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Working with the A/C service station > < Replace components of the refrigerant circuit

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Specified values
Specified values
High-pressure side:
From the output pressure (when connecting the pressure gauge) rising up to maximum 20 bar overpressure.
Low-pressure side:
   
From the output pressure (when connecting the pressure gauge) dropping to diagram value.
A -  High pressure (measured on the service connection) in bar overpressure.
B - Low pressure (measured on the connection with valve on the AC compressor or on the catch pan) in bar overpressure.
C - Permissible tolerance range.
D - Low pressure (measured on the connection with the valve for the low pressure switch or on the service connection) in bar overpressure.
E - Permissible tolerance range.
  A87-0006
Possible deviation from the specified value Possible cause of faultRectifying fault
l  High pressure remains constant or increases only slightly (above the pressure while engine is not running),
l  Low pressure drops quickly to diagram value or lower,
l  the required cooling capacity is not reached.
Too little refrigerant in the circuit.
–  Search for leaks with the leak detector, eliminate.
–  Newly filling the refrigerant circuit.
l  High pressure is normal,
l  Low pressure corresponds to the diagram value,
l  the required cooling capacity is not reached.
  
l  High pressure is normal,
l  Low pressure is too low (see diagram),
l  the required cooling capacity is not reached.
  

  Note
If no fault is found for this complaint, clean the refrigerant circuit → Chapter.
   
Possible deviation from the specified value Possible cause of faultRectifying fault
l  High pressure increases only slightly above the pressure while engine is not running,
l  Low pressure drops only slightly,
l  the required cooling capacity is not reached.
AC compressor defective.
–  Clean the refrigerant circuit → Chapter.
–  Replace AC compressor.

   
Possible deviation from the specified value Possible cause of faultRectifying fault
l  High pressure increases above the specified value,
l  Low pressure drops quickly to diagram value or lower,
l  the required cooling capacity is not reached.
Constriction or blockage in the refrigerant circuit.
–  Feel the temperature gradient of the refrigerant circuit with the hand.
l  A temperature gradient is noticed on a component:
–  In case a hose line or a pipe is bent or constricted, replace this component.
–  In case of a blockage, clean the refrigerant circuit → Chapter.
l  If no fault is found:
–  Clean the refrigerant circuit → Chapter.
l  High pressure and low pressure is normal at first, after a certain period of time:
–  the high pressure increases above the specified value,
–  the low pressure drops to diagram value or lower,
–  the required cooling capacity is no longer reached.
Humidity in the refrigerant circuit.
–  Check catch pan (with dessicator) and throttle and replace if necessary, then evacuate the refrigerant circuit for at least 3 hours (see notes).
–  Clean the refrigerant circuit → Chapter.
l  High pressure and low pressure is normal at first.
l  After a longer operating time, the low pressure drops remarkably low (evaporator ices up).
  

  Note
t  If problems arise due to humidity in the refrigerant circuit only after a longer operating time or only occasionally (the low pressure drops below the specified value and the evaporator ices up), it is adequate to replace the dessicator (the volume of refrigerant oil must be adapted → Chapter). Subsequently the refrigerant circuit must be evacuated for at least 3 hours.
t  First of all, it is not required to clean the refrigerant circuit for this complaint (to flush with refrigerant R134a or to blow through with compressed air and nitrogen), as normally only a small amount of humidity is present in the system which can be removed through a longer period of evacuation.
   
Possible deviation from the specified value Possible cause of faultRectifying fault
l  High pressure is normal or too high,
l  Low pressure is too high (see diagram),
l  the AC compressor makes noises (especially after switching it on),
l  the required cooling capacity is not reached.
Too much refrigerant in the circuit.
–  Drain the refrigerant from the refrigerant circuit.
l  The volume of refrigerant suctioned out corresponds approximately to the specified filling capacity:
–  Replace AC compressor.
l  The volume of refrigerant suctioned out is considerably more than the specified filling capacity:
–  Newly filling the refrigerant circuit.
–  Repeat check.

   
Possible deviation from the specified value Possible cause of faultRectifying fault
l  High pressure and low pressure is normal,
l  the required cooling capacity is not reached.
Too much refrigerant oil in the circuit.
–  Drain the refrigerant from the refrigerant circuit.
–  Clean the refrigerant circuit → Chapter.
l  High pressure and low pressure is normal,
l  the AC compressor makes noises (especially after switching it on),
l  the required cooling capacity is reached.
 
–  Newly filling the refrigerant circuit.
–  Repeat check.

  Note
t  Over-filling with refrigerant oil can occur if, for example, the AC compressor was replaced without adapting the volume of refrigerant oil.
t  In case too much refrigerant oil is present in the refrigerant circuit, the AC compressor must be emptied and the catch pan with dessicator must be replaced. After the refrigerant circuit was cleaned → Chapter, fill the correct volume of refrigerant oil into the circuit.
   

Working with the A/C service station > < Replace components of the refrigerant circuit