Air Conditioning (A/C) (Engine management system - Td5)

Air Conditioning (A/C)

The ECM controls operation of the A/C compressor and the engine's electric cooling fan in response to requests from the Automatic Temperature Control (ATC) ECU.

A/C request When the ATC ECU supplies the ECM with an A/C request, the ECM energises the compressor clutch relay. The compressor clutch relay is located in the engine compartment fuse box. It is a four pin normally open relay. This means that the relay must be energised to drive the compressor clutch. During periods of high driver demand such as hard acceleration or maximum rev/min the ECM will disable the compressor clutch for a short time. This is to reduce the load on the engine.

The operation of the A/C request is via a switch being connected to earth. Voltage is supplied via pin 9 of connector C0658 of the ECM, at the point at which the switch is pressed the connection to the earth path is made and the compressor clutch is engaged.

The ECM provides the earth for the relay windings to allow the compressor clutch relay contacts to close and the compressor clutch drive 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 closes down the earth path, the return spring in the relay will pull the contacts apart to shut down the compressor clutch drive. Fuse 6, located in the engine compartment fuse box, provides voltage to the compressor clutch relay switching contacts. The relay windings are supplied with battery voltage from the main relay, also located in the engine compartment fuse box. The earth path for the relay windings is via pin 29 of the ECM connector C0658. When the relay is energised the output from the switching contacts is directly to the compressor clutch.

Cooling fan request The A/C fan request is an input to the ECM from the ATC ECU to request that the engine's electric cooling fan is activated to provide additional cooling for the A/C condenser.

The cooling fan relay is located in the engine compartment fuse box and is also controlled by the ECM. It is a four pin normally open relay. This means that the relay must be energised to drive the cooling fan. The cooling 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 sensor fails.

The operation of the cooling fan request is via a switch being connected to earth. Voltage is supplied via pin 23 of connector C0658 of the ECM, at the point at when the switch is pressed the connection to the earth path is made and the cooling fan is engaged.

The ECM provides the earth for the cooling fan relay windings 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 closes down 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 A/C cooling fan relay switching contacts is via fuse 4 located in the engine compartment fuse box. The relay windings are supplied with battery voltage from the main relay, also located in the engine compartment fuse box. The earth path for the relay windings is via pin 4 of the ECM connector C0658. When the relay is energised the output from the switching contacts is directly to the cooling fan motor.