Theory Of Operation

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 Starter Relay 3 in the DTC descriptions refers to the Power Control Relay. 

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 see 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 Power Control Relay (PCR)  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 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 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 Power Control Relay is closed (de-energized), the Main Battery is charged through the Power Control Relay. Therefore, if the Power Control Relay 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 Power Control Relay  is temporarily energized when 12.0 volts is supplied from the output of the PCR Control Relay.  When the Power Control Relay is energized it breaks the connection between the Batteries and 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 20-40 ms 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:

• 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. An alternative method of fault detection for an open or short to ground  that is used is to monitor current draw when the internal driver is switched on. If the module does not detect any current draw it determines that the component or circuitry is open. Excessive current draw detected would indicate a short to ground.
• 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.

  Courtesy of CHRYSLER GROUP, LLC