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.
Mechanically Driven A/C Compressor
The A/C compressor is driven by a ribbed belt -B- or an input shaft, which is driven by the vehicle engine.
A/C Compressor with A/C Clutch -N25-
An electromagnetic clutch -A- attached to A/C compressor provides the power link between the ribbed belt pulley -B- and A/C compressor crankshaft with A/C system switched on.
An overload safeguard attached to the clutch plate or in the A/C compressor solenoid coil is tripped if the compressor does not move freely, thus protecting the belt drive or the drive unit against overload.
So that the A/C compressor does not get damaged when the refrigerant circuit is empty, the A/C Compressor Regulator Valve -N280- and the A/C Clutch -N25- is no longer activated (the A/C compressor only runs when the A/C Clutch -N25- is activated at idle with the engine).
An A/C compressor with an A/C Clutch -N25- (without A/C Compressor Regulator Valve -N280-) is not activated by an empty refrigerant circuit and not driven.
A/C Compressor without A/C Clutch -N25-
- An overload safeguard attached to the belt pulley -B- or in the A/C compressor drive unit is tripped if the compressor does not move freely, thus protecting the belt drive or the drive unit against overload.
- An A/C compressor with A/C Compressor Regulator Valve -N280- (without A/C Clutch -N25-) is switched to internal lubrication by way of a valve when the refrigerant circuit is empty.
On A/C compressors without an A/C Clutch -N25-, the engine is only to be started if the refrigerant circuit is completely assembled.
So that the A/C compressor does not get damaged when the refrigerant circuit is empty, the A/C Compressor Regulator Valve -N280- is no longer activated (A/C compressor runs at idle with engine).
All A/C Compressors
The A/C compressor extracts the refrigerant gas from the evaporator, compresses it and relays it to the condenser.
The A/C compressor contains refrigerant oil, which can be mixed with refrigerant R1234yf at any temperature.
The type plate lists the type of refrigerant required for the A/C compressor. A regulator valve regulates the pressure of the low pressure side inside a specific specified range (control characteristic). Refer to REFRIGERANT CIRCUIT (R1234YF 2 OF 3) , or REFRIGERANT CIRCUIT (Climate control) , or REFRIGERANT CIRCUIT (R1234YF 3 OF 3) .
Depending on the version of the A/C compressor different refrigerant oil are filled in the A/C compressor. Refrigerant oil which was only developed for the refrigerant R134a, may not be used for the refrigerant R1234yf. Refer to TECHNICAL DATA .
A/C compressors with or without a A/C Clutch -N25- are currently activated from the outside by a A/C Compressor Regulator Valve -N280- -C-. Refer to REFRIGERANT CIRCUIT (R1234YF 2 OF 3) , or REFRIGERANT CIRCUIT (Climate control) , or REFRIGERANT CIRCUIT (R1234YF 3 OF 3) .
The refrigerant for which the A/C compressor is designed is listed on the manufacturer's plate. A A/C Compressor Regulator Valve -N280- regulates the pressure of the low pressure side inside a specific specified range (control characteristic). Refer to REFRIGERANT CIRCUIT (R1234YF 2 OF 3) , or REFRIGERANT CIRCUIT (Climate control) , or REFRIGERANT CIRCUIT (R1234YF 3 OF 3) .
The A/C Compressor Regulator Valve -N280- is activated from the outside. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function for the A/C system.
The engine on a vehicle with an A/C compressor without an A/C Clutch -N25- cannot be activated, when the refrigerant circuit is under vacuum (for example when evacuating). Refer to WORKING WITH A/C SERVICE STATION .
The engine can be restarted on a vehicle with an A/C compressor without a A/C Clutch -N25- when the refrigerant circuit is completely assembled.
So that the A/C compressor does not get damaged when the refrigerant circuit is empty, the A/C Compressor Regulator Valve -N280- is no longer activated (the A/C compressor runs at idle with engine) if the refrigerant circuit is too low (smaller than 2 bar (29 psi)).
An A/C compressor with A/C Compressor Regulator Valve -N280- is switched to internal lubrication by way of a valve when the refrigerant circuit is empty.
Depending on the A/C compressor version, there may be a valve installed on the high pressure side of the A/C compressor, which prevents the liquid refrigerant from flowing back into the compressor once the A/C is turned off. If an A/C compressor with this valve is installed in a vehicle with a refrigerant circuit having an expansion valve, then it may take some time until the pressure in the high pressure side decreases (the expansion is cold and the pressure in the low pressure side quickly increases after it is turned off, the expansion valve closes and the refrigerant flows slowly into the low pressure side). If the A/C compressor is switched on, the pressure on the low pressure side reduces, the expansion valve opens and the refrigerant can flow to the low pressure side.
On A/C compressors with an electro-magnetic clutch -A- and a regulator valve -C-, the electro-magnetic clutch -A- is, for the most part, only activated when the regulator valve -C- is activated. Refer to REFRIGERANT CIRCUIT (R1234YF 2 OF 3) , or REFRIGERANT CIRCUIT (Climate control) , or REFRIGERANT CIRCUIT (R1234YF 3 OF 3) .
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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.