Theory Of Operation
The terminology of the relays in this procedure can be confusing. To add clarity the Power Control Relay is the relay that makes and breaks the connection between the two Batteries. The PCR Control Relay is in the PDC and is controlled by the Powertrain Control Module (PCM) using a Low Side Driver (LSD). The relay can be removable or a Printed Circuit Board (PCB) relay depending on vehicle configuration. The PCR Control Relay provides the power supply to operate the Power Control 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 and charging systems are connected directly to the Main Battery. When the Power Control Relay is closed (de-energized), the Auxiliary Battery is charged from the Main Battery. Therefore, if the Power Control Relay is stuck on (open), the Auxiliary Battery will not charge.
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 in the PDC . When the Power Control Relay is energized it breaks the connection and allows 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.
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.
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.