the BCM can identify parasitic drain.

Honda applications use an electrical load detector (ELD), which is typically located in the underhood fusebox. The function of the electrical load detector is very similar to a battery sense module.

The PCM supplies five volts to the electrical load detector. The electrical load detector grounds the circuit, providing a voltage signal between 0.3–4.50 volts to the PCM. The higher the voltage, the lower the charging system output, while a lower voltage indicates a higher charging system output.


Starting around 2004, GM introduced its regulated voltage control (RVC) charging system. You can identify the system by looking for the battery sensor or a 2-wire connection at the alternator. GM has two versions of this system: regulated voltage control and standalone regulated voltage control (SARCV) (figure 1).

The standalone regulated voltage control system wasn’t used for long. Standalone regulated voltage control systems used its own module to control the charging system, known as a generator battery control module (GBCM).

The GM regulated voltage control system can vary from 11.5 –15.5 volts. As with other systems in use today, the GM system has a fuel economy mode, which is designed to lower the charging system voltage by reducing the alternator field strength.

Technicians often diagnose this mode as a faulty charging system, only to find out that the vehicle operates the same even with new parts installed. Simply applying maximum load to the vehicle’s electrical system will cause the PCM/ECM to respond, raising charging system voltage.

GM also designed this system to identify battery sulfation. In response, the PCM/ECM will command the charging system’s voltages to increase to as high as 15.5 volts, even when there’s no electrical load. Sulfation mode will typically last less than five minutes at a time, so, as the timer expires, the system will return to normal charging operation.

The BCM is the brains of the operation, but the PCM is the module that actually controls charging system operation. The PCM controls the signal to terminal L of the alternator to control system output.

Terminal L — Terminal L is designed to control the charging system. Terminal L feeds the signal to a regulator inside the alternator. The PCM/ECM sends five volts to alternator terminal L. When the system requires an increase in output, the PCM/ECM will change the circuit duty cycle, which will cause the regulator to change the voltage set point.

The duty cycle can range from 10% to as high as 90%, with the higher duty cycle creating a higher voltage charging rate. If an open circuit occurs, the system will default to a charging voltage of 13.2–13.8V.

Terminal F — Terminal F is a duty cycle signal that the PCM/TCM monitors. The duty cycle percentage represents the operation of the alternator field. The PCM/ECM monitors the duty cycle to determine the load the alternator is placing on the engine. They then use this input for idle speed control and the alternator voltage set point.


Ford Smart Charge was introduced on the 1999 Windstar. Like the GM system, the regulator is mounted in the alternator. The PCM then controls the regulator to control system output. It’s similar to the operation of the GM system, with two main control connections to the alternator: generator monitor (GEN MON) and generator command (GEN COM). GEN MON and GEN COM are similar to terminals F and L on a GM system (figure 2).

GEN COM — Generator command controls the system output. The PCM sends a 128 HZ PWM signal to the alternator GEN COM terminal. The duty cycle varies from 3% to 95% with the high duty cycle corresponding to higher alternator output.

If the battery voltage is correct, there won’t be a signal on the GEN COM circuit. If an open circuit occurs, the output voltage default will be 13.5- 13.7 volts after engine speed exceeds 2500 RPM the first time.

GEN MON — Generator monitor sends a signal to the PCM based on the alternator load and output. The PCM sends a signal to the alternator GEN MON circuit. The regulator then grounds or ungrounds the circuit at a fixed frequency (generally 128Hz), but varies the duty cycle from 5-95% based on the alternator load. The GEN MON circuit will have a signal present anytime the engine is operating.

SENSE — the alternator receives the current battery voltage from the sense circuit.


Honda uses a dual-mode charging system on most 1990-2012 applications (figure 3). This system decreases drag on the engine while cranking and reduces charging system output (engine load) for improved fuel economy. Similar to other systems, the PCM controls the alternator. The PCM receives the system voltage and amperage demand from the electronic load detector in the fusebox.

Cranking (Low Output Mode) — during cranking, the PCM will set the output target between 12.4-12.9 volts. This mode will be active when the system meets a given set of criteria, leading the PCM to command the alternator to reduce the charging system output. Don’t misinterpret low output mode as a bad alternator.

Normal Mode — with the engine running and a specific set of criteria met, the PCM will command a target output voltage between 13.5 and 14.9 volts. With the ignition switch on, the alternator should receive a signal on the IG circuit greater than 12 volts, which it uses to activate the alternator.

PIN C — the PCM looks at the alternator PIN C to determine voltage output. With the circuit grounded, the alternator will be in low output mode.

PIN FR — The PCM supplies a 5-volt signal to PIN FR. The alternator regulator toggles the circuit to ground the varying duty cycle based on alternator load.

PIN IG — Ignition

PIN C — Computer; when output is demand is low, the PCM will ground PIN C, forcing the regulator into low output mode.

Starting in 2013, Honda introduced an updated design that uses a single control wire.


The Chrysler charging system is slightly different from the other systems (figure 4). Chrysler has used three different charging system designs over the past few years: local interconnect network (LIN), analog controls (used only with 1.4L engines), and their most popular system, which is PCM controlled.

Most Chrysler alternators don’t typically contain a regulator, the exception being the analog-controlled application. Chrysler controls the alternator externally. A PCM controls the field current. The voltage regulator electronics are housed within the PCM.

The PCM will typically receive a battery voltage input on two circuits: one from the alternator B+ sense circuit (Kelvin sense) and one from the totally integrated power module (TIPM).

Chrysler may control one or both sides of the field (power and ground). The field current flow will vary based on electrical demand, battery state of charge, and engine RPM.

Unlike some other manufacturers, unplugging a Chrysler alternator will result in zero output. Two types of field control are available with a Chrysler system: A circuit controls and B circuit controls.

A Circuit Control — controls the power to the field. It may accomplish this through the shutdown relay or it may be a direct input from the PCM to the alternator field. The PCM controls the second field terminal, which controls the field ground.

B Circuit Control — introduced in 2008. One field pin grounds internally within the alternator, while the PCM controls the other pin. Because this type of system grounds through the alternator case, mounting to the engine is critical for proper operation.

The PCM senses the battery voltage via terminal B, also referred to as Kelvin sense. The PCM controls a PWM signal to the field (F terminal) to control the field strength and alternator output.

As you can see, all of the alternators are similar in operation. Charging systems have become a major player in proper transmission operation.

Next time we’ll look at some simple steps you can use to isolate charging system issues. Until then, remember: “If it doesn’t challenge you, it won’t change you.”