Theory Of Operation

General Operation:  The Engine Stop/Start (ESS) system uses a Power Control Relay (PCR)  and two Batteries to eliminate the voltage drop in the system voltage during an AutoStart event. The purpose of this is for the consumer electronics and vehicle modules subsystems to continue to function in a normal manner. The Power Control Relay is connected in-line between the Main (Cranking) Battery and the Auxiliary (Vehicle) Battery. The Power Control Relay is normally closed when de-energized, connecting the two Batteries. When the Power Control Relay is energized, the relay opens and breaks the connection between the Batteries. The Power Control Relay (PCR)  is energized when 12.0 volts is supplied from the output of the PCR Control Relay in the PDC  . The PCR Control Relay is controlled by the Powertrain Control Module (PCM) using a Low Side Driver (LSD).

Key Start and normal driving:  The engine starting and charging systems are connected in parallel to the Main Battery and Auxiliary Battery and charges both Batteries simultaneously. The starting and charging systems are connected directly to the Main Battery. The starting and charging systems are connected to the Auxiliary Battery when the Power Control Relay is de-energized (closed). Therefore, if the Power Control Relay is stuck on (open), the Auxiliary Battery will not charge. The Power Control Relay remains closed during all non-AutoStart crank events and while driving.  With the Power Control Relay closed during a non-AutoStart  crank event, the Auxiliary Battery is connected to the starting system and will have a voltage drop during cranking. Before allowing a Key Start, the Power Control Relay is opened to diagnose a disconnected or depleted Auxiliary Battery.

AutoStart Event:  When in ESS mode, and an AutoStart crank event is initiated  , the Power Control Relay is temporarily energized which opens the relay, breaking the connection and allowing the Batteries to work independently. When this occurs, the Main Battery is used to crank and start the engine. The Auxiliary Battery, which is now solely connected to the Power Distribution Center (PDC) and is not affected by cranking, can maintain full Battery voltage to the rest of the vehicle modules and subsystems with the exception of the Electric Power Steering (EPS) Module.

Diagnostics:  It is important for the Power Control Relay to operate properly for the Auxiliary Battery to charge correctly with the engine running and be disconnected during an AutoStart event, the PCM will run diagnostics to determine if the Power Control Relay is stuck in the On (open) or off (closed) positions.

• Disconnected Battery Detection: Before allowing a key crank event, the voltage supply from the Auxiliary Battery to the PCM is monitored. If the voltage drops low enough to cause a PCM reset, the Auxiliary Battery state of charge is determined to be too low.
• There are two checks before allowing the key crank: 1. If the PCM resets, the Auxiliary Battery is disconnected. 2.If the PCM doesn't reset and the voltage is below calibration, then deplete the Auxiliary Battery.
• After the engine is started and running the Auxiliary Battery system voltage is monitored and compared to the charging system feedback voltage. If the Auxiliary Battery voltage is below the charging system voltage by more than a calibrated threshold, the Power Control Relay is determined to be stuck open.
• The Power Control Relay is monitored for a stuck closed condition during an Auto Start Event. If the difference between the Auxiliary Battery voltage and the Main Battery voltage is not more than a calibrated threshold during an Auto Start cranking event, the Power Control Relay is determined to be stuck closed.