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 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.
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 PDC and is not affected by cranking, can maintain full Battery voltage to the rest of the vehicle modules and subsystems.
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.
- During 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 tool low.
- 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 circuitry that connects the Main Battery to the Auxiliary Battery is determined to be open. The Power Control Relay is only one part of this circuit.
- 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.
The terminology on the relays in this procedure can be confusing. For clarity, the Power Control Relay is the relay that makes and breaks the connection between the two Batteries. The PCR Control Relay is controlled by the PCM using a typical Low Side Driver (LSD) and is the relay that provides the power supply to operate the Power Control Relay.
Typical Low Side Driver Operation and Fault Detection: This type of driver circuit is generally used for relay control, controlling a solenoid or a similar type of driver device. The PCM provides a ground to operate the device when switched on. The PCM also provides fault detection for the device, wiring and internal driver. For diagnostic purposes the PCM uses an internal pull down diagnostic resistor connected in series and a voltage reference (V-Ref) comparator for fault detection:
- Circuit Open and Circuit Low Detection: The PCM monitors for an open circuit and short to ground when the driver is switched off. When switched off, the available voltage passes through the device and the internal pull down resistor connected in series. The voltage at the comparator circuit should be close to Battery voltage since the majority of the voltage drop occurs through the diagnostic resistor. If the available voltage is less than the V-Ref, a fault is set. In this scenario the V-Ref would be slightly below Battery voltage.
- Circuit High Detection: The PCM monitors for a short to voltage when the driver is switched on. When the driver is switched on providing a path to ground through the transistor, the available voltage should be pulled low, near zero volts since the comparator circuit is monitoring the ground side of the device. If the voltage is greater than V-Ref, a fault is detected. In this scenario V-Ref would be slightly above zero volts.
A load that has a resistance that is below manufacturer specification, or a second load device shorted to the low side driver circuit can cause excessive current draw on the internal driver. The driver will be switched off to protect against overheating and damaging the driver. In this instance the Circuit High fault may be detected because the available voltage on the comparator circuit is above V-Ref.