Electrical Power Management Systems

Have you had a customer come in to your repair facility complaining that the voltage gauge on their vehicle is acting erratically? The customer will state that the gauge sometimes will read near 18 volts while, at other times, will be closer to the 12 volt mark on the gauge. You perform a charging system test and report that the system is operating properly but the customer continues to say the gauge is erratic.

Welcome to the Electrical Power Management System that several vehicle manufacturers are currently using. A traditional charging system set the generator charging points based on an internal temperature sensor. So, when the generator is cold, the system will raise the voltage output and, conversely, when the generator is hot, it will lower the charging system set points. This type of system tends to overcharge the battery on long highway trips and undercharge it on short trips.

The Electrical Power Management Systems base the generator output on the estimated battery temperature and the state of charge rather than the generator’s internal temperature. The main benefits of the Electrical Power Management System are extended battery life, switch and lamp life and improved fuel economy.

The purpose of the Electrical Power Management Systems is to maintain the battery state of charge at 80% or higher while supplying the required electrical voltage for the vehicle’s current loads. The Engine Control Module controls the generator output using a Pulse Width Modulated (PWM) signal on the generator control circuit. During normal operation the duty cycle will range between five -95 per cent. The chart below shows the approximate output voltages for the commanded duty cycle.

The main components of the Electrical Power Management System are the battery temperature/current sensor, the generator, the Engine Control Module and the Totally Integrated Power Module (TIPM)/Generator Battery Control Module (GBCM). These components operate together in order to determine the correct charging system voltage for the vehicle load and current battery state of charge. The system also uses the vehicle’s Body Control Module and Instrument Panel Cluster to warn the driver of a charging system failure either by illuminating a warning lamp or displaying a fault message in the Driver Information Display.

The engine control module receives a voltage input signal from both the generator and the TIPM/GBCM. The ECM will compare these voltages to the desired voltage values that have been programmed into its memory. If there is a difference between the actual and desired voltage, the ECM will increase or decrease the PWM signal to the generator’s output control circuit. Feedback signals are sent back to the ECM from the internal voltage regulator.

A system failure can set a diagnostic trouble code. First, begin diagnosis by checking the ECM for any current or history “P” codes. This will indicate if there is a generator, circuit or control module fault. These systems can also be diagnosed using a digital voltage meter that can monitor frequency. With the engine running monitor the generator control circuit PWM signal and battery state of charge. The generator output voltage should measure close to the values in the chart above based on the duty cycle.

If there are no trouble codes stored perform a basic charging system test. The vehicle may also require a test drive in order to duplicate the failure.

For more information on automotive technology visit CARS OnDemand training at: www.cars-council.ca