Circuit/System Description

WARNING: This page is about a different car, the 2008 GMC Yukon and 2008 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.

The drive motor generator battery control module is also referred to as the battery energy control module (BECM). The BECM will diagnose its own systems and determine when a fault condition is present. Diagnostics and system status is communicated from the BECM to the hybrid powertrain control module (HPCM) through the Hybrid LAN communication circuit. The HPCM is the host controller for diagnostic trouble code (DTC) information.

The drive motor generator battery contains 3 high voltage (HV) contactor relays that allow the HV direct current (DC) batteries to be connected to the vehicle or safely contain the DC high voltage within the drive motor generator battery assembly. The three relays are a positive HV contactor relay, negative HV contactor relay and precharge current limit relay. These relays close and open in a specific sequence controlled by the BECM. Both the BECM and the HPCM monitor for proper system operation.

The power-up sequence for the HV DC system occurs in the following order:

The BECM closes both the negative and pre-charge contactor relays. Because the pre-charge contactor relay routes the HV current through a precharge current limit resistor, the hybrid battery terminal voltage should increase at a steady, predictable rate.
When the BECM observes that hybrid battery terminal voltage and hybrid battery calculated voltage are at about the same voltage, the BECM closes the positive contactor relay.
With both the negative and positive contactor relays closed, the BECM opens the pre-charge contactor relay.

The BECM calculates a total hybrid battery voltage based upon internal hybrid battery voltage sense leads. The BECM measures the actual voltage at the hybrid battery terminals and compares this value to the calculated voltage.

The HPCM measures the actual voltage within the drive motor generator power inverter module (PIM) and compares this value to the calculated voltage. The time it takes for terminal voltage to equal hybrid battery calculated voltage is measured and recorded. If the pre-charge event does not occur as expected, a DTC will set.

If the BECM detects 2 consecutive failed high voltage precharge events, a precharge wait time is activated by the BECM. This precharge wait time limit is to allow the Precharge circuit internal to the drive motor generator battery assembly to cool down. The precharge wait time counts down in seconds and is displayed on the scan tool. The contactor relays will not close until the precharge wait time has timed out.