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 HVAC 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.
Hvac Control Head
The hvac 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.
The AC actuator moves air distribution doors to two or more positions to direct air flow for heating and A/C vents. The AC actuator is located underneath the dashboard attached to air distribution doors in the hvac box.
Doors can control hot air/cold air blending, recirculated/fresh air control, or dual zone splitting. The motors can move the doors into many different positions, or just two.
What are the symptoms of a bad actuator?
Air coming out from the wrong vents, clicking noises coming from behind the dashboard or not being able to control the temperature(Climate control set on cold position but blowing hot air) or re-circulated or fresh air mode in your car is a sign of a bad air door actuator motor. Exhaust or outside road fumes inside the cabin.
What are the symptoms of a bad temperature blend door actuator?
Air conditioning temperature can’t be regulated. The driver, passenger or both, depending on the design won’t be able to control the temperature on their climate zone. Decreased air conditioning performance due to improperly mixed of hot and cold air inside the hvac box.
How to test the blend door actuator motor?
There are some simple tests that could be done in order to determine if the vehicle is having issues with the blend door actuator:
If you can’t set the temperature from the head control set selector, there is a problem with the blend door actuator, the blend door, the head control set or the circuit connecting them.
For every movement of the knob on the climate control set corresponds a movement on the blend door. If there is no movement, there is a problem with the blend door actuator, the blend door, the climate control set or the circuit connecting them.
As the blend door open or closes to the heat or cold position it opposes the flow of air coming from the blower motor and makes a change in the intensity of the air noise. If you can’t hear this change in air noise intensity there is a problem with the blend door actuator, the blend door, the climate control set or the circuit connecting them.
After car startup A/C blows cold then sometimes blow hot.
The low-pressure / temperature sensor will interrupt compressor operation if system pressure drops to the point that a loss of refrigerant charge to the compressor occurs.The high-pressure cut-out sensor interrupts compressor operation in case of extremely high system pressure.
Cycling clutch pressure switch controls compressor pressures operation.It keeps the refrigerant pressure between 47 psi-26 psi.This cycling action maintains the evaporator at 33 °F.
Some air conditioning systems use a thermostatic clutch cycling switch that cycles the compressor on and off in relation to evaporator outlet temperature.
Diagnosing a bad or failing Ambient Temperature Sensor.
The air conditioning “AUTO MODE “ won’t work
Inconsistent cooling or not cooling at all.
A/C compressor inoperative
Incorrect reading on temperature gauge. Usually a default negative temperature.
The most common variable resistor is a thermistor.Thermistors change resistance based on temperature changes. There are two different styles of thermistor: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient).The difference is in how they react to temperature changes.
As temperature rise:
-PTC thermistor-resistance increases
-NTC Thermistor-resistance decreases
The NTC thermistor is more common than the PTC thermistor.
Here are some examples of thermistor in automotive circuits:
Engine temperature sensor
Evaporator temperature sensor
Ambient temperature sensors-In-car and outside sensors
A/C High Side Refrigerant Temperature Sensors
A/C Low Side Refrigerant Temperature Sensors
Sun Load Sensors (Not all sun load sensors are thermistor some are photoresistors.A photoresistor measures the amount of light hitting it .As the light intensity increases , sensor resistance decreases. That’s the why a photo resistor works so well as a sun load sensor. Some manufacturers use a photodiode. A photodiode works in a manner similar to the photoresistor.
What does the ambient temperature sensor do?
It prevents the compressor from engaging when ambient temperatures get too low. The oil that resides inside of a compressor can get very cold during the winter months. If the compressor’s clutch is energized while the oil is cold and thick, improper compressor-lubrication would result.
Where the ambient temperature sensor is usually mounted?
Mounted to the radiator support, in front of the condenser where it can sense outside temperatures without being affected by engine heat.
How does ambient temp sensor work?
A lot of air conditioning systems are supposed to run when you turn the defroster on, to help remove moisture from the air. But when outside temperatures are very low refrigerant oils won’t flow and lubricate the system properly.
The ambient temperature sensor prevents the compressor from running when it’s too cold for the system to work safely. These switches open when the temperature drops below a preset temperature,usually below 32° F
The A/C cycling switch cycles the compressor clutch on and off to maintain evaporator temperatures above freezing.
Where is the clutch cycling switch located?
Thermostatic cycling switch: Some vehicles mount it in the engine compartment, near the evaporator case. Others mount theirs under the dashboard near the evaporator.
Pressure cycling switch: The A/C pressure cycling switch is located in the low-side. Some vehicles mount theirs on the accumulator, others on the lines.
Some vehicles use a temperature-controlled cycling clutch switch to maintain evaporator temperatures. The switch opens or closes the circuit to the compressor clutch, based on evaporator temperature. The switch has a temperature sensing bulb that sits on the evaporator or along the evaporator outlet line.
The bulb is filled with a gas or refrigerant similar to one in the system. If the evaporator core gets too cold, the pressure of the refrigerant in the sensing bulb drops. That allows the contacts in the cycling clutch switch to open, which releases the compressor clutch and stops the compressor.
As soon as the compressor stops turning, the pressures in the system begin to equalize. The evaporator pressure increases and warms up. That warms the sensing bulb enough to let the cycling clutch switch close. The compressor clutch engages, and the cycle begins again.
Cycling the compressor clutch based on evaporator temperature is just one way to control evaporator temperature. Since refrigerant temperature can be controlled by adjusting the pressure, some manufacturers use a pressure switch on the accumulator to monitor evaporator pressure and cycle the compressor clutch.
Low-side refrigerant pressure sits on one side of a diaphragm, spring pressure on the other. When refrigerant pressure exceeds a predetermined value about 45 PSI (310 kpa) it overcomes the spring pressure, forcing a set of electrical contacts to complete the circuit, and energizing the compressor clutch.
As the compressor runs, low-side pressure drops. When low-side pressures drop below about 23 PSI (159 kpa), the spring tension overcomes refrigerant pressure; the contacts in the switch open, shutting the compressor clutch off. Pressures rise, and the cycle begins again.
Electronic cycling clutch switch
Electronic cycling switch monitors evaporator temperature, and turns the compressor on and off to maintain evaporator temperatures over 32° F (0ºC).
How do I know if my AC cycling switch is bad?
The AC system is not blowing as cold as it used to, that may be a sign that the switch has failed, or is beginning to fail
Your AC system will completely cease to blow cold air without the cycling switch to activate the compressor’s clutch.
The A/C compressor cycles on and off too often and out of pressure specifications. You can hear the clicking noise of the A/C compressor clutch engaging and disengaging.
Engine coolant temperature sensor (ECT) and,ECM air conditioner compressor control.
If the coolant temperature rises to a predetermined level, the ECM will disengage the compressor clutch. This helps an overheating engine by allowing the condenser to cool down and by removing the mechanical load of driving the compressor.
What is the function of vacuum controlled actuators.
Vacuum motors move the various doors to direct the airflow to the proper vents. Vacuum air doors are controlled by a selector switch in the control head. This unit directs the vacuum signal to the proper motor.
Vacuum system malfunction.
If the selector switch is defective, vacuum will not be supplied to the proper door. If there is a leak in the vacuum system, the door wills not move properly due to no or a low vacuum signal. A vacuum leak anywhere in the control circuit will affect the operation of the entire system.
How to test a vacuum controlled door actuator.
If the system is sound, the vacuum motor should be checked. When testing a vacuum actuated door, the vacuum gauge readings should remain steady for at least one minute. If the gauge reading drops slowly, the actuator is leaking and should be replaced