MK3 2013 focus Aircon issue

Hello everyone.

So recently, my a/c stopped working in my focus 2.0 tdci 140 auto, after being sat for 2 weeks whilst on holiday. It was working before I went away.

Had it regassed as I thought the pressure might be low, garage said it was almost full and barely took any gas to top it off so was not the problem.

He showed me that the clutch plate wasn't engaging, just free wheeling and an auto electrician would need to diagnose.

I did some diagnosis myself as follows :

Fuse fine. Relay fine, swapped out it with horn relay to check. Checked voltage at compressor connector, got 18v.

on that basis, I've ordered a used compressor, and my garage will fit it for me and re-gas.

My questions are:

when I checked for voltage at the connector, there are two terminals, and I got 18v across these when the AC switched was pressed, and 0v when switch was off. Are these two terminals + & -??, I was expecting only 1 terminal and the ground would be through the chassis of the compressor? Have I tested correctly? Is 18v right, should it not be nearer 12?

Twice since it ceased working, it has randomly started working when I started the engine after being sat overnight. When I turned the switch off and back on again, it stopped working on both occasions.

Can anyone assist me in saving some money if there's something glaringly obvious I've overlooked?

Thanks in advance if you can!