Monitoring Functions, Components

WARNING: This page is about a different car, the 2002 Volvo C70, 2001 Volvo C70, 2000 Volvo C70, 1999 Volvo C70, and 1998 Volvo C70. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Identifying Linear Pressure Sensor And Combined Unit With Multiple Integrated Pressure Switches

Courtesy of VOLVO CARS OF NORTH AMERICA.

Which functions/components there are, how they are designed and where they are located depends on the type of system, car and model year and the engine variant.

Pressure switches

Are located on the high pressure side and in principle consist of switches which open/close a current circuit or a pressure sensor which transmits a signal to the engine control module (ECM).

These can be:

A: A linear pressure sensor. In this case, all the functions are always governed via the engine control module (ECM) and the current pressure can be read off using the Volvo Scan Tool (ST).
There tasks are:
- to start the engine cooling fan (FC) at 1st or 2nd speeds at high pressure. The air flow through the condenser is then increased. More heat can be given off by the refrigerant to the air, the temperature/pressure drops and the capacity is maintained.
- to disengage the compressor if the pressure falls too low (= safety switch), in the event of leakage for example.
- to disengage the compressor if the pressure rises too much (= safety switch), if the pressure switch or engine cooling fan (FC) is not functioning for example.
B: Separate units.
There tasks are:
- to start the engine cooling fan (FC) at 1st or 2nd speeds at high pressure. The air flow through the condenser is then increased. More heat can be given off by the refrigerant to the air, the temperature/pressure drops and the capacity is maintained.
- to disengage the compressor if the pressure falls too low (= safety switch), in the event of leakage for example.
- to disengage the compressor if the pressure rises too much (= safety switch), if the pressure switch or engine cooling fan (FC) is not functioning for example.
C: A combined unit with multiple integrated pressure switches.
There tasks are:
- to start the engine cooling fan (FC) at 1st or 2nd speeds at high pressure. The air flow through the condenser is then increased. More heat can be given off by the refrigerant to the air, the temperature/pressure drops and the capacity is maintained.
- to disengage the compressor if the pressure falls too low (= safety switch), in the event of leakage for example.
- to disengage the compressor if the pressure rises too much (= safety switch), if the pressure switch or engine cooling fan (FC) is not functioning for example.

On the majority of pressure switches from and including 1992 model years there is a valve case (Schraeder valve) in the connection. This enables the pressure switches to be removed without needing to drain the system.

NOTE: If the connection has internal threads (as at C in the illustration) there is no valve casing.

Safety Valve

Is located on the high pressure side, on the compressor or on a pipe.

The valve opens and releases refrigerant if the pressure is too high in the system. It then shuts again when the pressure has returned to normal.

This function protects the system from exploding if, for example, the safety switch is not functioning.

Temperature switch

Is located on and gauges the compressor temperature.

Opens the compressor current circuit at approximately 105°C (221°F). Closes the circuit when the temperature has fallen to approximately 90°C (194°F).

This function protects the compressor from overheating/breakdown in the event of, for example, refrigerant leakage (= low quantity).

Other functions that may be present

The compressor can be engaged 10 seconds after the engine is started at the earliest.

Often governed by a timer circuit in the A/C relay or in the engine control module (ECM).

This function prevents the engine stopping immediately after start. The engine is subjected to a heavy load when the compressor engages.

Disengagement of the compressor at:

wide open throttle (WOT) (often time governed disengagement).
heavy acceleration and low speed (usually occurs on diesel engines).
high engine coolant temperature.

Usually governed via the engine control module (ECM).

These functions are to provide optimal acceleration and to prevent the engine from overheating.

Values for pressure switches/safety switch/safety valve

The values vary depending on, amongst other things, the type of system, refrigerant and compressor.

The following values are only an example. They apply to a Volvo 900 with R134a refrigerant and Sanden SD-7H15 compressor.

The engine cooling fan (FC) low speed is engaged at 18 bar and disengages at 14 bar.
The engine cooling fan (FC) high speed is engaged at 23 bar and disengages at 19 bar.
The compressor disengages at 30 bar and engages at 20 bar.
The safety valve opens at 38 bar.