Specified Values for the Refrigerant Circuit Pressures

Pay attention to the test requirements and check the pressure. Refer to Pressures, Checking .

Checking:

-- Bring the engine speed up to 2000 RPM.

-- Pay attention to the pressure indicator (for example the pressure gauge) of the A/C service station. Refer to the A/C service station Owner's Manual.

NOTE:

The switching pressures for the activation of the A/C Compressor Regulator Valve -N280- and the radiator fan are vehicle-specific. use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function for the A/C system.

Pressures must be measured at the service connections; the component location for these connections are vehicle-specific. Refer to Refrigerant Circuit .

High-Pressure Side:

Increasing from initial pressure (when connecting the pressure gauges) to a maximum of 20 bar (290 psi).

Low-Pressure Side:

Decreasing from initial pressure (when connecting the pressure gauges) to a value between 1.5 and 2.3 bar (21.76 to 33.36 psi) absolute pressure (depending on the required cooling output).

NOTE:

If high cooling output is needed (for example, the outside temperature is very high, the blower speed it set on high but the engine speeds are low), then the A/C compressor will not bring the pressure on the low pressure side to the specified value (for example, for a certain time after turning on the A/C). The A/C compressor is actuated with maximum specified control current, however delivery volume is no longer sufficient at this engine speed to reduce pressure on low-pressure side to the specified value. To check the A/C compressor control under these conditions, for example, the fresh air blower is activated only with approximately 40% of the maximum output (voltage), check the pressures at a lower fresh air blower speed. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function for A/C system. Refer to COOLING OUTPUT, CHECKING .

If there is a low cooling output (for example an exterior temperature of 20 °C (68 °F) and a low fresh air blower speed) the pressure on the high pressure side increases only to a value of 6 to 7 bar (87 to 101.5 psi) (the absence of energy is low, the refrigerant is quickly cooled in the condenser). To check the A/C compressor control and the pressure in the refrigerant circuit under these conditions for example the activating the fresh air blower with maximum voltage, adjusting the A/C system to maximum heat output and recirculating air mode. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system. Refer to COOLING OUTPUT, CHECKING .

Under unfavorable conditions (very high ambient temperatures, high humidity), pressure on high-pressure side may increase to max. 29 bar (421 psi).

The specified operating current for the A/C Compressor Regulator Valve -N280- is displayed as the measured value of the Front A/C Display Control Head -E87-, the A/C Control Module -J301- of the Climatronic Control Module -J255-. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system.

The measured high pressure from the High Pressure Sensor -G65-, from the Refrigerant Circuit Pressure Sensor -G805- or the A/C Pressure/Temperature Sensor -G395- is displayed as the measured value of the Front A/C Display Control Head -E87-, the A/C Control Module -J301- or the Climatronic Control Module -J255-. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system.

Low pressure settles depending on control current for A/C Compressor Regulator Valve -N280- and control characteristic of expansion valve within compressor output range in tolerance range. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system.

Under unfavorable conditions (very high ambient temperatures, or high humidity) the output of the A/C compressor may not always be sufficient to attain the specified pressure values within the specified time (depending on the engine speed it can take longer until the specified low pressure can be reached).

If the A/C compressor load is greater than 90%, the pressure on the low pressure side can be greater than the specified tolerance range. The A/C compressor load is no longer sufficient to reduce the pressure on the low pressure side to the value of approximately 2 bar (29 psi) under predominant ambient conditions (ambient temperature, humidity, A/C compressor settings, engine speed etc.).

The specified operating current for the A/C Compressor Regulator Valve -N280- must be greater than 0.3 A in order to ensure reliable A/C Compressor Regulator Valve -N280- actuation. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system.

In "maximum cooling output" setting, control current for A/C Compressor Regulator Valve -N280- is regulated to approximately 0.65 A (up to 0.85 A). The measured value is vehicle-specific and is displayed in the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system.

At absolute pressure, 0 bar (0 psi) corresponds to absolute vacuum. Normal ambient pressure corresponds to 1 bar (14.5 psi) absolute pressure. 0 bar (0 psi) pressure corresponds to an absolute pressure of 1 bar (14.5 psi) on most pressure gauges (indicated for example by -1 bar (-14.5 psi) below 0). Depending on the version of the corresponding control module the pressure is displayed as a measured value on the basis of the absolute pressure and relative pressure (difference between the display 1 bar (14.5 psi)). Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system. Refer to COOLING OUTPUT, CHECKING .

The temperature of the air downstream from evaporator, the current A/C compressor speed and the pressure of the refrigerant on the high pressure side as well as the specified operating current for the A/C Compressor Regulator Valve -N280- is displayed for example as the measured value of the display control head of the Climatronic Control Module -J255-. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system.

Possible Deviation from Specified Value, the Cause and Its Elimination
The requested cooling output is not attained. the high pressure remains constant or increases only slightly (above pressure with engine stopped), The low pressure quickly drops to target value or lower.		Refer to Possible Deviation From Specified Value During The Pressure Test (1 Of 9)
The requested cooling output is not attained. The high pressure pipe corresponds to the specified value The low pressure corresponds to the specified value or is too low		Refer to Possible Deviation From Specified Value During The Pressure Test (2 Of 9)
Required cooling output is not attained The high pressure does not increase or only increases slightly above pressure with engine stopped The low pressure does not drop or drops only slightly		Refer to Possible Deviation From Specified Value During The Pressure Test (3 Of 9)
Required cooling output is not attained The high pressure increases above specification The low pressure quickly drops to target value or lower		Refer to Possible Deviation From Specified Value During The Pressure Test (4 Of 9)
The required cooling is reached next after some or a long running time the cooling output is no longer sufficient. The high pressure and low pressure are normal. After some time the high pressure raises above the specified value, the low pressure corresponds to the specified value or it falls under the specified value.		Refer to Possible Deviation From Specified Value During The Pressure Test (5 Of 9)
The specified cooling output is reached, then after a shorter or longer period of time the specified cooling output is no longer reached. The high pressure and the low pressure are normal at first, after some time high pressure increases above the specified value and the low pressure falls to the specified value or lower. The high pressure and the low pressure are normal, then after a period of driving time the low pressure falls to the specified value or lower, the evaporator ices over.		Refer to Possible Deviation From Specified Value During The Pressure Test (6 Of 9)
The required cooling performance is obtained. The high pressure pipe corresponds to the specified value The low pressure is too low (lower than the target value)		Refer to Possible Deviation From Specified Value During The Pressure Test (7 Of 9)
The A/C compressor noise (particularly after switch-on) The required cooling performance is obtained. High pressure normal or too high The low pressure is normal or too high (the specified value is not always reached,		Refer to Possible Deviation From Specified Value During The Pressure Test (8 Of 9)
The requested cooling output is not attained. The high pressure and the low pressure corresponds to the specified value		Refer to Possible Deviation From Specified Value During The Pressure Test (9 Of 9)