Test Requirements

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Radiator and condenser are clean (clean if necessary).
The thermal insulation at expansion valve is OK and properly installed.*
Ribbed belt is OK and properly tensioned. Ribbed belt for A/C compressor and generator are OK and correctly tensioned.*
All air ducts, covers and seals are OK and properly installed.
Fault finding on the electrical equipment and vacuum system found no malfunction.* Vehicle Diagnostic Tester ("OBD" or "Guided Fault Finding" for the A/C system) and [For engine(s) CTUA, CEUC, CRDB, CGXB, CPNB] System Overview - Refrigerant Circuit (vehicle-specific repair information).

Air conditioner On Board Diagnostic (OBD) has not revealed any faults (with engine running and air conditioner switched on), no compressor shutoff criterion displayed in measured value block (vehicles with "Air conditioner" On Board Diagnostic only)*. Refer to Vehicle Diagnostic Tester ("OBD" or "Guided Fault Finding") and [For engine(s) CTUA, CEUC, CRDB, CGXB, CPNB] System Overview - Refrigerant Circuit vehicle-specific Repair information).

Air flow through dust and pollen filter not obstructed by dirt*
Air conditioner unit not drawing in secondary air at maximum fresh-air blower speed* Evaporator and heater not drawing in secondary air at maximum fresh-air blower speed*
Air doors in air conditioner unit, heater and evaporator reach end position*
Fresh-air intake ducts beneath hood and in passenger compartment as well as corresponding water drain valves OK*. Refer to Air Conditioning (vehicle-specific repair information).

The engine is warm.
Vehicle is not exposed to direct sunlight. Refer to Repair Information (vehicle-specific repair information).

The ambient temperature is greater than 15 °C (59 °F).
All instrument panel vents are open.

-- Start the engine.

Adjustment at A/C Control Head -E87- or Climatronic Control Module -J255- (and Rear A/C Control Head (Climatronic) -E265- in vehicles with two A/C units):

-- Preselect "Auto" mode (A/C compressor on).

-- Set temperature pre-selector switch to "LO" for driver's side and front passenger's side (and the rear of the vehicle interior, left and right in vehicles with two A/C units).

Setting on heater controls:

-- Press A/C button and Rec- or recirculated air button.

-- Turn rotary temperature control towards "cold" stop.

-- Set rotary fresh-air blower control to "4".

The following system test conditions should be met:

One or more Coolant Fan -V7- operated (at least speed 1)*

NOTE:

For some versions, the fan is switched on only once the pressure in refrigerant circuit has exceeded a specified value.

Fresh Air Blower -V2- (and Rear Fresh Air Blower -V80- in vehicles with two A/C units) running at maximum speed.
Recirculated/fresh-air door set to "Recirculated air mode" (within one minute, after starting vehicle, air-flow door is closed and recirculated-air door is opened)*
The coolant shut-off valve is closed.*
The valves of pump valve unit are closed and there is no coolant circulation pump delivery.*
Compressor is actually driven (A/C Clutch -N25- operated, overload safeguard (if installed) not tripped).*

NOTE:

The A/C compressor is driven by different components depending on the engine (belt or input shaft). The belt pulley or the drive unit has an overload protection to protect these components and the engine, if the A/C compressor is runs with resistance. Refer to A/C Compressor Regulator ValveN280, Removing and Installing (vehicle-specific repair information).