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 power inverter control module (PIM) contains two motor control modules (MCMs). Each MCM measures hybrid battery high voltage (HV) with several internal sensors. The MCMs test for loss of isolation (LOI) between either the HV positive circuit or HV negative circuit and vehicle chassis. The MCMs test for isolation when the HV contactor relays are closed. The battery energy control module (BECM) only tests the hybrid battery assembly for HV loss of isolation when the HV contactor relays are open.

MCM LOI is detected through the use of two high-impedance resistors and voltage measuring circuitry. The two resistors are connected in series between the HV positive and HV negative circuits. The center connection of the two resistors is also connected to vehicle chassis. The MCM then measures the voltage drop across one of the resistors. Without a loss of isolation, the MCM should measure about half of the high voltage potential. This is referred to as mid-pack voltage. The Mid-pack voltage value is then doubled by the software and displayed on a scan tool as Motor 1 Isolation Voltage or Motor 2 Isolation Voltage. When a loss of isolation is present, the motor isolation voltage display will indicate voltage that is more or less than actual hybrid battery high voltage. The MCM monitors the LOI voltage measuring circuitry for correct operation.