High Pressure Safety Valve HVAC: Description and Operation
High Pressure Sensor (G65) Signal
- A - Pressure on the high pressure side in the refrigerant circuit in bar (absolute pressure)
- B - square-wave signal duty cycle
- C - Characteristic curve.
• As soon as there are no conditions for switching off the A/C compressor, the Climatronic control module (J255) switches the A/C compressor on (by activating the A/C compressor regulator valve (N280) ). Refer to Vehicle diagnosis, testing and information system (VAS5051B) in "Guided Fault Finding".
• The Climatronic control module switches the compressor on at a duty cycle larger than approximately 12% (corresponds to approximately 1.2 bar absolute pressure) and less than approximately 78% (corresponds to approximately 32 bar absolute pressure), if no other shut off criterion is in effect.
• The compressor is not switched on if the duty cycle is smaller than 12% or greater than 78% ( the Compressor Regulator Valve (N280) is not activated).
• The duty cycle and pressure calculated by the Climatronic control module is displayed in the measured value block. Refer to Vehicle diagnosis, testing and information system (VAS5051B) under "Guided Fault Finding".
• In terms of absolute pressure, 0 bar corresponds to an absolute vacuum. Normal ambient pressure equals approximately 1 bar absolute pressure. On the scales of most pressure gauges, 0 bar corresponds to an absolute pressure of one bar (can be seen from -1 mark below 0).
• The Climatronic control module sends the calculated refrigerant circuit pressure vie the Data bus. When the A/C compressor is activated, the coolant fan (V7) requests the engine control module for increased RPM's, and activates the coolant fan directly or via the Coolant fan control module (J293). Depending on version with a certain pressure in the refrigerant circuit or not depending on pressure as soon as the A/C compressor is switched on. The request from the Climatronic control module is displayed in the measuring value block. Refer to Vehicle diagnosis, testing and information system (VAS5051B) under "Guided Fault Finding".
• The request to activate the coolant fan is currently only sent to the databus system by the Climatronic control module at a specified pressure in the refrigerant circuit. If the pressure in the refrigerant circuit is greater than approximately 9 bar (6 bar, if the pressure was previously greater than 9 bar) and less than 20 bar, a request from 30 to 100 % takes place, depending on pressure.
• If the pressure in the refrigerant circuit is less than 9 bar (6 bar if the pressure was greater than 9 bar before), there is no request at this time to activate the coolant fan from the Climatronic control module.
• If the pressure in the refrigerant circuit is greater than approximately 20 bar, there is no request at this time to activate the coolant fan from 100% from the Climatronic control module (depending on the version of the engine control module/ Coolant fan control module (J293) , the actual activation may be less than 100%).
• The signal generated from high pressure sensor is also used to control the engine. The Climatronic control module sends the information to the Engine Control Module (ECM) via the databus system (the necessary torque to drive the A/C compressor depends on the pressure in the refrigerant circuit). Depending on the version of the engine control module, the signal in the measured values block will be displayed as a duty cycle. Check using Vehicle diagnosis, testing and information system (VAS5051B) under "Guided Fault Finding", (the A/C system and, depending on engine, the fuel injection and ignition system or the diesel direct injection and glow plug system).
• Depending on the version of the high pressure sensor, an insufficient measuring value for the pressure in the refrigerant system will be sent by the high pressure sensor at low outside temperatures (lower than 5 °C) (even though the A/C system is OK and the refrigerant circuit is properly filled). At lower outside temperatures (ambient temperature lower than 5 °C), -the Climatronic control module with part number "8J0 820 043" through to index "AJ"-might therefore make an incorrect DTC memory entry (DTC memory entry: high pressure sensor ", and fall below the lower threshold"). If the fault is displayed as a sporadic fault at an outside temperature greater than 5 °C, calculated by the A/C system, and if the A/C function is OK, then clear the DTC memory (no additional steps are necessary). This error does not occur with - the Climatronic control module, part number "8J0 820 043" beginning with index "AK".
This pattern appears on oscilloscope screen (for example on Vehicle diagnosis, testing and information system (VAS5051B).) when the following conditions are met.
- Ignition switched on (B+ at terminal 3 and Ground (GND) terminal 1 on high pressure sensor present)
- Making the adjustment on the oscilloscope: 5 volts/Division. = (5 volts per unit DC voltage) 5 ms / Div. (5 milliseconds per unit)
- Measuring lead (signal wire) connected to terminal 2 on high pressure sensor.
- Measuring lead (shielding) connected to terminal 1 (Ground -GND- on high pressure sensor).
• The display shows a signal that is transferred when the pressure in the refrigerant circuit is approximately 7 bar, it corresponds to an approximate duty cycle of 25% (with compressor not engaged, an ambient temperature of 30 °C and properly filled refrigerant circuit).
• The impulse width - A - depends on the pressure in the refrigerant circuit (if pressure increases, area - A - gets wider).
• The signal distance - B - is always 20 milliseconds (corresponds to frequency of 50 Hertz).
• The duty cycle is determined by the ratio of impulse width - A - and signal distance - B -.
• Use the measuring leads form the Connector test set (V.A.G 1594/C) or an adapter lead for this test. To do this, for example, use one connector each - A - and - B - (part number 1J0 973,703 and 1J0 973 803 and relevant terminal contacts), two standard sockets for banana connectors - C - and three wires with 0.5 mm 2 diameter.
