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

WARNING: This page is about a different car, the 2024 Jeep Grand Cherokee L. However, it is still accessible from the selected car via links, so may be relevant.

PCM DIAGNOSTICS: It is important for the Dual Battery Switch Module (DBSM) to operate properly to allow the Main Battery to charge correctly with the engine running and disconnect the batteries during an Engine Stop/Start (ESS) Auto-start event. The PCM runs continuous diagnostics to determine if the DBSM is stuck in the On (open) or Off (closed) positions. To detect a disconnect the Auxiliary Battery voltage is monitored on pin C1-68 of the PCM and the Main Battery voltage is monitored on pin C1-87 of the PCM. The Main Battery Fused B+ circuit is used as a sense circuit for ESS diagnostics.

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.

DUAL BATTERY SWITCH MODULE (DBSM) DIAGNOSTIC:

DIAGNOSTIC 1 - Auxiliary Battery Disconnected: Prior to an initial key start, the DBSM is briefly commanded on (open) and the Auxiliary Battery SOC is evaluated. If the Fused B+ voltage 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 may cause a no crank condition. On the next key cycle the DTC will be active. The PCM will de-energize the DBSM 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 DBSM is temporarily commanded on (open) disconnecting the Main and Auxiliary Batteries from each other. 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.
NOTE:
For the 3.6L engine, the PCM uses the two Fused B+ circuits to determine the difference (delta) in voltage between the two batteries (main and auxiliary). The Fused B+ that is supplied from the main battery is a voltage sense circuit used for Diagnostic 4 and Diagnostic 5 below.
DIAGNOSTIC 4 - DBSM 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, 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 DBSM is commanded off (closed), connecting the Main and Auxiliary Batteries together to allow the Main Battery to charge. If the difference (delta) between Auxiliary Battery voltage and Main Battery voltage is more than a calibrated threshold, the PCM will set P2AF5 and ESS will be disabled.