Refregeration Cycle Schematic. Unless you know air-conditioning or have a couple Air-conditioning knowledgable friends, this is not a project for you. Before proceeding I highly recommend that you read Zay Brand\'s Tech article about GMC air conditioning at http://www.gmcws.org/Tech/air_conditioning/air_conditing.html . You should add your auxilliary unit by splicing into the high and low pressure lines both sides of the existing evaporator. The add on evaporator should have its own expansion valve. The unit I show in this upgrade came out of a late model chevrolet van. It was located under the back seat and ducted forword to cool the passengers. I was advised in making this upgrade to either install a larger condenser or add an auxilliary fan to the existing condenser. I chose to add an additional fan.