Air Conditioning Request

Air Conditioning Request

Operation of the A/C system is controlled by the engine control module (ECM). The ECM decides when to operate the A/C compressor clutch on receipt of an A/C request signal. The request signal takes the form of an earth signal applied to the ECM connector (C126-44).

The earth signal is applied from the ECM (C126-44) on a blue and red wire to the trinary switch (C279-2) low and high pressure switch contacts. For the low pressure contact to be made, a refrigerant pressure of at least 1.4 bar (20.3 psi) must be present. A refrigerant pressure of 1.0 bar (14.5 psi) or lower will break the contact. For the high pressure contact to be made, a refrigerant pressure of at least 22.5 bar (326.3 psi) must be present. A refrigerant pressure of 28.5 bar (413.3 psi) or lower will break the contact.

The earth signal is transferred from the contacts of the trinary switch (C279-1) on a slate and pink wire to the A/C thermostat (C423-1). For the thermostat contact to be made the thermostat switch must be closed. The thermostat switch contacts close at temperatures above 1°C and open at temperatures below 1°C. The earth signal passes from the A/C thermostat (C422-1) to the A/C switch (C275-4 LHD/C275-1 RHD) on a slate and pink wire.

With the A/C switch pushed in, the earth signal passes from the switch (C275-1 LHD/C275-4 RHD) on a slate and white wire to the blower motor switch (C058-4). If the blower motor switch is in the off position, the circuit will not be made and the A/C will not operate.

When the blower motor switch is operated, the earth signal passes through the switch contacts (C058-5) and is connected by a black wire to earth.