On my '89 240, the A/C is a knob, as opposed to newer cars.

I can't quite understand why (functionally) the temp is regulated via the knob.

1.. As far as I know, there is a temp sensor running from the knob to the evap to detect how cold it is, right?

2.. Does this control the temp threshold at which the copressor kicks on? i.e. to regulate a certain amount of compression hence a certain amount or coldness to the evap?

3.. And most of all, from the 6 o'clock position to the 9 o'clock position it is RED (opposite to cold BLUE). Heat? Or is this just almost no turning on of the compressor? Why was this added to that control at all?

4.. Why did they not just put an A/C ON/OFF switch, and regulate the temp from the main temp control (heater control valve)?


If anyone understands the true reasoning behind this design, pls let me know.

Thanks,
Greg Mustang
Montreal - Ottawa
Canada

Greg, This is the 'Good' A/C regulation system. The switch regulates the compressor clutch cycling time. Red is always on. The system allows for freeze-up by never having the fan off if the switch is turned past 12 oclock.
This operation was abandoned by Volvo due to simplicity, and some owners not liking the engine 'lag/lunge when the compressor cycles on/off. The A/C compressor lasts much longer and presents a lot less load to the engine with the cycling, and better mileage when the A/C is on.
The first 240's had these switches and in the 'early 80's went to 'always on', then for a few years, back to the switch in the late 80's.
Modern climate control requires 'always on' to regulate the temp accurately, so thh A/C system is taxed more secerely with these type of systems.
I personally like the cycling switch. It gives more complete temp manual control, and the compressor is original in my '89 now with 240k mi.
--
'89 245 Sportwagon, VX cam,K&N, Bil's, IPD susp & exhaust

Greg_mustang
The main purpose is to prevent the evaporator from freezing up. The sensor will sense when the evaporator is at or near freezing and turn off the compressor. Putting the knob into the red will cause the compressor to not turn off. The owners manual says not to keep it in this area to long, or the evaporator will freez. You can turn the knob down to bring the amount of cooling down. Turning the knob down will cause the compressor clutch to cycle more often. Hope this helps.
Joe

All I know is the red section is for "rapid cooling". The compressor does not cycle it is just locked on. The manual says don't leave it there long.

Yes, the owner's manual does caution not to leave the A/C control in the red (compressor on and not cycling) for a long time, to prevent evaporator freezing. That makes sense to me. But:

a) What do you folks think is a "long time"?

b) If the compressor is locked on, and the evaporator freezes, I assume we'd know because the air coming out of the vents wouldn't be cooled. Would operating the A/C system until this occurs damage or stress its components?