Pulse Width Modulated Alternators

    In the 1980’s Chrysler came out with an alternator system that utilized a 
voltage regulator as part of the Powertrain Control Module (PCM). Then Ford 
and GM came out with their own version of a “Smart” alternator in the 90’s 
as well as most of the other manufacturers. These smart alternators helped 
to lower engine loads which in turn allowed for smaller displacement engines 
while still maintaining a high degree of performance.

    With the GM and Ford applications there is still a voltage regulator inside 
the alternator casing (Chrysler alternators do not use an internal regulator).  
What they all have in common is how the regulations of current flow is 
controlled. A pulse width modulated signal generated by the PCM, is used to switch the current to the rotor’s field coils on and off. The PCM, in turn, directs the duty cycle on and off as needed, based on the operating strategies programmed into its operating logic. Things like battery voltage, battery temperature, the load on the electrical system, to the engine load are all taken into account as to how much or when the alternator needs to produce energy.

    Today’s regulators can read temperature, they know how hot or cold the battery is. These temperature changes can vary the voltage settings and output of the alternator. Typically, the “set” voltage (the predetermined voltage level pre-set in the regulator) is higher in cold temperatures than it is in warmer temperatures. This allows for a more even battery recharge in the winter and reduces the chances of overcharging in the hot summer months. 

    As the alternator duty cycle changes, so does the alternator’s output. 
    An example of the various strategies are:

    Charge Mode – Bumps up the charging voltage when the battery is low or when there are unusually high loads on the system.

    Fuel Economy Mode – Lowers charging output to just under 13 volts to reduce the alternator’s load on the engine.

    Voltage Reduction Mode – Reduces charging output when the battery is fully charged and electrical loads are low.

    Start Up Mode – Momentarily fixes charging output at a steady 14.5 volts for 30 seconds following an engine start.

    Windshield De-ice Mode – Increases charging output when the defrosters are on.

    Battery Sulfation Mode – Increases charging voltage after 45 minutes if the battery is low.

    On some of these systems that utilize the Body Control Module (BCM) as the main module controlling module, the BCM signals the PCM when more or less charging output is required and the PCM responds by changing the duty cycle to the voltage regulator in the alternator. It’s a multi-step process that involves feedback from the alternator field duty cycle signal circuit, and input from a battery current sensor connected to the negative or positive battery cable at the battery. On other GM applications with “Stand Alone Regulated Voltage Control” (SARVC), the Body Control Module does not control charging. Rather, a separate module on the negative battery cable performs this function along with sensing the battery voltage, load and temperature.

     In 2004, GM began using a smart charging approach called “Regulated Voltage Control” (RVC) on some of its cars and trucks. Several versions of RVC have been used. On GM applications with Regulated Voltage Control, the Body Control Module actually oversees the charging system and determines the best charging rate for different operating modes.

​    The operating strategy of the SARVC and the RVC systems is to maintain the battery at an 80 percent or higher state of charge while modifying the charge level to optimize fuel economy. Ford, Chrysler and other auto makers use a similar operating strategy with their charging systems. How they accomplish this will vary depending on the design of the charging system, the year, and the make/model of the vehicle.

    More importantly from a mechanics point of view, the charge indicator on the dash isn’t directly connected to the alternator as was the case with the older systems. Today, the CAN system (Controller Adaptable Network) monitors and distributes the needed signals to the different components in the vehicle. Which means, the testing procedures have somewhat changed from the previous types of charging systems.

    Operating Theory

    First off, the regulator uses “Pulse Width Modulation” to switch the current to the rotor’s field coils on and off. By increasing the duty cycle the regulator increases charging output. The PCM, in turn, tells the regulator how much duty cycle is required based on its programmed operating logic, battery voltage, battery temperature, and the load on the electrical system. This allows the computer to change the charging output instantly or gradually as the situation dictates.

     Even though, alternator output tests are basically the same, to test the PCM’s influence on the alternator is a completely different matter. This will require the mechanic to dig through the manufacturer’s information to find out how that particular vehicle gets its PCM information.  A scope is extremely helpful in seeing the actual PCM signals, although a good scanner can give you the values as well. 

    Basically, the PCM sends a series of data streams to the alternator as a way of turning on the voltage regulator. The regulator in turn, sends a series of data commands streaming back to the PCM as a confirmation that it is charging. In most cases the “sent” information isn’t continuous, but is sent every few seconds. (I’ve found Ford sends their packet of data about every five seconds.) On some models this return data is about half of the output voltage. The return voltage (data) has about a 2 volt spread up or down from the output voltage which the PCM uses to determine if the charge light should be turned off or a service code should be set. Note: This is only a “generic” overview of the process. Check the manufacturer’s information for the actual process. 

    Testing the Alternator System

    For the most part, output tests are no different. Checking the voltage and amperage ranges can be accomplished with the same equipment as we’ve used before. Additionally, just to be sure of the results are accuracte, the battery must be at least 75 percent charged before load testing. A battery that tests bad needs to be replaced before continuing, as well as checking the battery cables and connection quality. The big difference is the “when” you are testing the alternator output, since the majority (or higher) charge level will be during the short window of time right after start up. The amount of possible charge after that will depend on the battery level, the temp, and any accessories that are on. Even with the windshield heater on if the battery level is full, and the temperature is within range the actual output may not be the actual output under other conditions.

    Alternator Output Quick Test

    One advantage of these new alternator designs is the ability to run the alternator without the use of the PCM. Basically, on GM and Ford type alternators you can disconnect the small connector to check if the alternator itself has the ability to create an output voltage or not. (Chrysler alternators do not use an internal regulator. All field current is directly controlled by the PCM. Disconnecting the small 2-wire plug on these alternators will result in no output.)

    GM PCM controlled alternators will charge at a default rate of approximately 13.2 to 13.8 volts with the regulator unplugged. Ford PCM controlled alternators will charge at a default rate of about 13.5 to 13.7 volts, but only if engine is rev’d over 4500 RPM for 3 seconds. If these alternators charge at the default rate the stored DTC is the result of an issue outside of the alternator (in most cases). 

    Secondary Battery Installation 

    One of the draw backs is when you are installing a second battery for a trailer or using a battery that does not conform to the manufacturer’s specifications. The PCM can’t tell the difference between an extra battery or the original one. If only one battery is connected to the battery temperature sensor the results could be inadvertently reducing or even stopping the alternator output for both batteries.  

    Since most of these type of alternators produce about 14.2 volts when the engine is cold, but decreases to about 13.2 volts as the engine warms, the secondary batteries or a non-conforming to manufacturer’s specifications battery such as an AGM type battery may not receive enough voltage to maintain a reasonable charge rate.