Dual Battery Ess Starting System
DESCRIPTION: The Engine Stop/Start (ESS) system stops and starts the engine when the vehicle is at a stop and certain conditions are met to lower Carbon Dioxide (CO2) emissions. The Powertrain Control Module (PCM) is the main module controlling operation of the ESS starting system. The system defaults to active on each ignition cycle. There is a customer selectable Stop/Start off switch that will disable the system. The switch is a smart device that sends signals over a LIN bus circuit to the Body Control Module. The Powertrain Control Module (PCM) receives the switch signal message over the CAN bus. The generic graphic below shows most of the system components and the relation between the battery, starting and charging systems. The graphic is depicting a system with a smart alternator. However the ESS starting system operates the same on vehicles equipped with a standard alternator and charging system.
There are four primary components that connect the starting, charging and vehicle electrical systems on the dual battery system. These are the main battery, auxiliary battery, power control relay and dual battery system fuse. The power control relay separates the system into two sides, the main battery side and auxiliary battery side. The generic graphic below shows a simplified diagram of the components for ease in understanding the system configuration.
IMPORTANT RELAY NAMING INFORMATION: The information referenced in the list explains the functionality of the relays and how the Powertrain Control Module (PCM) diagnostics are calibrated to view these relays when displaying faults. When diagnosing a battery/starting/charging system issue there can be confusion on the relay naming when referencing the PDC cover and wiring information due to some of the starting system relay names varying between platforms. There are two starter relays wired in series controlling starter motor operation. One is controlled by the Body Control Module (BCM), the other is controlled by the PCM.
- The first relay in series is controlled by the BCM, but is always considered to be starter relay 2 in the PCM diagnostic calibrations. When diagnosing any failure mode or PCM DTC referenced as starter relay 2, diagnostics should be focused on the relay that is controlled by the BCM.
- The second relay in series is controlled by the PCM and is always considered to be starter relay 1 in the PCM diagnostic calibrations. When diagnosing any failure mode or PCM DTC referenced as starter relay 1, diagnostics should be focused on the relay that is controlled by the PCM.
- The standalone relay that connects the main battery and auxiliary battery on the dual battery systems is referred to as the power control relay . This relay is referred to as starter relay 3 in the PCM DTC descriptions.
- The relay located in the Power Distribution Center (PDC) that is directly controlled by the PCM, to indirectly control the power control relay is referred to as the PCR control relay .
STARTING SYSTEM OPERATION: The initial engine cranking event occurs when the start button is pressed and the engine crank request is sent. Pressing the KIN, a request is sent to the RFH via a private Local Interface Network (LIN) bus for authentication. The RFH receives the request from the KIN and then looks for the valid key fob by using the LF antennas. If the RFH detects a valid key fob and the brake is engaged, the RFH sends a Controller Area Network (CAN) request to the BCM to change the ignition position to START/CRANK. The BCM controlled relay (starter relay 2) and PCM controlled relay (starter relay 1) are closed. The power control relay is momentarily energized by the PCM which separates the connection between the main battery and auxiliary battery for approximately 20-40ms (see graphic below). When separated, only the auxiliary battery is connected to the Power Distribution Center (PDC) and vehicle modules. This allows the PCM to check the State of Charge (SOC) of the auxiliary battery from the Fused B+ circuit voltage at the PCM. After the engine starts (engine speed above a calibrated threshold), the PCM turns off starter relay 1 and the starter is disengaged. Each starter relay is cycled off and on to check for a stuck relay. If both relays test good, starter relay 2 remains closed by the BCM and starter relay 1 is opened. The PCM controls engine cranking for all subsequent ESS start events by closing starter relay 1. The power control relay is momentarily energized by the PCM during all ESS crank events. This allows the auxiliary battery to maintain full battery voltage to the PDC, vehicle modules and accessories during the initial starter in-rush so that there is no noticeable difference in vehicle features to the driver. The PCM can also continue to monitor the auxiliary battery voltage on the fused B+ circuit.
On ATX equipped vehicles the PCM sends a signal to TCM to indicate it is ready for an ESS event. The TCM responds it is ready for ESS and enters a hydraulic neutral state while in an auto-stop event and stays in hydraulic neutral during the auto-start event to avoid vehicle movement during engine cranking.
On MTX equipped vehicles the PCM will allow an ESS auto-stop event with the MTX in neutral and the PCM receives the clutch signal from the clutch pedal sensor that the clutch is dis-engaged (pedal released). If the PCM does not see both inputs it will trigger a cluster message to alert the driver to either press the clutch in or shift the transmission into neutral. The PCM allows an ESS auto-start event to occur when the PCM receives a signal that the clutch is engaged (pedal pressed).
LISTED BELOW ARE THE OTHER SYSTEMS AND COMPONENTS THAT CONTRIBUTE TO ESS SYSTEM FUNCTIONALITY