Re: M-zone fuel saver magnets
It's absolutely true on a 164! Let's say the ambient air is about 20 degrees C. You turn on the aircon and there's a "tick" under the bonnet as the compressor cuts in. The compressor continues to run, the evaporator gets cold and the condenser gets hot. After a while (if the car's not moving) the engine cooling fan cuts in to draw some air through the condenser. Meanwhile, the evaporator in the heater box continues to get colder and condensation starts to form on it. This starts dripping out through the heater box drain tubes in the usual way. If the compressur continues to run, the evaporator will get colder and colder. Eventually, the condensation on it will start to freeze and then the airflow through it will be reduced. If the air from outside the car stops flowing through the evaporator, there will be no warming effect on it at all and it will get even colder. Meanwhile, in the cabin, you're cursing because there's no air coming out of the vents! To stop this hapenning, there is an anti-icing switch - just a fluid filled probe that's shoved into the evaporator fins. At a preset temperature (usually just above freezing), it cuts the power to the compressor and the compressor switches off. Once this happens, the air flowing through the evaporator and into the cabin starts to warm it up again. As soon as it gets past the temperature where the anti-icing switch cut the power to the compressor, it will allow power back to the compressor again and the evaporator will start cooling down again. This process just repeats itself continuously, keeping the evaporator just above freezing, but preventing it from turning itself into a block of ice.
On a hot day - say 35 degrees C of ambient air, the compressor will probably be running pretty much all the time. The cabin fan will be working at a high speed drawing lots of 35 degree air through the evaporator, so the chances of it getting cold enough to trip the anti-icing switch are pretty slim. In this mode, it will use quite a lot of power (I guess a couple of BHP - maybe 5BHP) almost constantly and will correspondingly increase fuel consumption a bit.
On a cold day (say 10 degrees ambient), the temperature of the incoming air will be pretty cold to start with, so it won't warm the evaporator up much. Also, you probably won't have the interior fan on very high, so it won't be drawing much air through the evaporator anyway. That being the case, the compressor won't have to run for very long before the evaporator gets cold enough for the anti-icing switch to turn it off. What's more, it will remain off for much longer before the evaporator warms up enough to trip the switch and turn the compressor back on again.
On a really cold day (like we've had in Northern England - it hasn't got above freezing all day today!), the aircon pump won't cut in regardless what I do with the switch on the dashboard because the anti-icing switch is already tripped out! I notice that my wife's 156 is the same. In fact it's aircon light won't even come on when you press the dashboard switch.
The important thing from the power usage point of view, is that the compressor is either "on" or "off". When it's "on", it's drawing however much power it draws from the engine. When it's "off", it draws no power. All that happens on a hot day compared to a fairly cold one is that it spends more of its time being "on" than "off". The reverse is true on a cold day. When it's "on" it always draws the same amount of power - regardless of the abient temperature. This is uch like a conventional domestic fridge. When the temperature rises above a set oint, the compressor runs until the temperature is low enough again and then shuts down.
NOTE that there are newer types of aircon compressor these days that can do clever things like alter the amount of power they draw. I don't know if a 166 has this sort of compressor. Even so, the basic premise still holds, the greater the air temperature outside, the more work the compressor will do - either by working "harder" (like the more modern sones) or by running for longer (like the 164 ones).
It really winds me up when you get the government or environmental pressure groups quoting misleading dumbed-down statistics about "using the aircon increases your fuel consumption by X%"! The percentage by which it increases your fuel consumption will vary depending on how big your car is, how hot the day is, how cold you want the inside of your car, how much "solar gain" your car has, how many people are in it, how big your engine is....and so on and so forth.