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Fault Finding if readout does not match specification > < Air conditioner cooling output

A1

Checking air conditioner cooling output - vehicles with manually controlled air conditioner
Checking air conditioner cooling output - vehicles with manually controlled air conditioner
Special tools and workshop equipment required
t  Vehicle diagnostic tester
t  Commercially available thermometer (for temperature measurement; if applicable use thermometer with 2 probes for simultaneous measurement of temperature e.g. on right and left)
Test requirements
l  Ambient temperature above 15 °C
l  Radiator and condenser clean (clean if necessary)
l  Air conditioner compressor poly V-belt properly fitted
l  Air conditioner compressor actually being driven via poly V-belt and pulley
l  All air ducts, covers and seals OK and properly installed
l  Air flow through dust and pollen filter not impeded by contamination
l  Air intake (in fresh-air and air recirculation mode) not impeded by contamination or retrofitted components
l  Vehicle not exposed to sunlight
l  Engine warm
l  Event recorder of operating unit, air conditioning system control unit -J301- interrogated and erased, basic setting performed in „Guided fault-finding“ function → Vehicle diagnostic tester
   
–  Switch on the ignition and make the following settings on the operating unit, air conditioning system control unit -J301-:
l  Open all dash panel vents.
l  Close the front lid.
l  Set the rotary temperature control -1- to the „Cold“ stop.
l  Air conditioner compressor on - lamp in AC button -4- lights.
l  Air recirculation mode on - lamp in Air recirculation button -5- lights.
l  Set the rotary air distribution control -3- to „Dash panel vents“.
l  Set the rotary fresh air blower control -2- to speed „4“.
–  Start the engine and check the following functions:
l  Operation of radiator fan -V7- (and radiator fan 2 -V177-) (actuation and speed governed by pressure in refrigerant circuit and engine temperature).
  Note
t  Depending on the design of the operating unit, the radiator fan -V7- is switched in immediately or only as of a specific pressure in the refrigerant circuit (currently as of a pressure of 9 bar). To check actuation of the radiator fan, „read measured values“ in the „Guided fault-finding“ function → Vehicle diagnostic tester.
t  Depending on the vehicle model, a radiator fan 2 -V177- may also be fitted → Rep. gr.19.
l  Fresh air blower -V2- running at maximum speed
  A87-10864
l  Switching of air conditioner to air recirculation mode, fresh air intake closed and air recirculation intake -1- open (air is drawn in by the fresh air blower -V2- from the passenger compartment beneath the dash panel / glove compartment).
l  Operation of radiator fan -V7- (and radiator fan 2 -V177-) (actuation and speed governed by pressure in refrigerant circuit and engine temperature).
  Note
Depending on the vehicle model, a radiator fan 2 -V177- may also be fitted → Rep. gr.19.
Checking
–  Measure the ambient temperature (must be above 15 °C).
–  Close the front lid, doors and windows.
–  Open all dash panel vents.
–  Switch on the ignition.
  A87-10863
–  Press the AC button -4- to switch off the air conditioner at the operating unit.
l  The lamp in the AC button does not light.
–  Start the engine.
–  „Read measured values“ in air conditioner „Guided fault-finding“ function → Vehicle diagnostic tester.
Check the display in the display zones:
–  Read the actuation of the air conditioner compressor regulating valve -N280-.
l  The air conditioner compressor is deactivated, the display is 0 %.
–  Read the measured value of the high-pressure sender -G65-.
l  Pressure in refrigerant circuit is identical to or above value in table below (depending on ambient temperature).
  A87-10864
Ambient temperature in °CPressure display (in bar absolute)
154,0
205,0
256,0
307,0
358,0

  Note
t  At absolute pressure, 0 bar corresponds to an absolute vacuum. Normal ambient pressure thus corresponds to roughly 1 bar absolute. On the scales of most pressure gauges, 0 bar corresponds to an absolute pressure of 1 bar (can be seen from „-1“ mark below „0“).
t  At present, the measured values for this vehicle are only displayed as whole numbers. The display fluctuates between two values if the measured pressure is between the two.
t  The pressure in the refrigerant circuit depends on the ambient temperature. Due to the radiation of heat by components (e.g. the radiator), the pressure displayed with a warm engine is slightly higher than that given for the corresponding ambient temperature.
If the displayed pressure is lower than that given in the table:
–  Check the signal of the high-pressure sender -G65- → Chapter.
–  If no fault is found at the high-pressure sender -G65-, there is not enough refrigerant in the circuit. The vehicle is to be taken to a workshop equipped with the necessary tools where the work can be performed by appropriately qualified personnel.
If the pressure in the refrigerant circuit is OK:
   
–  Switch on the air conditioner at the operating unit as follows:
l  Set the rotary temperature control -1- to the „Cold“ stop.
l  Set the rotary air distribution control -3- to „Dash panel vents“.
l  Air conditioner compressor on - lamp in AC button -4- lights.
l  Air recirculation mode on - lamp in Air recirculation button -5- lights.
l  Set the rotary fresh air blower control -2- to speed „4“.
–  „Read measured values“ in air conditioner „Guided fault-finding“ function → Vehicle diagnostic tester.
–  Check the display in the display zones:
l  The current calculated by the air conditioning system control unit -J301- flowing via the air conditioner compressor regulating valve -N280- is displayed as being greater than 0.3 A (air conditioner compressor switched on).
l  The pressure displayed rises above the value with the air conditioner compressor switched off.
  Note
t  If the pressure displayed does not change and actuation of the air conditioner compressor is OK, check again whether the calculated current is actually flowing via the air conditioner compressor regulating valve -N280- and whether the air conditioner compressor is actually being driven. If the current is actually flowing via -N280- and the air conditioner compressor is being driven, there is a fault in the refrigerant circuit. The vehicle is to be taken to a workshop equipped with the necessary tools where the work can be performed by appropriately qualified personnel → Air conditioner with refrigerant R134a (there may be problems with air conditioner compressor control). Inform the workshop of the problem detected.
t  The air conditioner compressor regulating valve -N280- is actuated by the air conditioning system control unit -J301- such that the temperature of the air downstream of the evaporator reaches the specified value (approx. 2 to 5 °C).
t  After starting the vehicle, a value greater than 0.55 A is displayed for actuation of the air conditioner compressor regulating valve -N280- depending on the measured temperature, engine speed and electrical system voltage. As soon as the temperature measured by the evaporator output temperature sender -G263- approaches the specified value, actuation is cancelled and the air conditioner compressor output thus reduced.
t  Under certain operating conditions, residual moisture in the refrigerant circuit may lead to the formation of ice at the air conditioner compressor regulating valve -N280- (and at the expansion valve). Such ice formation impedes air conditioner compressor control. The evaporator is excessively cooled and ices up. Icing-up of the evaporator may give rise to various problems → Chapter.
t  If no or an insufficient current is displayed in the measured value block for actuation of the air conditioner compressor regulating valve -N280-, check actuation of -N280- → Chapter.
  A87-10864
–  Press the Air recirculation button -5- on the operating unit, air conditioning system control unit -J301-.
l  The „Air recirculation mode“ lamp in the button lights.
–  Set the engine speed to 2000 rpm (start of time measurement).
–  „Read measured values“ in air conditioner „Guided fault-finding“ function → Vehicle diagnostic tester.
  A87-10864
–  Compare the measured value displayed for the evaporator output temperature sender -G263- to the values in the graph.
A - Air temperature measured by evaporator output temperature sender -G263-
B - Ambient temperature
C - Permissible tolerance range
l  Depending on the ambient temperature, the measured ambient temperature must be within the stated tolerance range after 5 minutes.
If the required values are not attained:
–  Use a thermometer to measure the temperature of the air emerging from the dash panel vents.
l  Compare the measured value to the measured value displayed for the evaporator output temperature sender -G263-.
  Note
t  If the two measured values only differ slightly (measured value of -G263- and thermometer), perform the measures to be taken if the readout does not match the specification → Chapter. If the measured value of -G263- is higher than your measured value, check for proper installation of -G263- and perform the electrical checks for this sender → Vehicle diagnostic tester.
t  The operation of the air conditioner can be detected e.g. from the fact that the refrigerant pipe on the low-pressure end (thick pipe) cools down.
If the measured value of -G263- (and thus the system cooling output) is OK:
–  Use a thermometer to measure the temperature of the air emerging from the left and right dash panel vents.
l  Compare the measured values of the thermometer and -G263-.
–  The values measured must not be lower and after 5 minutes not more than 3 °C higher than the value of -G263-.
  Note
t  If the measured values obtained are lower, check -G263- for proper installation and the wiring for contact resistance. Replace the sender if defective
t  If the required values are not attained, check actuation and operation of the temperature flap. To do so, perform the measures to be taken in the event of temperature increase downstream of the evaporator → Chapter and → Vehicle diagnostic tester.
  A87-0373

Fault Finding if readout does not match specification > < Air conditioner cooling output