I put a new ac compressor on my audi I just bought. I bought it wrecked and the front of the ac compressor was broken so I dont know if it worked before hand. I put the new(to me) compressor on and I filled it up with close to 19oz of refrigerent. Compressor doesnt seem to kick on because the pressures did not equalize. Researching brought up a possible pressure switch problem so I just want to know where that pressure switch for the A/C is. I haven't released the idea of having gotten a bad compressor, but I want to try this first. Its a 2008 Audi a4 2.0T

 

That pressure switch plugs into the AC rail. On a 2008 I think it is located on the passenger side behind the bumper next to to the radiator.

This should be very helpful:
How To Change the G65 Aircon Thrust/Pressure Sensor