3-10
04/14/2009 NP10-V8JLR: AJ133 5.0-Liter DFI V8 Engine
Technical Training
Relays
Engine Management System
RELAYS
Main Relay
The main Engine Management System (EMS) relay is
not a main power input; it is used to initiate the power-up
and power-down routines within the ECM.
This input comes from the engine junction box (EJB).
When the ignition is turned on 12V is applied to the igni-
tion sense input. The ECM then starts its power-up rou-
tines and turns on the ECM main relay, the main power
to the ECM and its associated system components.
When the ignition is turned OFF, the ECM will maintain
its powered-up state for several seconds (or up to 20
minutes in extreme cases when cooling fans are
required) while it initiates its power-down routine and,
on completion, will turn off the ECM main relay.
The main relay is located in the EJB. The operation of
the main relay is controlled by the ECM, which provides
a ground path for the main relay coil, energizing the
relay and closing the relay contacts.
The main relay supplies battery voltage to the following
engine sensors and actuators:
• Throttle position (TP) sensor (through ECM)
• Fuel injectors
• Ignition coils
• Coil capacitor
• CPS solenoids
• All heated oxygen sensors
• Evaporative emission (EVAP) canister purge valve
• Diagnostic Monitoring of Tank Leakage (DMTL)
Failure Modes
• Relay drive open circuit
• Short circuit to battery voltage or ground
• Component failure
Failure Symptoms
• Engine will not start
Starter Relay
The starter motor relay is located in the EJB. Operation
of the starter motor relay is controlled by the ECM,
which provides a ground path for the relay coil, energiz-
ing the relay and closing the relay contacts. When the
relay contacts are closed, battery voltage is supplied,
through the starter motor relay, to the starter module
solenoid coil.
The starter solenoid is energized and connects the starter
motor with a direct battery feed to operate the starter
motor.
Once the engine has started, the ECM removes the
starter motor relay ground, opening the relay contacts
and terminating the battery feed to the starter solenoid,
which in turn stops the operation of the starter motor.
NOTE:
Diagnose using Jaguar Land Rover approved
diagnostic equipment.
