Category: A/C electrical & electronic systems

Nissan vehicle interior. Sun load sensor.

How does the sun load sensor in my car work?

Varying amounts of exposure to sunlight will modify its electrical return signal to the programmer to adjust system output toward cooler air; less sunlight will cause the programmer to adjust system output toward warmer air. Some multizone climate control systems have more than one sun load sensor.

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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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ambient temperature sensor

Diagnosing a bad or failing Ambient Temperature Sensor.

  1. The air conditioning AUTO MODE won’t work
  2. Inconsistent cooling or not cooling at all.
  3. A/C compressor inoperative
  4. 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

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The purpose of the cycling switch in a cycling clutch system is to control the evaporator temperature above the freezing point by controlling the the operation of the a/c compressor clutch. Based on pressure input from the low side of the a/c system the cycling switch will open power to the a/c compressor clutch coil when the pressure/temperature is so low that could cause the evaporator to freeze up. Once the pressure/temperature has stabilized the cycling switch closes its contacts again allowing power to the air conditioning compressor clutch coil and compressor operation is allowed until it lowers the pressure/temperature near the evaporator freezing point, where the cycle repeats again.

auto ac repair near me

Thermal switches are used on both the low and high side of AC systems. A Thermal switch may be connected in series with the compressor clutch.

A/C compressor cycling control .

A thermostatic switch de-energizes the clutch and stops the compressor to keep the evaporator from freezing up. When the temperature of the evaporator approaches the freezing point (or the low setting of the switch), the thermostatic switch opens the circuit and disengages the compressor clutch. The compressor remains inoperative until the evaporator temperature rises to the preset temperature, at which time the switch closes and compressor operation resumes.

A/C Compressor temperature control.

A thermal limit switch may be mounted on or near the compressor and is used to stop compressor operation before the temperature of the compressor reaches a point of damage.


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ac compressor oil

Car A/C compressor cycling on and off

The car a/c compressor cycles on and off according to the air conditioning system temperature and pressure. Normal on and off cycling happens at a manufacturer predetermined a/c system temperature and pressure. Car ac compressor cycling is one of the logics used by car manufacturer to protect the car a/c system by preventing excessive high pressure or excessive low temperature.

Rapidly car ac compressor cycling on and off.

Rapidly car a/c compressor cycling on and off could indicate a problem in the car air conditioning system. As a safety measure the compressor cycling clutch devices interrupt power to the compressor clutch coil to avoid damage to the compressor, evaporator coil freezing up or ac hoses damage. Any car ac compressor operation under extreme parameters of temperature and pressure will affect the  compressor oil quality and circulation back to the compressor and could exceed the design work pressure and temperature for the ac components in the vehicle.

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How can i make my car ac colder

What are the symptoms of a bad fan clutch?

We need the fan clutch to cool down the vehicle ac condenser in order to allow the refrigerant condensing and subcooling process to happen as efficiently as possible. A bad fan clutch will decrease the vehicle a/c performance.

Symptoms of a bad fan clutch and air conditioning impact.

  • Excessive high a/c system pressure.
  • Hvac compressor safety relief valve venting refrigerant.
  • Low a/c system refrigerant condition.
  • A/C system underperformance.
  • Air conditioning hose and lines damage.
  • A/C compressor damage.
  • Subcooling and de-superheat inefficient
  • Overall poor a/c performance.

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ac vacuum controlled door

AC vacuum controlled door.


The air delivery system controls the movement of heated and cooled air into and throughout the passenger compartment. The Hvac housing is the plenum unit located and beneath the instrument panel. It contains the evaporator, heater core, the blower motor assembly, blower speed control device and the vacuum controlled door actuators.

The plenum houses the air doors and door actuators that direct airflow to the correct ductwork. The doors may be controlled by cables, vacuum, or electrical actuators. Single zone systems are usually equipped with three doors.


hvac box showing harness ,door actuators and vents.

AC vacuum-controlled door actuator main fault

Automotive ac climate control panel levers may operate a rotary switch to select the various vacuum or air routings to the motors that control A/C modes. The main fault experienced with vacuum-controlled door actuators is vacuum leaks.


Do you need assistance with the ac vacuum-controlled door in your vehicle?

We have auto ac mechanics in your area in Miami-Dade County. Some ac door actuators may be located underneath the dashboard of the vehicle and to get access to them could be labor intensive. Give Us a Call Today!

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What does an ac actuator do ?

What does an A/C actuator do ?

An A/C door actuator is a small  motor that the auto ac air conditioning delivery system uses to control the movement of heated and cooled air into and throughout the passenger compartment and to give the driver control over the position of the vents from where the cooled or heater air will come out.

Where is the A/C actuator located in my car ?

The plenum houses the air doors and door actuators that direct airflow to the correct ductwork. The doors may be controlled by cables, vacuum, or electrical actuators. Single zone systems are usually equipped with three doors

The Hvac housing is the plenum unit located and beneath the instrument panel. It contains the evaporator and heater core, and the blower motor assembly and the blower speed control device.

Do you need assistance with the A/C door actuator in your vehicle ?

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