Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Checking With Expansion Valve, Reservoir And A/C Compressor Regulator Valve N280
Connecting service station, refer to SERVICING STATION, CONNECTING FOR MEASURING AND TESTING .
Observe test requirements, refer to PRESSURES, CHECKING .
-- Bring engine speed up to 2000 RPM.
-- Check the pressure gauge on the A/C service station.
Switching pressures for actuation of A/C compressor regulator valve -N280- and radiator fan -V7- are vehicle-specific.
The pressure should be measured at the service connections. The location of these connections depends on the vehicle. Refer to [For engine(s) BGP, BPY, BGQ, CBTA, CBUA, CCTA, CBFA] Description and Operation .
Target values
High-pressure side
Increasing from initial pressure (when connecting pressure gauges) to 20 bar.
Low-pressure side
Decreasing from initial pressure (when connecting pressure gauges) to the value in the graph.
A - Low pressure (measured at service connection) in bar absolute
B - Control current for A/C Compressor Regulator Valve -N280-.
C - Permissible tolerance range
Under unfavorable conditions (very high ambient temperatures, high humidity), pressure on high-pressure side may increase to max. 29 bar.
The control current -B- is shown in the measured value block.
The high pressure measured from the High Pressure Sensor -G65- is not shown in the measured value block. Refer to [For engine(s) BGP, BPY, BGQ, CBTA, CBUA, CCTA, CBFA] Diagnosis and Testing .
Low pressure settles as a function of control current for A/C compressor regulator valve -N280- within compressor output range in tolerance range.
Under unfavorable conditions (very high ambient temperatures, high humidity), compressor output may not always be sufficient to attain the specified value.
The specified operating current for the regulator valve must be greater than 0.3 A in order to ensure reliable valve activation.
In the setting "maximum cooling output" control current is regulated to approximately 0.65 A up to 0.85 A (vehicle specific, displayed in measured value block). Refer to [For engine(s) BGP, BPY, BGQ, CBTA, CBUA, CCTA, CBFA] Diagnosis and Testing .
At absolute pressure, 0 bar corresponds to absolute vacuum. Normal ambient pressure corresponds to 1 bar absolute pressure. 0 pressure corresponds to an absolute pressure of one bar on most pressure gauges (indicated by -1 bar below 0).
| Possible Deviation From Specification | Possible Cause Of Fault | Corrective Action |
|---|---|---|
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-- Check activation of A/C Compressor Regulator Valve -N280-. -- Localize leak with leak detector and eliminate -- Charge the refrigerant circuit |
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-- Extract refrigerant from refrigerant circuit
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-- Replace expansion valve -- Charge the refrigerant circuit -- Repeat test |
If no malfunction is found with this complaint, flush (clean) refrigerant circuit with refrigerant R134a. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH REFRIGERANT R134A . If that is not possible in this workshop, flush the refrigerant circuit with compressed air and remove moisture with nitrogen. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH COMPRESSED AIR AND NITROGEN .
Check measured values from Evaporator Vent Temperature Sensor -G263- or Evaporator Temperature Sensor -G308- and activation of A/C Compressor Regulator Valve -N280-. If the measured value of the High Pressure Sensor -G65- is faulty, the evaporator can ice up or the cooling output is not reached.
If the function of the air conditioning system is not OK when the test is repeated after replacing expansion valve, re-install old expansion valve and flush refrigerant circuit with compressed air and nitrogen. Then replace A/C compressor and reservoir.
With this malfunction, 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). Because refrigerant cannot flow through the expansion valve, the correct cooling output cannot be reached. The high pressure may not increase or may only increase slightly.
| Possible Deviation From Specification | Possible Cause Of Fault | Corrective Action |
|---|---|---|
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-- Check activation of A/C Compressor Regulator Valve -N280-. |
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-- Extract refrigerant circuit. -- Flush (clean) refrigerant circuit. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH REFRIGERANT R134A . -- Replace A/C compressor. -- Charge the refrigerant circuit | |
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-- Check activation of A/C Compressor Regulator Valve -N280-. |
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-- Run hand over refrigerant circuit to check for differences in temperature
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-- Extract refrigerant circuit. -- Replace expansion valve and receiver. -- Charge the refrigerant circuit |
With this malfunction, 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). Because refrigerant cannot flow through the expansion valve, the correct cooling output cannot be reached. The high pressure may not increase or may only increase slightly.
| Possible Deviation From Specification | Possible Cause Of Fault | Corrective Action |
|---|---|---|
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-- Check activation of A/C Compressor Regulator Valve -N280-. |
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-- Extract refrigerant circuit. -- Refrigerant Circuit, Flushing with Compressed Air and Nitrogen. -- Replace reservoir with dryer. -- Evacuate refrigerant circuit for at least 3 hours. -- Charge the refrigerant circuit -- Repeat test, if function is not OK. -- Flush (clean) refrigerant circuit. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH REFRIGERANT R134A . -- Charge the refrigerant circuit. -- Repeat test |
If this complaint occurs after a long operating time or only sporadically (the low pressure falls under the specified value and the evaporator ices up), replacing the dryer (installed in the reservoir) is sufficient. The quantity of refrigerant oil should be adjusted. Refrigerant circuit is then to be evacuated for at least 3 hours.
First, it is not necessary to flush the refrigerant circuit with compressed air and nitrogen in case of this complaint because generally only a small amount of moisture is in the system and this can be removed by a long evacuation.
With this malfunction, evaporator may ice up although quantity of refrigerant in circuit is OK.
| Possible Deviation From Specification | Possible Cause Of Fault | Corrective Action |
|---|---|---|
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-- Check activation of A/C Compressor Regulator Valve -N280-. |
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-- Extract refrigerant circuit. -- Flush (clean) refrigerant circuit. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH REFRIGERANT R134A . -- Replace expansion valve and receiver. -- Charge the refrigerant circuit. -- Repeat test if function is not OK. -- Replace A/C compressor. | |
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-- Extract refrigerant circuit. -- Flush (clean) refrigerant circuit. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH REFRIGERANT R134A . -- Replace A/C compressor. -- Charge the refrigerant circuit. -- Repeat test |
Note the following when the malfunction "high pressure normal, low pressure too low" is detected: faulty, the evaporator can ice up or the cooling output will not be reached.
With this malfunction, evaporator may ice up although quantity of refrigerant in circuit is OK.
Check measured values from Evaporator Vent Temperature Sensor -G263- or Evaporator Temperature Sensor -G308- and activation of A/C Compressor Regulator Valve -N280-. If the measured value of the High Pressure Sensor -G65- is faulty, the evaporator can ice up or the cooling output is not reached.
If the problem is with the A/C Compressor Regulator Valve -N280- (regulating valve is not actuated but compressor operates nevertheless), refrigerant circuit does not have to be flushed. With this malfunction, replacing the A/C compressor is sufficient (adjust refrigerant oil quantity in new A/C 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). Because refrigerant cannot flow through the expansion valve, the correct cooling output cannot be reached. The high pressure may not increase or may only increase slightly.
Refer to [For engine(s) BGP, BPY, BGQ, CBTA, CBUA, CCTA, CBFA] Removal and Installation .
| Possible Deviation From Specification | Possible Cause Of Fault | Corrective Action |
|---|---|---|
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-- Check activation of A/C Compressor Regulator Valve -N280-. -- Extract refrigerant from refrigerant circuit
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-- Discharge refrigerant circuit
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-- Discharge refrigerant circuit. -- Replace expansion valve and receiver. -- Charge the refrigerant circuit. -- Repeat test if function is not OK. -- Replace A/C compressor. | |
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-- Discharge refrigerant circuit. -- Flush (clean) refrigerant circuit. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH REFRIGERANT R134A . -- Replace A/C compressor.
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If the problem is with the A/C Compressor Regulator Valve -N280- (regulating valve is not actuated but compressor operates nevertheless), refrigerant circuit does not have to be flushed. With this malfunction, replacing the A/C compressor is sufficient (adjust refrigerant oil quantity in new A/C compressor).
Overfilling with refrigerant oil can occur if, for example, the compressor has been replaced without adjusting the quantity of refrigerant oil.
If the A/C compressor is not replaced, the refrigerant oil should be drained from the A/C compressor via the oil drain plug. Out of the entire quantity of refrigerant oil MAINTENANCE, DIAGNOSIS , 50 grams should be put in the A/C compressor and the rest in the refrigerant circuit.
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). Because refrigerant cannot flow through the expansion valve, the correct cooling output cannot be reached. The high pressure may not increase or may only increase slightly.
| Possible Deviation From Specification | Possible Cause Of Fault | Corrective Action |
|---|---|---|
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-- Check activation of A/C Compressor Regulator Valve -N280-. -- Flush (clean) refrigerant circuit. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH REFRIGERANT R134A . |
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-- Discharge refrigerant circuit. -- Flush (clean) refrigerant circuit. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH REFRIGERANT R134A . -- Charge the refrigerant circuit -- Repeat test |
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-- Replace expansion valve |
Overfilling with refrigerant oil can occur if, for example, the compressor has been replaced without adjusting the quantity of refrigerant oil.
If the A/C compressor is not replaced, the refrigerant oil should be drained from the A/C compressor via the oil drain plug. Out of the entire quantity of refrigerant oil, 50 grams should be put in the A/C compressor and the rest in the refrigerant circuit. Refer to MAINTENANCE, DIAGNOSIS
If expansion valve is malfunctioning (permanently open), evaporator temperature is no longer regulated such that only refrigerant in gaseous state 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).
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.