Functional Description - Powertrain Control Module (Pcm)

The PCM determines the target charging voltage based on several inputs. Some of the main contributors used to determine the target charging voltage are:

• Battery voltage
• Battery temperature
• Alternator sense voltage
• System electrical loads
• Engine speed
• System faults detected

The PCM directly controls the charge rate of the alternator through the Pulse Width Modulated (PWM) high side driver control circuit. The pulse width is continually adjusted to regulate the system voltage to the desired set point. For this to occur the voltage at high side field terminal of the alternator is toggled on and off by the Powertrain Control Module (PCM). During normal operation at idle or steady state, the duty cycle control typically operates between approximately 30-40 percent. The PCM will adjust the duty cycle because mechanical loads on the engine such as rpm sag of the engine or an in-rush current from a large electrical load.

The PCM diagnostic monitor compares the battery system voltage and the sense voltage from the alternator. The PCM sets faults if the alternator sense voltage is not detected or if there is a difference between the two voltages that is more than a calibrated threshold. The PCM also monitors the duty cycle control circuit for faults. To protect from overcharging the PCM will use a default duty cycle when voltage sense is lost to prevent from overcharging the battery.

When the PCM determines there is a charging system failure the PCM will send a message to the instrument cluster. The instrument cluster will signal an audible chime and illuminate the check gauges lamp (if equipped) or display a faulty charging system message to alert the driver with the engine running.