A/C Unit, Checking Cooling Output
A/C Unit, Checking Cooling Output
Special tools, testers and auxiliary items required
• Vehicle Diagnosis, Testing and Information System (VAS 5051) or the Service Information System (VAS 5052)) => Vehicle Diagnosis, Testing and Information System VAS 5051.
• Standard thermometer (for temperature measurements, if necessary a thermometer with 2 measuring probes for simultaneous measurement e.g. for temperature at right and left)
=> [ Test Requirements ]
=> [ Checking ]
=> [ Specified Values Not Obtained, Determining Malfunction ]
=> [ Temperature Increase Downstream from Evaporator, Determining Malfunction ]
Test Requirements
• Ambient temperature greater than 15 °C
• Radiator and condenser are clean (if necessary clean)
• Ribbed belt for compressor drive is OK and correctly tensioned and belt pulley actually drives the compressor (on vehicles with 6 cyl. engine).
• Compressor drive unit is mounted correctly, compressor is actually driven (on vehicles with 8 or 10 cyl. engine) => [ Compressor Drive Unit ] Compressor Drive Unit.
• All air ducts, covers and seals OK and properly fitted
• Air flow through dust and pollen filter not obstructed by dirt. => [ Dust and Pollen Filter ] Dust and Pollen Filter.
• Vehicle not exposed to sunlight
• Engine is warm
• DTC memory of A/C control head Climatronic Control Module (J255) was checked and erased, basic setting was performed => Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding" function.
• The coding and adaptation of the Climatronic Control Module (J255) A/C control head was checked => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
• All instrument panel vents open
• Hood closed
• Perform the following settings at the "Basic" version of the Climatronic Control Module (J255) A/C control head:
- Lowest possible setting for the driver and front passenger's side temperature preset (both temperature rotary switches are in the "left stop" position).
- The compressor is on (the "Econ" function is not active or the "A/C" function is on).
- The rotary switch for air distribution is in position "A" (automatic air distribution).
- The rotary switch for the fresh air blower speed is in position "A" (automatic control of fresh air blower speed).
• Perform the following settings on the "Comfort" version of the Climatronic Control Module (J255) A/C control head:
- Operating mode "Auto" is set.
- Temperature selection "LO" for driver's and passenger's side.
- The compressor is on (the "Econ" function is not active or the "A/C" function is on).
• Check the following functions with the engine running:
- The Coolant Fans (V7) are running (the speed depends on pressure in refrigerant circuit and engine coolant temperature).
• Depending on version of A/C control head Climatronic Control Module (J255), the request occurs at a specific temperature in the refrigerant circuit or independently of the pressure as soon as the compressor is switched on.
- Fresh Air Blower (V2) runs at maximum speed.
- The A/C system goes into recirculation mode (approximately 1 minute after starting the vehicle, the back pressure/fresh air doors are closed and the recirculation door is opened) the Fresh Air Blower (V2) extracts the air from the passenger compartment below the instrument panel/glove compartment.
- No operation of Coolant Pump (V50).
- Left Heat Regulation Valve (N175) and Right Heat Regulation Valve (N176) are closed (coolant hoses or coolant pipes to heat exchanger for heating in the A/C system cool down).
• If one of these test requirements is not fulfilled, check the DTC memory, perform the output diagnostic test, and read the measured value block in the Climatronic Control Module (J255) A/C control head, => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
Checking
Test requirements are met => [ Test Requirements ]
- Measure ambient temperature (must be greater than 15 °C).
- Close doors, hood, windows and sunroof.
- Open all dash panel vents.
- Switch ignition on.
- Set the "Econ" operating mode on the Climatronic Control Module (J255) A/C control head "Basic" version (or switch off the "A/C" operating mode).
- In vehicles with the "Comfort" version of the Climatronic Control Module (J255) A/C control head, set the "Econ" operating mode by pressing the "Econ" button, or via the rotary push-button of the MMI control unit => Owner's Manual and => "Infotainment / MMI" Owner's Manual (or switch off the "A/C" operating mode).
• The "Econ" or "A/C" operating mode is switched on when the indicator lamp in the corresponding button is on.
- In vehicles with the "Comfort" version of the A/C system, close the vents in "B".
- Using the MMI Terminal, set the temperature preset for the variable temperature center vent to "Warm" (in vehicles with the "Comfort" version of the A/C system).
- Start the engine.
- Initiate a Guided Fault Finding on the A/C system => Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding" function.
- In the "Read measured value block" function, select the display group with the following values (A/C Compressor Regulator Valve (N280) activation and A/C Pressure/Temperature Sensor (G395) measured value) and check the display => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
• The compressor activation via the A/C Compressor Regulator Valve (N280) is switched off (the compressor is switched off and 0% is shown in the display field).
• The pressure in the refrigerant circuit determined by the A/C Pressure/Temperature Sensor (G395) is the same or greater than the value in the table at the measured ambient temperature.
• In terms of absolute pressure, 0 bar corresponds to an absolute vacuum. Normal ambient pressure equals approximately 1 bar absolute pressure. On the scales of most pressure gauges, 0 bar corresponds to an absolute pressure of one bar (can be seen from -1 mark below 0).
• Pressure in refrigerant circuit is governed by ambient temperature. On account of heat given off by components (e.g. radiator), pressure displayed with warm engine will be slightly higher than that given for the respective ambient temperature.
• If the displayed pressure is less than what is in the table, check the signal of the A/C Pressure/Temperature Sensor (G395) => Vehicle Diagnosis, Testing and Information System VAS 5051. If no fault is determined at the (G395), there is insufficient coolant in the circuit. Bring vehicle to a workshop that has the necessary tools and in which the work can be performed accordingly by qualified personnel => Refrigerant R134a - Servicing.
• If pressure in refrigerant circuit is OK:
- Switch on the compressor (to do this select operating mode "Auto" on the Climatronic Control Module (J255) A/C control head "Comfort" version or switch off the "Econ" function on the "Basic" version).
- Select the temperature preset "Lo" (for the diver and front passenger sides) at the Climatronic Control Module (J255) A/C control head "Comfort" version, or set the lowest possible temperature in the "Basic" version.
- Set direction of air flow to "instrument panel vents" at the Climatronic Control Module (J255) A/C control head.
- For the Climatronic Control Module (J255) A/C control head "Basic" version, check the fresh air blower speed rotary switch; it has to be in position "A" (automatic fresh air blower speed regulation).
- Select "read measured values block"=> Vehicle Diagnosis, Testing and Information System VAS 5051 in "Guided Fault Finding" function.
- Check the indicators in the various display fields on the tester display:
- A duty cycle greater than 30% is shown in the display field for the A/C compressor regulator valve (N280) activation (the (N280) is activated, the compressor is switched on).
- Current greater than 0.3 A is shown in the display field for the current to the A/C compressor regulator valve (N280) (the current flows via the (N280), the compressor is switched on).
- The pressured measured and displayed by the A/C Pressure/Temperature Sensor (G395) increases over the value with the compressor switched off.
• The A/C compressor regulator valve (N280) is activated by the Climatronic control module (J255) A/C control head so that the temperature of the air downstream from the evaporator reaches the specified value (approximately 2 to 5°C):
• A value greater than 75 % (0,55A) is displayed after vehicle start, depending on temperature, engine speed and vehicle voltage measured.
• As soon as the temperature measured by Evaporator Vent Temperature Sensor (G263) approximates the specified value, activation of the A/C Compressor Regulator Valve (N280)is rescinded and thus the compressor output is reduced.
• The connector to A/C Compressor Regulator Valve (N280) and the connector to the electric engine mounts are identical in construction. If these connectors are interchanged, no entry appears in DTC memory, however since the compressor is continuously actuated (by the engine control module), the evaporator may ice-up.
• If no voltage or voltage that is too low is shown in the display field for the A/C Compressor Regulator Valve (N280) activation, check the (N280) activation.
• If the pressure measured and displayed in the A/C Pressure/Temperature Sensor (G395) does not change and compressor activation is OK, check whether the compressor is actually being driven (an overload protection is installed between the compressor and the belt pulley/drive unit. The compressor will not be driven if it has been triggered). If no malfunction is found, there is a malfunction in the refrigerant circuit. Bring vehicle to a workshop that has the necessary tools and in which the work can be performed accordingly by qualified personnel => Refrigerant R134a - Servicing (it is possible that compressor regulation is not OK). Bring the problems determined to the attention of the workshop.
- Press the button for "recirculating air mode" at the Climatronic Control Module (J255) A/C control head (the symbol for "recirculating air mode" lights up in the button).
- Set engine speed of 2000 rpm (start of time measurement).
- Select the display group with the evaporator vent temperature sensor (G263) in the "Read measured values block" function => vehicle diagnosis, testing and information system VAS5051.
- Compare the displayed value (of the (G263)) with the values in the diagram.
A Air temperature measured by the (G263)
B Measured ambient temperature
C Permissible tolerance range
Depending on ambient temperature, the air temperature measured at the Evaporator Vent Temperature Sensor (G263) must be within indicated tolerance range after 5 minutes.
• If the required values are not reached, check the measured value of the Evaporator Vent Temperature Sensor (G263), the Left Vent Temperature Sensor (G150), and the Right Vent Temperature Sensor (G151), and compare the displayed measured values with one another => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
• If the measured values of (G150) and (G151) differ only slightly from the measured value of (G263) => Vehicle Diagnosis, Testing and Information System VAS 5051. Perform "Identifying malfunctions if specified values are not obtained" => [ Specified Values Not Obtained, Determining Malfunction ].
• If the measured value of (G263) is greater than the measured value of (G150), and (G151), check the (G263)for proper installation and perform an electrical test on this sensor => Vehicle Diagnosis, Testing and Information System VAS 5051.
• The function of the air conditioning is recognizable, e.g. the refrigerant pipe on the low pressure side (thick line) cools off.
If measured value of (G263) (and thus the A/C system cooling output) is OK:
- Compare the measured value for the Evaporator Vent Temperature Sensor (G263) with the measured values for the Left Vent Temperature Sensor (G150) and Right Vent Temperature Sensor (G151).
The measured values of (G150) and (G151) after 5 minutes may not be be greater than maximum 3 °C than the value of the Evaporator Vent Temperature Sensor (G263).
• If the required values are not reached, compare the displayed measured values of (G150), (G151), and (G263) with one another. If the measured values for the (G150) and/or (G151) are not OK, check the activation and function of the Left Heat Regulation Valve (N175) and Right Heat Regulation Valve (N176), and the Coolant Pump (V50) => [ Temperature Increase Downstream from Evaporator, Determining Malfunction ] and => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
• If no malfunction can be found in the Left Vent Temperature Sensor (G150), Right Vent Temperature Sensor (G151), Left Heat Regulation Valve (N175), Right Heat Regulation Valve (N176) and Coolant Pump (V50), determine the malfunction with temperature increase downstream of the evaporator => [ Temperature Increase Downstream from Evaporator, Determining Malfunction ] and => Vehicle Diagnosis, Testing and Information System VAS 5051.
• If the measured value of one or both sensors ( (G150) and / or (G151)) is less than the measured value of the Evaporator Vent Temperature Sensor (G263): Check the (G263), (G150) and (G151) for proper installation and check their electrical connectors for contact resistance. Replace the faulty sensor if necessary => [ Temperature Increase Downstream from Evaporator, Determining Malfunction ] and => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
• You can measure the temperature of the air flowing from the "center" instrument panel vents using a commercially available thermometer (depending on the vehicle equipment, it can also be read from the measured value block) => Vehicle Diagnosis, Testing and Information System VAS 5051.
• If air flowing out of the variable temperature center vent (only available with the "Comfort" version A/C system, at "Lo" setting with the temperature rotary switch set to the "cold" stop on the driver and front passenger sides of the Climatronic Control Module (J255) A/C control unit) when the variable temperature center vent is set to "cold"in the MMI, is colder than for the setting "warm", check the function of the Left Heat Regulation Valve (N175) and Right Heat Regulation Valve (N176) => Vehicle Diagnosis, Testing and Information System VAS 5051.
• If cooling output on one side (e.g. front passenger's side) is still adequate but values on the other side (e.g. driver's side) are no longer attained, this is an indication for insufficient coolant in the coolant circuit (stratification in evaporator due to lack of coolant).
Problems relating to differences between temperature of air emitted by vents with identical settings in the air conditioner control mode may be due to the following:
• The cold-air doors or warm-air doors in air ducts do not close completely or do not reach their end positions => Vehicle Diagnosis, Testing and Information System VAS 5051.
• There is air in the heat exchanger (effecting different flow through the different sides of the heat exchanger and uneven heat distribution)
• The foam seal may have come loose when installing the heat exchanger allowing air to flow past the heat exchanger.
Specific problems can occur even though cooling output is OK. This can be due to the following:
- After a long drive, A/C system repeatedly or sporadically fails (no cooling or heating performance), after switching off vehicle and after a short time, A/C function is OK again.
- After a long drive, windows fog up from inside, windows are also not cleared by then pressing the "Defrost" button, after switching off vehicle and after a short time, A/C function is OK again.
Under certain operating conditions, residual moisture in the coolant circuit can lead to an ice build-up at the A/C Compressor Regulator Valve (N280). This impedes compressor regulation. Evaporator is excessively cooled and ices up. The ice on the evaporator may cause these problems.
• Corrective action:
- Check the measured value of Evaporator Vent Temperature Sensor (G263) via the "Read measuring value block" function => Vehicle Diagnosis, Testing and Information System VAS 5051.
- If measured value of sensor is too high under operating conditions reported by customer (even though A/C system is operating without difficulty at greater than e.g. 10 °C), check Evaporator Vent Temperature Sensor (G263) (an incorrect measured value may cause evaporator to ice-up).
- If the measured value of the sensor under the conditions of use specified by the customer is too low (at an ambient temperature above 0 °C, colder than 0 °C for an extended period of time, there is an problem in the coolant circuit), bring the vehicle to a workshop that has the necessary tools and in which the work can be performed accordingly by qualified personnel => Refrigerant R134a - Servicing.
- Check refrigerant line from evaporator to accumulator (thick tube, low pressure side) with engine running. If this line is thickly iced (a thin layer is permissible) when the malfunction occurs, this also indicates that the temperature in the evaporator is too low, bring the vehicle to a workshop that has the necessary tools and in which the work can be performed accordingly by qualified personnel => Refrigerant R134a - Servicing (it is possible that compressor regulation is not OK). Inform the workshop of the problem at hand, the necessary work can only be performed there (draining refrigerant circuit, replacing reservoir and then evacuating refrigerant circuit for minimum of 3 hours).
Specified Values Not Obtained, Determining Malfunction
The required cooling output is not reached during the cooling output test => [ Checking ]
- Select the "read measured values block" function => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
- Repeat cooling output test.
• Pressure in refrigerant circuit is governed by various factors of influence. However, at an ambient temperature between 20 to 25 °C the pressure should not exceed more than 20 bar. Due to high coolant performance, pressures up to 31 bar may occur under extreme operating conditions (e.g. in southern countries with high ambient temperatures, "stop and go" traffic and high humidity).
• At ambient temperatures below 25 °C, pressure in refrigerant circuit does not normally exceed 16 bar (radiator fan runs and cools condenser).
• The Climatronic Control Module (J255) A/C control head calculates the pressure in the coolant circuit via the A/C Pressure/Temperature Sensor (G395) and displays it => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
• At an absolute pressure less than approximately 1.8 bar and greater than approximately. 32 bar, the A/C Control Head Climatronic Control Module (J255) does not switch on the compressor (the A/C Compressor Regulator Valve (N280) is not actuated). The compressor is only switched on once the absolute pressure becomes greater than 1.8 bar or less than 16 bar.
• Additional information regarding pressure in the coolant circuit => Vehicle Diagnosis, Testing and Information System VAS 5051 in the "Guided Fault Finding" function.
• If the pressure in the refrigerant circuit or the engine coolant temperature is too high, A/C control head Climatronic Control Module (J255) regulates the compressor performance (to avoid sudden compressor cut-off):
- When the pressure in the refrigerant circuit exceeds 30 bar (absolute pressure). Full compressor performance is only possible again once the pressure drops below 27 bar.
- As soon as the coolant temperature exceeds 115° C (full shutdown at 118° C).
- If the pressure in the coolant circuit exceeds 32 bar despite regulation, the Climatronic Control Module (J255) A/C control head does not switch the compressor back on until the pressure has dropped below 29 bar.
Temperature Increase Downstream from Evaporator, Determining Malfunction
The cooling output required by the cooling output test has been reached, but the temperature of the air flowing from the vents is too high => [ Checking ]
- Clamp off the coolant hose from the A/C unit heat exchanger to the engine or from the engine to the pump/valve unit e.g. using hose clamps up to 25 mm dia. (3094).
- Repeat cooling output test.
