Theory Of Operation

WARNING: This page is about a different car, the 2016 Dodge Durango. However, it is still accessible from the selected car via links, so may be relevant.

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 Auto Start event. The purpose of this is for the consumer electronics and vehicle subsystems to continue to function in a normal manner. The PCR is connected in-line between the Cranking Battery and the Vehicle Battery, which are connected in parallel. The PCR is normally closed when de-energized, connecting the two Batteries. When the PCR is energized, the relay opens and breaks the connection between the Batteries. The PCR is energized when 12.0 volts is supplied from the output of the PCR Control Relay in the PDC, which is being controlled by the Powertrain Control Module (PCM).

Key Start and normal driving: The PCR remains closed during all key crank events and while driving. Since the Batteries are still connected during a key start, the Vehicle Battery will have a slight voltage drop during cranking. The engine cranking and charging systems are only connected to the Cranking Battery. Therefore, the only way the Vehicle Battery can be charged is if the Batteries are connected through the normally closed Power Control Relay.

Auto Start Event: When in ESS mode, and an Auto Start crank event is initiated, the PCR is energized which opens the relay, breaking the connection and allowing the Batteries to work independently. When this occurs, the Cranking Battery is used to crank and start the engine. The Vehicle 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 electronics and subsystems.

Diagnostics: Because it is important for the Vehicle Battery to be connected to the Cranking Battery in order to charge with the engine running and disconnected during an Auto Start event, the PCM will run diagnostics to determine if the PCR is stuck in the open or closed positions.

During a key crank event, the voltage drop of the Vehicle Battery system voltage is monitored. If the voltage does not drop during cranking, the PCR is determined to be stuck open. After the engine is started and running the Vehicle Battery system voltage is monitored and compared to the charging system feedback voltage. If the Vehicle Battery voltage is below the charging system voltage by more than a calibrated threshold, the PCR is determined to be stuck open.
The PCR is monitored for a stuck closed condition during an Auto Start Event. If the Vehicle Battery system voltage drops during an Auto Start Event, the relay is determined to be stuck closed.

NOTE:

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 and is the relay that provides the power supply to operate the Power Control Relay.