What is the EC light and how do you know that this light is related to a/c control ? There are a lot of components that combine to control the compressor, and if you have the Klima relay like I have on my 87, 260E, then there is a step by step process I have posted to test the compressor circuit.
http://www.peachparts.com/shopforum/showthread.php?t=158591&page=5