Refrigerant Circuit, Checking Pressures with A/C Service Station: Notes
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NOTE:
- All test conditions marked * are vehicle-specific and are described in the Repair Information for the relevant vehicle.
- Check cooling output.
- Connections with valve and service connections for measurement and testing → 87 - Air conditioning
Under certain operating conditions, residual moisture in the coolant circuit may lead to the formation of ice on the compressor regulator valve. Such ice formation impedes compressor control. The evaporator is cooled excessively and ices up. Icing-up of the evaporator may be the cause of the following problems:
- Repeated or sporadic failure of the air conditioner (no cooling/heating output) after a lengthy journey; operation of air conditioner soon returns to normal after switching off engine
- Windows mist up on inside after a lengthy journey and situation is initially not even remedied by pressing "Defrost" button; air conditioner operation soon returns to normal after switching off engine
Corrective action:
- In the case of vehicles as of Model Year 2001 and a compressor with A/C Compressor Regulator Valve N280 , read measured value of evaporator outflow temperature Evaporator Vent Temperature Sensor G263 (by way of function "Reading measured value block" ). If sensor measured value is too low under usage conditions outlined by customer (at ambient temperature above 0 C, colder than 0 C for lengthy period although A/C Compressor Regulator Valve N280 not actuated) or too high (greater than approx. 10 C although air conditioner is functioning properly), evaporator may ice up due to the incorrect measured value , air conditioner "Guided Fault Finding function and → 87 - Air conditioning .
- On vehicles with no Evaporator Vent Temperature Sensor G263 use Footwell Outlet Temperature Sensor G192 for example to check vent temperature under the usage conditions described by the customer in the following settings: "Lo temperature" for drivers and passengers side, 4 or 5 bars for fresh-air blower speed, air outlet to footwell and fresh-air mode. If measured value of sensor is too low (at ambient temperature above 0 C, colder than 0 C for lengthy period):
- Check refrigerant pipe between evaporator and reservoir (thick pipe, low-pressure side) with engine running. If pipe is severely iced up when problem occurs (thin ice layer is permissible), this is a further indication that the temperature in the evaporator is too low.
- Discharge refrigerant circuit, replace reservoir or receiver with dryer and then evacuate refrigerant circuit for at least 3 hours.
Test conditions
- Radiator and condenser clean (clean if necessary)
- Thermal insulation at expansion valve OK and properly installed*
- Poly V-belt OK and properly tensioned / belts for compressor and alternator OK and properly tensioned*
- All air ducts, covers and seals OK and properly installed
- Electrical system and vacuum system fault-finding has not revealed any faults* , air conditioner Guided Fault Finding function and → 87 - Air conditioning
- 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 (OBD) only)* , air conditioner Guided Fault Finding function and → 87 - Air conditioning
- Air flow through dust and pollen filter not impeded by contamination*
- 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 flaps 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* → 87 - Air conditioning
- Engine warm
- Vehicle not exposed to sunlight → 87 - Air conditioning
- Ambient temperature above 15 C
- All instrument panel vents open
- Start engine.
Settings on 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 (compressor on).
- Set "LO" temperature for drivers and front passengers side (and left and right rear seats in vehicles with two A/C units).
Settings 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 then be met:
- Operation of Coolant Fan(s) V7 (at least speed 1)*
NOTE:
- With some versions, the fan is not switched on until the pressure in the refrigerant circuit has exceeded a specified value.
- Operation of Fresh Air Blower V2 (and Rear Fresh Air Blower V80 in vehicles with two A/C units) at maximum speed
- Recirculated/fresh-air flap set to "Recirculated air mode" (within 1 min. after starting vehicle, air flow flap is closed and recirculated-air flap opened)*
- Coolant shutoff valve closed*
- Valves of pump valve unit closed and no coolant circulation pump delivery*
- Compressor is actually driven ( A/C Clutch N25 energized, overload safeguard (if installed) not tripped)*
Pressures, Checking
- Switch off ignition.
- Connect A/C service station → A/C Service Station, Connecting .
- Take pressure gauge readings (two possible results).
| Ambient temperature (in degrees C) | Pressure in refrigerant circuit in bar |
| + 15 ° C | 3.9 |
| + 20 ° C | 4.7 |
| + 25 ° C | 5.6 |
| + 30 ° C | 6.7 |
| + 35 ° C | 7.8 |
| + 40 ° C | 9.1 |
| + 45 ° C | 10.5 |
NOTE:
- Temperature of refrigerant circuit components should be equal to ambient temperature (pressure will deviate from values in table if individual components of refrigerant circuit are warmer or colder).
- At absolute pressure, 0 bar corresponds to an absolute vacuum. Normal atmospheric 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).
- On vehicles with High Pressure Sensor G65 or A/C Pressure/temperature Sensor G395 for which measured pressure is displayed in measured value block, pressure measured should coincide with values in table , air conditioner Guided Fault Finding function and → 87 - Air conditioning .
- Pressure is measured in different units: 1 MPa (mega pascal) corresponds to 10 bar positive pressure or 145 psi, 1 bar absolute pressure corresponds to 0 bar positive pressure and thus to the ambient pressure (atmospheric pressure).
Pressure in refrigerant circuit lower than indicated in table
Not enough refrigerant in circuit
- Determine whether refrigerant circuit is leaking → Refrigerant Circuit, Determining Leaks .
- Check pressure relief valve.
If pressure relief valve has responded:
- Check actuation of coolant fans.
- Check for constricted refrigerant pipe and hose cross-sections caused by inadequate bending radii.
- Check refrigerant pipes and hoses for external damage.
- If no fault is found, clean refrigerant circuit (flush using refrigerant R134a → Refrigerant Circuit, Flushing with Refrigerant R134a or blow through using compressed air and nitrogen → Refrigerant Circuit, Flushing with Compressed Air and Nitrogen )
Pressure in refrigerant circuit in line with table or higher
- Start engine.
- Set air conditioning system to maximum cooling output.
NOTE:
- On vehicles with A/C Compressor Regulator Valve N280 , control current can be read out in measured value block , air conditioner Guided Fault Finding function and → 87 - Air conditioning .
If compressor is not driven with engine running or regulator valve is not actuated:
- Establish and eliminate cause e.g. by checking air conditioner DTC memory.
- Observe test conditions.
- Check power supply for A/C Clutch N25. If OK, service A/C clutch.
- Check actuation of A/C Compressor Regulator Valve N280 , air conditioner Guided Fault Finding function and → 87 - Air conditioning .
NOTE:
- If low-pressure switch has been removed to connect A/C service station, jumper electrical connections in relevant connector for pressure measurement.
- Compressor is driven by engine via A/C Clutch N25.
- A/C Compressor Regulator Valve N280 is actuated by A/C Control Head E87 or Climatronic Control Module J255 , air conditioner Guided Fault Finding function and → 87 - Air conditioning .
Continuation of Test Depending on Design of Refrigerant Circuit
→ Pressure Checking, Vehicles with Restrictor and Reservoir with Internally Regulated Compressor
→ Pressure Checking, Vehicles with Expansion Valve and Receiver with Internally Regulated Compressor