I have tested this on other vehicles, but not on a disco. Unplug the harness from the compressor and apply +12V to the solenoid pin, this will engage the clutch.
You can also check for voltage on the harness plug.

 

Yes, the clutch will audibly engage, disengage, and reengage. If it does not engage at all, it can be a bad clutch, bad relay, bad wiring, bad low pressure sensor, etc. But the most likely culprit, and what would be checked first by a garage is refrigerant. If it's low on refrigerant, it's not going to engage the clutch at all.

Get/borrow a set of manifold gauges and take a look at your refrigerant pressure.

This issue comes up this time every year, and I got tired of re-posting basic system charging and how to use a manifold gauge, so I put it here.

 

From RAVE electrical library 4.79:
Air conditioning compressor clutch operation
When the main relay is energised, a feed passes from the relay contacts to the coil of the air conditioning compressor clutch relay. The compressor clutch relay coil is earthed on a BSwire to the ECM.
When conditions are correct, the ECM provides the earth path which in turn energises the compressor clutch relay coil closing the contacts. The feed from fuse 6 in the engine compartment fusebox passes through the relay contacts and from the fusebox is connected on a BG wire to the air conditioning compressor clutch. The feed operates the air conditioning compressor clutch which is connected to earth header C0018 (Td5 engines) or earth eyelet connector C0807-1 (V8 engines).

 

So it is actually grounding it ti make it engage if I read that correct?

Not to sound like an idiot, but I want to make sure I have this right. The outside pulley with the green arrow is where the belt runs and is always spinning when the engine is running. When the A/C Clutch engages, it spins the inside pulley with the red arrow? This is my first Rover.

 

Right, the inside part (red) is stationary when the clutch is not engaged and spins when it is engaged. You need a ground and a +12V wire to manually engage the clutch.
Check on what DarylJ said, I'm clueless on a/c systems.

 

You could also located the low pressure switch (rave)
(which is usually on the left side of the fire wall just before the refrigeration lines go through the wall) unplug it usually has two spade connectors inside, jump them out with a wire or paper clip (just don’t ground it) if you hear the compressor clutch kick in, you low on refrigerant. But the $64000.00 question is where did it go?

 

Yeah, that is the question. I am gonna try and see what I can figure out. I would like to know if the compressor works, and at least eliminate that.

At least I know it is not what was making my carpets wet! Score. Sort of.

 

R-134A Refrigerant Gauge

Thinkone of these should give me my quick answer if there is anything left in there.

 

No so fast. Low pressure switches go bad all the time. This is why you need check pressures as well. If you really can't get your hands on gauges, running with the low pressure switch jumped for 30 to 45 seconds to see if the AC gets cold inside probably won't kill it (unless it has contaminated refrigerant or no oil in it), and will give you an idea of what's up.

But really, if you plan on working on cars, you should at least have a manifold gauge set, even if you don't plan on getting charging and evacuation equipment. It's what you need to be able to know you're getting the right service when you bring it to someone with the rest of the equipment to service AC.

 

You are absolutely correct but not everyone is going to run out and buy gauges, a vacuum pump and scale. If I was not in the HVAC business I would have them either, but at least this way the average guy will have an idea of what he's in for before he drops it off to be repaired. This certainly is not what I would expect a full blown repair shop to be doing, but for this purpose I think it’s OK