Is the A/C compressor always on?
I just went through replacing the climate control in my 81 300SD. While fooling around with the old one trying to get it to work, I noticed that the first three buttons would turn the compressor on even when the dial was set at max heat. So now with the new (from a wrecking yard, but works) installed, I find that the compressor still comes on with the first three buttons. Is that how it works or is something funny going on?