Engine management system - Td5 (Main relay): Description and operation

Main relay

The main relay is located in the engine compartment fuse box and supplies battery voltage to the following: l ECM. l MAF. l Fuel pump relay. l Cruise control master switch. l Cruise control RES switch. l Cruise control SET+ switch.

It is a 4 pin normally open relay and must be energised to provide voltage to the ECM.

Input/Output The earth path for the main relay is via a transistor within the ECM. When the earth path is completed, the main relay energises to supply battery voltage to the ECM. Interrupting this earth path de-energises the main relay, preventing battery voltage reaching the ECM.

Input to the main relay is via pin 1 of connector C0632, located at the engine compartment fuse box. Output from the main relay is via fuse 1 to the ECM connector C0658 pins 3, 22 and 27. The earth path is via pin 21 of ECM connector C0658.

The main relay can fail in the following ways: l Relay open circuit. l Short circuit to vehicle supply. l Short circuit to vehicle earth. l Broken return spring.

In the event of a main relay failure any of the following symptoms may be observed: l Engine will crank but not start. l If the engine is running it will stop.

For the ECM start up to take place the ignition 'on' (position II) voltage must be greater than 6.0 volts.