I just installed an AC compressor on my 2007 Chevrolet Impala and it does not work what can be wrong?

Going to the accumulator is the high pressure switch you can not miss it because it has two wires going to it unplug the switch and put in a jump wire in the connector this will by pass the switch and allow the compressor to come on charge system and once charged remove jump wire and plug back into switch it will since freon is in the system and keep running. The reason the compressor is not coming on the switch sensing there is no freon in system and by doing what I suggested will allow you to put freon in system.

Thank-you for your timely response. I have not yet tried this suggestion all tools and facilities being at a friends garage. We had the system running for the first month, then when we reclaimed the 134 we replaced the valve going into the firewall. The compressor seemed to engage at that time while we were trying to charge the system...would the low pressure still cause the switch to NOT allow us to charge the system fully?...

System is not going to charge instantly as long as compressor is clicking on and off it is changing. When compressor clicks on turn the can of freon up side down this will charge with liquid letting freon in system much quicker then turn can back to up right position when it clicks off continue doing this until compressor constantly stays on then it is full. If compressor not click on follow directions in first answer.

From a 08 impala ss and from a Pontiac Grand Prix gxp are the ac compressors the same ?