Air Conditioning Grant

The ECM uses the completed earth path as a signal and issues an A/C grant signal, when necessary changes to idle speed have been made. When the A/C request signal is removed, the ECM will delay the disengagement of the A/C compressor clutch until the necessary idle adjustments are completed. The request/grant process ensures that the idle speed remains as near constant as possible during engagement/disengagement of the compressor clutch.

Air Conditioning Grant

When the ECM decides that conditions are correct for engagement of the A/C compressor clutch, it applies an earth signal from the ECM. The grant takes the form of an earth signal applied from the ECM (C126-9) on a blue and black wire to the relay module (C173-5). The earth signal allows the A/C compressor clutch relay to be energized, closing the relay contacts.

A feed from fuse 2 (C572-2) on a brown and white wire passes to the relay module (C172-8) and to the A/C compressor clutch relay, whose contacts have been closed by the A/C grant. This feed is shared with the condenser fan relay. From the relay module (C172-2), the feed passes on a red wire to the A/C compressor clutch. The compressor clutch engages and the earth path is completed through the compressor mounting to the engine.

Air Conditioning Condenser Fan Request

When the ECM decides that conditions are correct to start the A/C condenser fan, it applies an earth signal from the ECM. The request takes the form of an earth signal applied to the ECM (C126-24).

The earth signal is applied from the ECM (C126-24) on a blue and green wire to the trinary switch (C279-4). The trinary switch normal pressure contact closes at a pressure of more than 22.5 Bar (326.3 psi) and opens at a pressure of less than 17.5 Bar (253.8 psi).

With the trinary switch normal pressure contact closed, the earth signal is transferred from the trinary switch (C279-3) to earth on a black wire. Completion of this earth path is used by the ECM as a signal to grant the A/C condenser fan.