Category: A/C compressor control

ac temperature out of the vents

How the a/c should be operating in order to cool down the interior of your car

Check inside the vehicle:

  • The A/C climate control should be set to Max Cold Position.
  • A/C Climate control set button On.
  • Recirculation/fresh air mode set to recirculate.
  • Engine temperature should be in the normal range (not overheating)
  • Outside ambient temperature should be sensing outside actual ambient temperature.(Ambient temperature should be above 37 F )
  • Engine RPM’s should in the normal everyday range.
  • With the A/C  blower on Max, the mass of air coming out from the vents should be strong.
  • Cabin air Filter should be clean.
  • A/C evaporator coil should be free from debris and leaks.

Check the engine compartment :

  • The front of the vehicle should be free from any road debris.
  • The radiator and A/C condenser should be free from leaves, bugs or dirt.
  • With the A/C system ON the radiator/condenser fan assembly should be operating.
  • The Drive belt should be running all the accessories and in good condition.
  • The A/C compressor CLUTCH should be engaged with A/C ON.
  • The A/C compressor ECV should be modulating the hvac compressor output for max cold.
  • All A/C lines and hoses should be free from dye, oil ,ice or damage.
  • The A/C system should be properly charged with exact amount of refrigerant specified by the manufacturer.
  • Hvac box drain tube should be draining water from underneath the vehicle.



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automotive ac compressor and refrigerant lines

Sun Load or Ambient Light Sensor

The sun load sensor function as an input for the module in charge of automatically controlling the headlight operation and air conditioning system control module.

How to test the Sun Load or Ambient Light Sensor ?

A bright LED light may be used to artificially force the sensor  to adjust for light intensity and angle. With the headlights in the automatic setting position, we should see the running lights coming on and off and the automatic climate control adjusting the air temperature out of the vents. Exposing the sensor photocell to the strong led light will increase the current flow from the sun load sensor while  covering the sensor photocell will decrease the current flow signal to the hvac control module and headlight control module. With a scan tool connected to the vehicle or a multimeter we should be able to see these variations in voltage when the sensor is operating properly. The most common voltage range is between zero and five volts, but is always a good idea to compare the readings against manufacturer specs.

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automotive ac compressor and refrigerant lines

Solar Sensor, Sun load sensor or Ambient light sensor.

The solar, sun load sensor, or ambient light sensor detects the heat load of the sun, elevation and azimuth. It is used to adjust the amount and direction of the cooled air through the dash vents and to control headlights operation of the vehicle.

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what could cause high ac compressor ratio

A high compression ratio can be caused by:

Overcharged A/C system.

Inefficient condenser.

Improper oil balance

A/C condenser insufficient air flow.

Dirty evaporator core.


Auto A/C Repair service and repair near me.AC compressor test.

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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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Purpose of the AC cycling switch in my car

Purpose of the A/C cycling switch in my car ?

The A/C cycling switch in my car will control the A/C evaporator core pressure  at a point where the evaporator core surface temperature will be maintained slightly above freezing. This prevents icing of the A/C evaporator core and blockage of airflow due to ice build-up.

Where is the cycling switch located in my car ?

The switch is  mounted on a Schrader valve-type fitting on the low side of the auto ac system usually on the top of the accumulator or close to it.

How the switch work in my car ?

A valve depressor, located inside the threaded end of the switch, presses in on the Schrader valve stem. This allows the suction pressure inside the accumulator to control the operation of the switch according to the temperature and pressure of the a/c refrigerant coming out from the evaporator core. The electrical switch contacts open when the suction pressure drops to (22-25 psi).The contacts close when the suction pressure rises to (39-47 psi). When the A/C cycling switch contacts close, the A/C compressor clutch field coil is energized and it pressurizes the refrigerant inside the Air conditioning system. When the switch contacts open, the A/C compressor clutch field coil is deenergized and compressor operation stops allowing the evaporator to pick up heat from the air inside the vehicle’s cabin and preventing excessive low temperatures on the evaporator core to start freezing up.

How Do A/C switches fail ?

The A/C cycling switch uses internal moving metal part contacts to complete and open the electrical circuit that controls the a/c compressor operation and the temperature and pressure control at the evaporator core and on the low side of the automotive air conditioning system. Over time the contacts wear out and the switch must be replaced. The internal parts of the switch can’t be replaced and is just better to replace the entire cycling switch.

How do you know if your ac switch is bad ?

Some of the symptoms of a failed a/c cycling switch are:

  • A/C system not cooling properly
  • A/C system is not cooling at all
  • Air flow out of the vents greatly reduced
  • Smoke coming out from the vents
  • A/C compressor clutch over cycling


How to replace the A/C cycling switch in your vehicle ?

It is not necessary to recover the system to remove the A/C cycling switch. If you need assistance replacing the A/C cycling switch in your vehicle, Call Us Today and one of our ASE and EPA Certified mechanics will come to you. We have auto ac mechanics near you.

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