Originally Posted by Diseasel300

I misspoke earlier when I said #10 was a ground. #10 is an output signal generated by the Klima and pulled down to ground by the ACC.

The Klima will output a voltage on #10 (~11V on my car) which the ACC pulls down to ground through the refrigerant pressure switch. It doesn't pull all the way to 0, I measure ~.8V like Frank does. The reason it doesn't go fully to 0 is because the switching is done by transistor in the ACC unit on '86+ models, they can't pull 100% to ground, the idea is to pull the signal "low" which tells the Klima to go ahead and engage the compressor.

So to recap, Klima generates voltage on #10 and if the pressure switch is closed (you have enough refrigerant), the ACC pulls #10 down to ground (low).