Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Functional Description - 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 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.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.