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Main relay, engine management system

Main relay, engine management system



When +15 from the ignition switch is applied to the engine control unit, the main relay will be activated and +30 is supplied from main relay pin 87 via fuse 14 to the following components.


A/C compressor relay (156)

Charge air control valve (179)

Mass air flow sensor (205)

EGR solenoid valve (606)

V6 diesel control module (655)

Power stage, injectors (656)


When the ignition key is turned to OFF, +15 will disappear from pin 24 and the running-on function in the engine control module will start. The control module will turn off the main relay, cutting the power to the control module, only when the running-on function has completed. This will take about 40 seconds after the ignition key has been turned to OFF. Diagnostic communication will work after the running-on until communication with Tech 2 has ceased.

Diagnostics

If a fault occurs in the main relay circuit, one of the following DTCs can be generated:

P1652 Main relay. Short to ground.

P1653 Main relay. Short to B+.


System reaction to a fault

Engine will not start