car's air conditioning system control

car’s air conditioning system control

Car’s air conditioning system control.

For many years, the air conditioning compressor was controlled directly from the dashboard; when the A/C button was pushed, voltage would typically travel from the control head through the evaporator switch, the protection switch and through the compressor clutch to ground.
But that changed with the advent of on-board control computers, and since the 1990s, the compressor clutch on most passenger vehicles has been controlled by or through the engine computer. Today car’s air conditioning system controls are being integrated into the vehicle’s total electrical system.
In modern control circuits, engaging the dash button only sends a request signal to the compressor clutch. When the request signal is received, the control module will check a number of data values to determine if the compressor clutch should be enabled. If all conditions are correct, the control module will ground the A/C control circuit relay which energizes the ac compressor clutch.

Car Air conditioning system Control Head

The A/C Control Head (Climate control set or A/C climate control) is used for operator input. It may be an electronic module that contains a microprocessor, or simply contain switches and knobs.
The control head A/C request signal may be sent to the body control module (BCM) and then to the power train control module (PCM) to activate the A/C compressor clutch.
When the operator selects A/C on the control panel, an A/C request is sent to the PCM (Power train Control Module).If the pressure/temperature in the Air conditioning system is within specifications, and the engine operating conditions are within the programmed parameters, the PCM will allow the compressor clutch to engage.
The PCM will constantly monitor inputs from various engine sensors, including coolant temperature, throttle position, intake air temperature, and power steering system pressure, but is not limited to this only.
The PCM will also monitor A/C system pressure using various sensors and switches. These inputs will determine if system pressure is in the correct range to allow the air compressor clutch to engage, and if the cooling fans should be operating


Relays are necessary on computer controlled systems. Computers are not designed to carry large current loads; the control circuit in the relay uses low current to close the load contacts. This small current flow allows the systems to use smaller gauge wires, and micro switches that do not have to carry large amounts of current. This saves weight and helps to reduce tailpipe emissions, as well as improving fuel economy.
The clutch will be controlled by a relay or a field effect transistor (FET).In most cases, the PCM will control air compressor clutch engagement through a relay. The control side of the relay is activated by a circuit in the PCM. When the control side is closed, the contacts in the relay close, and voltage is fed to the clutch.
Relays are remoted-controlled electrical switches. They are used to connect source voltage to an electrical load when the control circuit in the relay is energized.
When the control circuit is completed, and electromagnet pulls the movable contacts of the load circuit closed. When the control circuit is open, a spring opens the contacts on the load side.

Some automotive computer inputs. Engine management operation.

  • Potentiometers
  • Variable Resistors
  • Voltage Generators
  • Switches
  • Switches
  • Serial Data
  • Frequency Generators
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