After looking at a wiring diagram of the a/c system, it looks like someone previously bypassed the Klima relay.

#27 is the pressure switch. The disconnected blue wire at my drier obviously goes from #27 to pin 10 on the Klima relay (c & d is the wire pair connected on the other spade of the pressure sensor). Lastly, the yellow circled wire by the little red junction is from the Klima relay to the compressor. I don't know why this would have been bypassed but I do know the PBU has beer replaced with a Programa unit about 6 yrs ago.

I hope this helps somebody in the future.



Edit: I just re-read a post from Dr Bert & Brian Carlton and infact the Klima takes the signal from the Tach to cut the power to compressor but if tach isn't working, the compressor clutch won't engage. My tach has been dead for a while judging by the un-even discoloration on the needle. This could be why someone previously by-passed the Klima.