Pressure Checking, Vehicles With Expansion Valve, Receiver, and Externally Regulated A/C Compressor
Pressure Checking, Vehicles With Expansion Valve, Receiver, And A/C Compressor Regulator Valve With Externally Regulated Compressor
NOTE:
- Connecting A/C service station
- Observe test conditions.
- If a malfunction occurs at only one evaporator in vehicles with two evaporators, check pressures in the refrigerant circuit; if they are OK, check line connection between malfunctioning evaporator and line connection end at distribution point of refrigerant lines (for constrictions or blockage). If no malfunction can be detected, discharge refrigerant circuit and re-charge it with the specified refrigerant quantity. Then check pressures and cooling performance of A/C system again; if the malfunction occurs again, replace the expansion valve which is prematurely switched by the malfunctioning evaporator.
- Set engine speed to 2000 rpm.
- Observe pressure display, (e.g. pressure gauge) of A/C service station.
NOTE:
- Switching pressures for actuation of A/C Compressor Regulator Valve N280 and Coolant Fans V7 are vehicle-specific.
- Pressures must be measured at service connections; component locations of these connections are vehicle-specific.
Specifications
High-pressure side:
Increasing from initial pressure (on connecting pressure gauges) to to max. 20 bar
Low-pressure side:
Decreasing from initial pressure (on connecting pressure gauges) to value in graph
A - Low pressure (measured at service connection) in bar absolute
B - Control current for A/C Compressor Regulator Valve N280 in amps
C - Permissible tolerance range (applicable to compressor capacity utilization of 10...90%)
NOTE:
- Given high ambient temperature and a low engine speed, compressor may for example no longer be able to adjust pressure on low-pressure side to value given in graph. 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 value in graph, air conditioner Guided Fault Finding function.
- Under unfavorable conditions (very high ambient temperatures, high humidity), pressure on high-pressure side may increase to max. 29 bar.
- Control current - B - is displayed in measured value block of A/C Control Head E87 or control head, Climatronic Control Module J255.
- High pressure measured by High Pressure Sensor G65 or A/C Pressure/temperature Sensor G395 is displayed in measured value block of A/C Control Head E87 or control head, Climatronic Control Module J255 .
- Low pressure settles as a function of control current for A/C Compressor Regulator Valve N280 within compressor output range in tolerance band.
- Under unfavorable conditions (very high ambient temperatures, high humidity), compressor output may not always be sufficient to attain the specified value.
- If compressor capacity utilization is greater than 90 %, pressure on low-pressure side may be in excess of tolerance range "C" shown in graph (compressor output no longer suffices).
- Specified operating current for A/C Compressor Regulator Valve N280 must be greater than 0.3 A to ensure reliable valve actuation.
- In "maximum cooling output" setting, control current for A/C Compressor Regulator Valve N280 is regulated to approx. 0.65 A (up to 0.85 A). This measured value is vehicle-specific and displayed in measured value block.
- At absolute pressure, "0 bar" corresponds to an absolute vacuum. Normal ambient pressure corresponds to "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 bar" mark below 0" ), air conditioner Guided Fault Finding function.
NOTE:
- If no fault is found with this complaint, clean refrigerant circuit (flush using refrigerant R134a or blow through using compressed air and nitrogen).
- Check measured values of Evaporator Vent Temperature Sensor G263 and actuation of A/C Compressor Regulator Valve N280. If measured value of Evaporator Vent Temperature Sensor G263 is not OK, evaporator may ice up or cooling output is not attained.
- If air conditioner operation is not OK when test is repeated after replacing expansion valve, re-install old expansion valve, clean refrigerant circuit (flush using refrigerant R134a or blow through using compressed air and nitrogen). Then replace compressor and receiver.
- With this fault, evaporator may ice up although quantity of refrigerant in circuit is OK.
- If expansion valve is malfunctioning (permanently closed or does not open sufficiently), A/C Compressor Regulator Valve N280 is actuated to maximum output and low pressure drops to value in graph or below (compressor draws off refrigerant from low pressure side). As however refrigerant cannot flow via expansion valve, cooling output is not attained and high pressure may also not increase or only increase slightly due to the absence of energy conversion, air conditioner Guided Fault Finding function.
NOTE:
- If the function of the air conditioning system is not OK when the test is repeated, replace expansion valve and receiver.
- With this fault, evaporator may ice up although quantity of refrigerant in circuit is OK.
- If expansion valve is malfunctioning (permanently closed or does not open sufficiently), A/C Compressor Regulator Valve N280 is actuated to maximum output and low pressure drops to value in graph or below (compressor draws off refrigerant from low pressure side). As however refrigerant cannot flow via expansion valve, cooling output is not attained and high pressure may also not increase or only increase slightly due to the absence of energy conversion, air conditioner Guided Fault Finding function.
- If there is too much refrigerant oil in the circuit, the compressor must be drained and the receiver must be replaced. After cleaning the refrigerant circuit (flushing with refrigerant R134a or blowing through using compressed air and nitrogen), the correct quantity of refrigerant oil is filled into the circuit.
NOTE:
- It is not initially necessary to clean the refrigerant circuit (flush using refrigerant R134a or blow through using compressed air and nitrogen) when this problem occurs since normally, there is only a small quantity of moisture in the system which can be removed by lengthy evacuation.
- If problem involving moisture in refrigerant circuit only occurs after a lengthy operating period or only infrequently (low pressure drops below specification and evaporator ices up), it is sufficient to replace the dryer (adjust quantity of refrigerant oil). Refrigerant circuit is then to be evacuated for at least 3 hours.
- With this fault, evaporator may ice up although quantity of refrigerant in circuit is OK.
- Problem with Evaporator Vent Temperature Sensor G263 can also cause icing-up of refrigerant circuit. If this problem is encountered, also pay attention to measured value of Evaporator Vent Temperature Sensor G263, air conditioner Guided Fault Finding function.
NOTE:
- Pay attention to the following as regards fault High pressure normal, low pressure too low" : This fault may lead to icing-up of evaporator although quantity of refrigerant in circuit is OK.
- If the problem is with the A/C Compressor Regulator Valve N280 (regulator valve is not actuated but compressor operates nevertheless), refrigerant circuit does not have to be cleaned (flush with refrigerant R134a or blow through using compressed air and nitrogen). In this case, it is sufficient to replace the compressor (observe quantity of refrigerant oil in compressor).
- If expansion valve is malfunctioning (permanently closed or does not open sufficiently), A/C Compressor Regulator Valve N280 is actuated to maximum output and low pressure drops to value in graph or below (compressor draws off refrigerant from low pressure side). As refrigerant cannot however flow via expansion valve, cooling output is not attained and high pressure may either not increase or only slightly due to the absence of energy conversion.
- Check measured values of Evaporator Vent Temperature Sensor G263 and actuation of A/C Compressor Regulator Valve N280. If measured value of Evaporator Vent Temperature Sensor G263 is incorrect, evaporator may ice up or cooling output is not attained, air conditioner Guided Fault Finding function.
NOTE:
- This fault may also be caused by too much refrigerant oil in the circuit. Overfilling with refrigerant oil may occur if, for example, the compressor has been replaced without adjusting the quantity of refrigerant oil.
- If expansion valve is malfunctioning (permanently closed or does not open sufficiently), A/C Compressor Regulator Valve N280 is actuated to maximum output and low pressure drops to value in graph or below (compressor draws off refrigerant from low pressure side). As however refrigerant cannot flow via expansion valve, cooling output is not attained and high pressure may also not increase or only increase slightly due to the absence of energy conversion, air conditioner Guided Fault Finding function.
NOTE:
- Overfilling with refrigerant oil can occur if, for example, the compressor has been replaced without adjusting the quantity of refrigerant oil.
- If expansion valve is malfunctioning (permanently open), evaporator temperature is no longer regulated such that only refrigerant in gas form exits from the evaporator. Under certain usage conditions, liquid droplets may then be drawn in by the compressor and cause noise (liquid cannot be compressed).
- If there is too much refrigerant oil in the circuit, the compressor must be drained and the receiver must be replaced. After cleaning the refrigerant circuit (flushing with refrigerant R134a or blowing through using compressed air and nitrogen), the correct quantity of refrigerant oil is filled into the circuit.