I'm currently having Air conditioning troubles and while I was trouble shooting I found the air condioning system (well the electrical part) was not layed out the way I expected.
So I thought I might share my findings.
Now I'm not going to go into the gas compression cycle refrigeration system used in the C3, there are heaps of videos on YouTube and you can look it up on wikipedia.
However just quickly: the compressor causes heat to be "pumped" out of the cabin and transferred to the condenser at the front of the car where the heat is transferred to the atmosphere. When the compressor "kicks-in" the system cools the cars interior and when the compressor kicks-out the system stops cooling.
This is a basic representation of the electrical circuit for the air conditioning. The red lines are simple electrical connections and the black lines are communication lines.
The compressor. The compressor sits at the front of the engine, at the bottom off side. It has a clutch which is pulled in by an electromagnet to engage the compressor onto the constantly spinning accessory drive belt (fan belt). So the cooling of the cabin is done by this compressor kicking in and out to regulate the cabin temperature. On more modern cars the compressors are more complicated and can compress more or less gas to regulate the temperature. This gives a more constant temperature in the cabin.
A/C ECU. This is the control panel that the user interacts with. There are two types (for the older cars) manual air con and climate control. At their heart they are basically the same. The user presses a button to request the system on. There is a control to set the temperature. A knob in the case of the manual version and buttons to set the temperature on the climate control.
The climate control ECU also has a "sniffer" to determine the cabins actual temperature so the A/C ECU can compensate if the temperature fluctuates. It normally sits in the dash but here it is hanging outside.
In both of the cases, the manual and auto ECU's only send out an "on" request and temperature request to the BSI.
The BSI. The BSI is the fuse box in the glove box. As far as the air conditioning is concerned this guy is the work horse. More on him in a moment.
The temp probe. The temperature probe is placed close to the evaporator in the air distribution box. This photo is taken near the drivers foot. The evaporator is the bit that gets cold and the air distribution box is the box in the center of the dash that has the heater and fan and all the air distribution stuff.
The BSI uses info from the probe and from the A/C ECU to determine when the compressor should kick in and when it doesn't, to get the temperature to where the user requested it to be.
The resistance of the probe changes with temperature.
The BSM. The BSM is the fuse box in the engine bay. In this case the BSM actually provides power to the electromagnet on the compressor's clutch. So the BSI just tells the BSM to turn the power to the clutch on and off. This guy is just the muscles.
The engine ECU sits besides the battery under the battery box cover.
Press probe. This is a pressure probe in the high pressure refrigerant line of the air conditioning system. It is located inthe engine bay. The pressure in this line is proportional to the load on the A/C system. The higher the load, the higher the pressure in this line. Under extreme circumstances this pressure can get very high and cause damage to the system. Also if the refrigerant leaks out, the pressure will go down. If there is no gas in the system the engine ECU will stop the compressor from running. The pressure sensor monitors the pressure in the line and sends the info back to the engine ECU as an analogue voltage.
The engine ECU. The engine ECU basically runs the engine but as far as the air conditioning is concerned, it's like America and Russia in the U.N... It has the power of veto!!
It uses the pressure sensor and other sensor inputs to determine if the compressor should be allowed to run. The air conditioning compressor is quite a load on the engine and would load down the engine about the same as adding another passenger to the vehicle. So if the pressure in the "high line" is too high or too low or if the engine is already loaded down according to its other sensors, Or it senses som other trouble, then the engine ECU will tell the BSI to turn off the compressor (or not turn it on in the first place).
There are other devices involved in determining the temperature of the cabin like the heater matrix and mixer flap in the air distribution box but I was only concerned with the electronics in the refrigeration side of things.
Hope this was informative it's certainly more complicated than I thought but there is a logic in it I guess.
