Engine Management System (Ems) Main Relay

WARNING: This page is about a different car, the 2015 Jaguar XF, 2014 Jaguar XF, and 2013 Jaguar XF. However, it is still accessible from the selected car via links, so may be relevant.

The EMS main relay is used to initiate the power up and power down routines within the ECM. The EMS main relay is installed in the EJB.

When the ignition is turned on (power mode 6), battery voltage is applied to the ignition sense input. The ECM then starts its power up routines and energizes the EMS main relay.

When the ignition is turned off (power modes 4 or 0), the ECM maintains its powered up state while it conducts the power down routines. This can be for:

Up to 20 minutes in extreme cases, when the Diagnostic Module Tank Leakage (DMTL) system is running (NAS markets only) or the TCM is 'kept awake' because of a 'not in park' condition
Up to 5 minutes when cooling fans are required.

On completion of the power down routines the ECM de-energizes the EMS main relay.