Cooling fan relay (Engine management system - V8)

Cooling fan relay

The cooling fan relay is located in the engine compartment fuse box. It is a four pin normally open relay. The relay must be energised to drive the cooling fan.

The cooling fan is used to cool both the condenser in which the ATC refrigerant is held and the radiator. This fan is used especially when the engine is operating at excessively high temperatures. It is also used as a part of the ECM backup strategy if the ECT fails.

Input/Output The ECM provides the earth for the relay coils to allow the relay contacts to close and the cooling fan motor to receive battery voltage. The ECM uses a transistor as a switch to generate an open circuit in the earth path of the relay windings. When the ECM opens the earth path, the return spring in the relay will pull the contacts apart to shut down the cooling fan motor drive.

Input to the cooling fan relay switching contacts is via fuse 5 located in the engine compartment fuse box. The relay coils are supplied with battery voltage from the main relay, also located in the engine compartment fuse box. The earth path for the relay coils is via pin 31 of the ECM connector C0636. When the relay is energised the output from the switching contacts is directly to the cooling fan motor.

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

In the event of a cooling fan relay failure, the cooling fan does not work.