Compressor Control

Fig 1: Identifying Circuit For Compressor (Engagement/Disengagement)

Courtesy of VOLVO CARS OF NORTH AMERICA.

The current circuit for the compressor (engagement/disengagement) varies greatly depending on the type of system, engine variant and the car and model year.

The circuit may be very simple with few components or more complex with many components.

Example 1 

The example shows the circuit on a 1977 Volvo 240.

The circuit includes:

• a switch to manually engage/disengage the compressor.
• a thermostat governed switch.

The thermostat senses the temperature at the evaporator via a resistor.

At too low temperatures (= risk of ice formation) the switch opens and the compressor is disengaged

When the temperature increases the switch closes again and the compressor is engaged.

Fig 2: Identifying Air Conditioning Components

Courtesy of VOLVO CARS OF NORTH AMERICA.

1. Compressor
2. Air conditioning (A/C) relay
3. Engine control module (ECM)
4. Air conditioning (A/C) pressure sensor
5. Engine coolant temperature sensor
6. Throttle position (TP) sensor
7. Air conditioning (A/C) pressure switch (Pressostat)
8. Control module ECC
9. Passenger compartment temperature sensor
10. Outside pressure sensor
11. Engine coolant temperature sensor ECC
12. Sun sensor

Example 2 

The example shows the circuit on a 1996 Volvo 960 with Motronic 4.4 and ECC.

The compressor is governed via a relay by the engine control module (ECM) depending on a number of input signals. With this connection the engine control module (ECM) can quickly (in advance) compensate for the changes in load that occur when the compressor is engaged and disengaged (= less jerkiness).

Function 

The ECC control module calculates whether the compressor needs to be engaged to obtain the correct temperature in the passenger compartment. The calculations are based on the drivers requirements (= the settings of the control) and signals from a number of sensors.

If necessary, the control module transmits a signal (= requests engagement of the compressor).

The signal is sent to the engine control module (ECM) via the pressure switch (Pressostat).

The pressure switch (Pressostat) breaks the circuit if the pressure and therefore the temperature in the system (on the low pressure side) falls too low.

The engine control module (ECM) activates the A/C relay and the compressor is engaged.

(10 seconds after the start of the engine at the earliest - timer circuit in the control module).

The engine control module (ECM) disengages the compressor:

• when the request to engage from the ECC control module/pressure switch (Pressostat) ceases.
• at wide open throttle (WOT), for a maximum of 15 seconds (signal from the throttle position (TP) sensor).
• at high engine coolant temperature (ECT), above approximately 125°C (257°F) signal from the temperature sensor).
• at too high-pressure on the high pressure side, above approximately 30 bar (signal from pressure sensor).

The signals from the engine coolant temperature sensor and the pressure sensor are also used by the engine control module (ECM) to govern the engine cooling fan (FC).