My 2001 MS has 223K on it. My ac runs about 10 or 11 months of the year. It doesnt cool as well as when new,but still is fine with 105 to 118 in summer.
Just yesterday I started noticing a little more noise than usual when it cycles off/on but when going into the on mode. Unless Im imagining it,this is a clear sign to me of impending failure. Is this to be expected having been on for close to the mileage 223K?

 

It is possible that the compressor is ready to fail. The amount of miles you got on the Montero is impressive. I'd suggest to first check the refrigerant level in the system and more important to check if you have enough AC oil in the system.
You mentioned having more than usual noise when the compressor is cycling on/off. Does the noise occur when compressor is on or when it is off? The reason I ask this is i had a bearing in the ac pulley go bad and i was getting grinding noise when compressor was not running (pulley was freewheeling). When compressor engaged, the sound would go away completely. Puzzled me for some time and before i figured out the cycle pattern I though my compressor was going out (at around 170k). Turned out to be a simpler fix.

 

They fail eventually, but in different ways. First off, the refrigerant is probably a touch low, that's why doesn't cool as well. These systems only hold about 3-4 lbs of refrigerant and if even a little leaks out, performance suffers. And refrigerant does leak out through hoses, connections and so on. You can probably add a little and put a gauge in your vent, watch Stop when the temp goes down a little. Don't over charge. You'd need to take it to a shop to really get it dialed in, but ..

Most common thing I've seen is the compressor clutch bearing fails. That's because it always spins whenever the engine is running weather you are using your A/C or not (it actually doesn't turn when you use the A/C). You can actually replace most compressor clutch bearings if you have a press. It's a $5 or $10 part. Or replace the clutch.

The other main wear item is the clutch itself. And if there is noise when it's engaging, that is probably something to zero in on. If the noise is constant, check if you can to see if the clutch is slipping. If the noise occurs when the compressor is disengaged, but goes wasy when engaged, the bearing is going. Usually sounds like a whirrring or howling noise. In your case probably makes sense to replace the whole clutch. But a worn or slipping clutch can chatter. Is it oily??

And of course the compressors can fail. There are usually 4x pistons inside with rings that move inside a bore. Like any mechanical component, they will wear given enough use. But they are flooded with oil and it is a 'closed' system meaning no contaminants get in there. So, they can last a long time (just think how many years a refrigerator can last?). There are also little reed-type valves (typically) that could stop sealing properly. But compressors can last a long time. Put a stick or dowel to the compressor body, then to your ear and see if you can ID the sound coming from the compressor body itself.

Also be sure to check your condenser coil in front of the radiator. People don't usually think of this, but if you added lights or a bull bar that obstructs air-flow, or there is crud or bugs clogging the fins ..or bent fins, or radiator behind it is clogged ..it's not going to get the proper airflow and that's going to cut down on your transfer of heat. Be sure the condenser is well cleaned. Your system basically uses the change of refrigerant state (Liquid->Gas->Liquid) to transfer the heat from the interior of your vehicle to the outside of the car where it is removed when air passes over the condenser coil. Theoretically an AC system doesn't 'Cool', it removes heat. So, ANY reduction of airflow at the evaporator (inside the heater box - Rare) or up at the condenser coil (front of radiator - Common) is going to directly impact the system's ability to transfer heat properly. That's why you get better cooling when moving rather than sitting in traffic on a hot day. Since the condenser is exposed to the elements, mud, bugs, debris, etc .... just a little bit can make a noticable difference. There could also be obstructions internally in the coolant lines, but that's more rare.

The AC compressor on my wife's Jeep has over 300k on it. It doesn't get as much seasonal use as yours. I took it apart at 240k thinking I would need a new one. To my surprise it looked like new inside. I just replaced the front seal, compressor body seals (front seals commonly leak) and clutch bearing she's been good since. I'd buy a new one, but every reman compressor I have boughten has leaked within a year or two and brand new ones are quite expensive. I've read that it's common for reman companies to inspect the internals and just replace the front and body seals, (main leak points), replace the clutch and call it rebuilt. Most of the time internal components aren't the main wear items. Century auto or A/C from Tuscon sells all kinds of A/C parts most places don't handle. Good luck

 

Nice reply, where can I find a clutch rebuild kit? mine squeals really bad when I engage it...

 

Thanks jeep walker...Im impressed & what a thorough and organized reply!
.That would have taken me at least a couple hours to write up even if i knew everything about ACs. Well i did know that cooling is actually the removal of heat and the evaporation is a cooling process,but thats about it. Ill have to go over all of his points and see where i go from there. Oh, and to answer your question:no it doesnt appear to have any leaks.

 

You're welcome guys.
I don't know where to find a clutch rebuild kit for a mitsubishi. I guess I would try: Automobile Air Conditioning Parts of Tucson Arizona, Century Auto Air, AC Parts, AC Service and Hoses, www.centuryautoair.com. Otherwise, a detailed google search or Ebay (used) would be worth a check. How much is a new clutch assembly?

Jeepwalker.