Theory Of Operation

WARNING: This page is about a different variant/trim than selected.

The Engine Stop/Start (ESS) system utilizes two Starter Relays in series to operate the Engine Starter Motor. The Body Control Module (BCM) controls one relay with a Low Side Driver (LSD). When the BCM controlled relay is energized, it provides voltage to the other Starter Relay controlled by the Powertrain Control Module (PCM). The PCM controls engine cranking by energizing the second Starter Relay. The Starter Motor can be disengaged by de-energizing either relay. The relay names may not match the DTC descriptions which can be confusing when diagnosing a relay control fault in the PCM. In the PCM diagnostics, the relay in the PDC that is controlled by the PCM is always considered starter relay 1 in the DTC descriptions and the relay in the PDC that is controlled by the BCM is considered starter relay 2 in the DTC descriptions.

The control circuits for both relays are monitored for circuit faults. The PCM also monitors and performs diagnostics on the output to the Starter Motor on the Ignition Crank Sense circuit. In addition to the control circuit diagnostics for each relay, the PCM also performs diagnostics to detect if one of the relays is stuck in the closed position.

Ignition Crank Sense Diagnostics: The PCM monitors the Ignition Crank Sense circuit voltage when engine cranking is commanded. The Ignition Crank Sense circuit is connected to the Starter Solenoid Feed circuit inside the PDC. During an engine cranking event the PCM expects to detect Battery voltage at the Ignition Crank Sense circuit. If no voltage is detected a fault is set.
Stuck Relay Diagnostic: After the engine is initially started, the Powertrain Control Module (PCM) will command one of the Starter Relays closed and the other Starter Relay open and monitor the Ignition Crank Sense circuit for voltage. It will then open the closed relay and close the open relay and monitor the Ignition Crank Sense circuit for voltage. If there is voltage present on the feedback circuit during either of these tests, then the relay that was commanded off when voltage was detected is determined to be stuck closed and a performance fault is set against that relay.

The Engine Stop/Start (ESS) system uses a Dual Battery Switch Module (DBSM) and two Batteries to eliminate the voltage drop in the system voltage during an Auto-start event. This allows the consumer electronics and vehicle modules subsystems to continue to function in a normal manner. The DBSM is a smart device that is connected in-line between the Main (Cranking) Battery and the Auxiliary (Vehicle) Battery. The DBSM is normally closed when de-energized, connecting the two Batteries. When the DBSM receives a signal from the PCM, it energizes and the internal relay opens to break the connection between the Batteries.

The engine starting system is connected directly to the Main Battery. The engine charging system is connected to the Auxiliary Battery side of the system. When the DBSM is closed (de-energized), the Main Battery is charged through the DBSM. Therefore, if the DBSM is stuck on (open), the Main Battery will not charge. If the fuse in the Front PDC fuse array is open, the Main Battery will no charge as well.

When ESS is active, and an Auto-start crank event is initiated , the Dual Battery Switch Module (DBSM) is temporarily signalled by the low side driver control in the PCM. The DBSM will break the connection between the Batteries allowing them to work independently. When this occurs, the Main Battery is used to crank 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. This only occurs during the first 20-40ms of initial starter engagement.

Typical Low Side Driver Operation and Fault Detection: This type of driver circuit is generally used for relay control, solenoid control or a similar type of driver device. The PCM provides a ground to operate the device when switched on. The ground could be constant or Pulse Width Modulated (PWM). The PCM also provides fault detection for the device, wiring and internal driver. Fault detection can be done by monitoring voltage on the circuit, current draw, or a combination of both. 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:

The Body Control Module (BCM) uses LIN communication with the DBSM for load shedding.