Electrical Load Management

WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.

Electrical load management is designed to maintain battery voltage and covers more than load-shed. The dash integration module (DIM) will request increases in idle speed from the powertrain control module (PCM), when the vehicle is in park or neutral, as well as turn off loads in order to manage the electrical system and preserve the vehicle electrical power availability. The DIM calculates the battery temperature, voltage and charging rate at all times while the engine is running.

The DIM calculates the battery temperature by factoring in:

the current intake manifold air temperature compared to the last temperature recorded when the ignition switch was turned OFF
the current battery voltage compared to the last battery voltage recorded when the ignition switch was turned OFF
the length of time since the last battery temperature calculation

If the calculated battery temperature is below -15°C (5°F), the PCM institutes steps to control the load. The DIM calculates the voltage of the battery by making constant voltage measurements and using the measurements to calculate the true battery voltage. If the DIM detects a low voltage, the DIM institutes steps to control the load.

The DIM calculates the net charge rate on the electrical system by making constant voltage measurements and using the measurements to calculate the charge rate in amp/hours. If the DIM detects a negative charge rate, equal to a discharge from the battery, the PCM institutes steps to control the load.

The DIM will either request an increase in the engine idle speed from the (PCM, VCM, ECM) or the DIM will turn off loads, called the load-shed function, in order to preserve the vehicle electrical system operation. The criteria used by the DIM to regulate this electrical load management are outlined below.

Fig 1: Identifying Criteria Used By DIM To Regulate Electrical Load Management

Courtesy of GENERAL MOTORS CORP.

Each load management function, either idle boost or load-shed, is discrete. No 2 functions are implemented at the same time.

During each load management function, the DIM checks the battery temperature, battery voltage and amp-hour calculations and determines if the DIM should implement a different power management function.

The highest loads on the electrical system are the resistance load of heating elements. The DIM controls the heating elements in the outside rear view mirrors, the rear window and the heated seats, either directly or by sending messages to any module controlling power to these devices.

The second highest load on the electrical system are the blowers used in the HVAC system. The DIM will send messages to the HVAC system controller that will result in reducing the blower operation on vehicles equipped with automatic HVAC systems.