Diagnostic Overview - Stop/Start System And Auxiliary Battery State Of Charge (Soc)

PCM DIAGNOSTICS:  It is important for the Power Control Relay to operate properly to allow the Auxiliary Battery to charge correctly with the engine running and be disconnected during an Engine Stop/Start (ESS) Auto-start event. The PCM runs continuous diagnostics to determine if the Power Control Relay is stuck in the On (open) or Off (closed) positions.

DIAGNOSTIC OVERVIEW OF SYSTEM: 

• Ignition Crank Sense Diagnostic:  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. When engine cranking is commanded the PCM expects to see battery voltage at the Ignition Crank Sense circuit. If no voltage is detected a fault is set.
  • If the engine is able to crank when the feedback circuit performance fault is active it eliminates the battery supply, Starter Relays and Starter as possible causes for the fault setting. If the engine does not crank when the feedback circuit performance fault is active the Ignition Crank Sense circuit is not the possible cause.
• Stuck Starter 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.

POWER CONTROL RELAY DIAGNOSTIC:  DISCLAIMER: The generic graphic below is used for reference to show how the components are configured in the system. It is not specific to any particular vehicle. Refer to the wiring and service information for the specific layout of the components for the vehicle that you are working on.

• Diagnostic 1 - Auxiliary Battery Disconnected:  During an initial key start, the Power Control Relay is commanded on (open)  and the Auxiliary Battery SOC is evaluated. If the Fused B+ voltage (C) to the Powertrain Control Module (PCM) is so low that the PCM experiences a reset during the initial key-on or engine crank event, the PCM will set P00FD and ESS will be disabled. 
  NOTE:

  This will cause a no crank condition. On the next key cycle the DTC will be active. The PCM will de-energize the Power Control Relay with an active DTC which will connect the Main Battery to the Auxiliary Battery, allowing the PCM to power up and allow engine cranking.

• Diagnostic 2 - Auxiliary Battery Depleted:  During an ESS Auto-start event, the Power Control Relay is temporarily commanded on (open) disconnecting the Main and Auxiliary Batteries from each other (E and F). This allows the Auxiliary Battery SOC to be separately monitored by the PCM. If the Auxiliary Battery voltage drops below a calibrated threshold (approximately 8.0 volts) during the ESS Auto-start event, the PCM will set P00FD and ESS will be disabled. 
• Diagnostic 3 - Auxiliary Battery is Depleting:  During the ESS Auto-start event, if the Auxiliary Battery voltage drops below a calibrated threshold the PCM will increment a counter by 1. ESS will be disabled for a short period to allow the Auxiliary Battery to charge before being re-enabled and tested again. If the voltage returns above a calibrated threshold on the next Auto-start event, the counter will decrement by 1. If the voltage is still below the calibrated threshold on the next Auto-start event, the counter will increment by 1 again. If the counter reaches 10 counts, the PCM will set P00FD and ESS will be disabled. 
• Diagnostic 4 - Power Control Relay Stuck Off:  During the ESS Auto-start event, if the PCM detects little or no difference (delta) between the Auxiliary Battery voltage and Main Battery voltage (C and IBS LIN feedback), the PCM will set P2AF6 and ESS will be disabled. 
• Diagnostic 5 - Auxiliary Battery Disconnected from Main Battery:  During normal engine operation, when the vehicle is not in an ESS Auto-start event, the Power Control Relay is commanded off (closed), connecting the Main and Auxiliary Batteries together (E and F) to allow the Auxiliary Battery to charge. If the difference (delta) between Auxiliary Battery voltage and Main Battery voltage (C and IBS LIN feedback) is more than a calibrated threshold, the PCM will set P2AF5 and ESS will be disabled. 

Courtesy of CHRYSLER GROUP, LLC

SIMPLE DIAGNOSTIC STRATEGY: 

• The diagnostic for the P00FD fault  is basically designed to detect a discharging, discharged or disconnected Auxiliary Battery any time there is a disconnect between the two Batteries. It detects this if the Fused B+ voltage to the PCM drops low enough to cause a PCM reset when disconnected from the Main/Cranking Battery. This fault will detect a faulty Auxiliary Battery, or wiring issue between the Auxiliary Battery, PDC and Battery Supply circuit (B and C) and the PCM.
• The P2AF5-Power Control Relay Stuck On description can be misleading.  Since the diagnostic is using a comparison of the voltage between the two Batteries as an indicator, it is more likely to have an issue in the wiring between either Battery and the Power Control Relay (E or F). The only way for the relay to be stuck on (open)  would be if the internal contacts were physically stuck on, or the Fused B+ output from the PCR Control Relay were shorted to voltage. Both of these conditions are possible but not very likely. A quick way to check for the Power Control Relay being stuck on (open)  would be to measure the voltage at each of the posts at the Power Control Relay with the engine running and the charging system active. If the Power Control Relay is stuck open the voltage on the Auxiliary Battery side will be lower than the voltage on the Main Battery side of the relay. If the voltage is the same on both sides, the relay is not open. If the voltage is equal to the charging system voltage at both terminals, the wiring issue is most likely on the Auxiliary Battery side of the relay. If the voltage is equal to the Auxiliary Battery voltage at both terminals, the wiring issue is most likely on the Main Battery side of the relay.
• The P2AF6-Power Control Relay Stuck Off diagnostic  is checking for the disconnection between the Batteries to occur when the Power Control Relay is energized during an initial Auto-start event. Anything that can cause the relay not to energize can set this fault,  including a wiring issue with either the Fused B+ output from the PCR Control Relay in the PDC or the Power Control Relay Ground circuit. These are the two circuits that control the Power Control Relay operation. They do not have separate control circuit faults associated with them. If there is an open or short in one of these circuits the stuck off rationality fault sets.