1990 740 GL Sedan automatic, no operator-controlled temperature setting on the air conditioning.

There are many times in stinky Los Angeles when I want interior air, but I don't want the air conditioner on. I wish my Volvo had a A/C on-off switch. With the A/C off, I could move the vent sliders to Max A/C to get recycled air without activating the A/C system.

I'd like to install a switch on the dash that defeats/allows the A/C system, regardless of the setting of the vent control sliders. Ideally, I would intercept one wire in the dash, wiring it to a simple on-off switch. Could it really be this simple? If so, which wire? Also, is the any chance of damaging any A/C system components when using such a switch? Thanks!

That same thing bothers somewhat. I don't know why modern cars have gotten away from the little thermosadt/switch that controls the compressor. It seems really inefficient that I have to regulate the amount of cold air I dump into the cabin by mixing warm air with it, rather than the compressor cycling off and on.

My Dodge pickup (and some Japanese cars I've seen) has a little push button that engages the compressor. When you start getting too cold, you punch the button again and it disengages the compressor. Getting warm? - punch it again, etc. Even that's not as good as the old style thermostadts, but it's much less cumbersome than the controls in our 740.

I don't know why you couldn't put one in your Volvo. You'd have to make certain it could handle the current demanded by the compressor clutch, or work a relay into the equation.