2006 Audi A6 4.2 Quattro AC Compressor not engaging
 

I have an A6, wonderful car, but had to replace the engine. Which I did, all good. However, my mechanic is having an issue with the AC compressor. It's not coming on. The AC always worked, so I don't think that it's the unit. Im thinking it must have been something with the swap. Is there something we may have missed? Anything?