geocacher (djcache) replied:

Hi Austravel,

Usually it works one of two ways.

Generally the airconditioner dries and cools the air all the time it's running and the pressure in the system is constant - with the pressure switch controlling the compressor cutting in and out to maintain it.

The air is warmed either by controlling engine water flow to the heater element and warming the air passing through it, or by opening a different flap to allow airflow through a heater core that has a constant flow of water through it. The latter is more common on later model vehicles (and is harder to fit a shower to as it requires an additional flow control valve to be installed).

There were some aftermarket units that controlled compressor cycling and I'm really not sure how some of the Toyota ones that had an Economy mode worked. I think but I'm not sure they reduced the compressor use by using a second lower pressure switch setting.

Having said that in vehicles with climate control a computer controlled thermostat in the climate control system uses a sensor in the cabin (or two in the ones that do mum's side separately) to control air and/or water flow and whether the air con is running in fresh or recirc mode to regulate the temperature in the cabin. They also sense solar load (usually on the dash at the base of the windscreen) to compensate early for increased heat provided by sun on the windscreen thus enabling a more constant temperature with less hysteresis.

What sort of vehicle does the question relate to? Why do you ask?

There may be a fridgey on the forum who can provide a more conclusive answer. The above is open to correction.

Dave

Top Cat replied:

Are you talking about the actual air conditioner.......or........the heater.......the two should not be confused.

To my knowledge most if not all heaters in a vehicle have there heat controlled by the amount of hot water that is allowed to pass through the core............although this can be in turn be controlled via a thermostat.

Member - Brian (Gold Coast) replied:

My understanding is on basic models a thermostat sensing evaporator coil temp (The COOLING element) cycles the compressor on/off. When you slide the temp control toward warmer settings the heat valve allows how water to pass thru the HEATING element to mix warm air with cold air. On cars with climate control/dual climate control, sensors either side of the car do a similar job to mix the hot/cold air to suit.

I am not an expert in this field, this is simply my understanding of it.... I may be wrong :-)

Austravel replied:

Thanks for all the replies, I'd initially thought this was the way the temp was controlled but thought that in modern cars they might have used a better control system. Looks like they don't, seems a waste of engine power to have the air con running continuously.

Bob Y. - Qld replied:

On 75/78 & 79 series tojo's, the aircon temp can be governed by a control mounted "under" the aircon fan control. This changes amount of cycling that the compressor does. Gives a multitude of temp/fan combinations, to suit nearly everyone, together with fresh/recycle option.

Don't know what cooling in a troopie is like, but in the utes it is very effective, especially with tinted windows.

Went to an aircon bloke in Penrith once, with a HJ60, and he suggested that I wind the thremostat down, to get about 2 degrees C, at vents. The thermo, is behind the glovebox in that model, and is black box about size of ciggy packet.

80 & 100 series aircon temps are controlled by the heater temp control, but think someone has mentioned that. Had the use of a newish Magna earlier this year, and it had climate control. Fell that the temp would governed by the amount that the compressor run, or cycled. Not from any input from the heater circuit. Can't substantiate that either, cause what happens when one wants some heat?

Hooroo...