In idle state such as in slow traffic or during stop, the engine output is small.
In this state, driving the compressor applies overload to the engine, which results in overheating or engine stalling. So, an idle-up device is installed to make the idle speed a little higher to use the A/C.
An engine ECU that receives an A/C switch-on signal opens the idle speed control valve a little to increase intake air to make the engine revolve at the appropriate speed.
Electric Fan Control
An electric fan cools the condenser when the A/C is in operation to increase the cooling capability.
In the vehicle that cools the radiator by electric fan, combining two fans for radiator and for condenser controls the cooling capability in three levels (stop, low speed, high speed). When the A/C is in operation, the connection of the two electric fans switches to series connection (low speed) or to parallel connection (high speed) depending on the condition of the refrigerant pressure and the coolant temperature.
When refrigerant pressure is high or engine coolant temperature is high, two electric fans are connected in parallel and rotate at a high speed.
When refrigerant pressure is low or engine coolant temperature is low, two electric fans are connected in series and rotate at a low speed.
Recent models have not only the type that switches fan connection by relay (series connection, parallel connection) but also the type that adjusts the current flow into the electric fan by engine ECU and cooling fan ECU.
The connecting method between the relay and fan and ON/OFF operation of the relay vary depending on the model.