Cooling system (not B8444S)

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Fig 1: Air Compressor

Courtesy of VOLVO CARS OF NORTH AMERICA.

The engine control module (ECM) controls the A/C compressor. When the air conditioning (A/C) compressor on the dashboard environment panel is activated, a request is transmitted via the control area network (CAN) to the engine control module (ECM) about the activation of the compressor. The engine control module (ECM) determines when the compressor must operate. If any of the following conditions are met the compressor is disconnected:

high engine coolant temperature (ECT)
wide open throttle (WOT)
the engine has just been started
overpressure in the air conditioning (A/C) system.

When the engine is started the engine control module (ECM) activates the air conditioning compressor for a short period even if the climate control module (CCM) has not transmitted a request for this. This is carried out for diagnosis. This check is carried out at all temperatures exceeding -6 °C.

The advantage in allowing the engine control module (ECM) to control the air conditioning compressor is that it can delay compressor activation somewhat, if the idling speed must be compensated for the extra load for example.