Here's a schematic of a X200 air conditioning system. The AC refrigerant (R132A) on this Jaguar AC has nullified, through a thoroughly corroded pipe. R132a isn't as dangerous as R12 was to our climate, though, still take care of our environment, don't de-pressurize the system yourself! let it be pumped out through an AC specialist. In this case, I decided to change all the O-rings myself, than, I already had the engine room apart, so it was easy to come by. Get yourself the recommended O-rings (set), go to your friendly A/C specialist, ask, if you can have a few CC of AC compressor oil (R134a Jaguar specification W??? PAG Polyalkyleneglycol or PAO oil 68).

O-ring:
Do one connection at the time, work clean and fast (close ends immediately, (dryer bottle, compressor oil saturation!), don't scratch, damage the new O-ring, nor its O-ring pipe sealing surface, are pipe sealing surfaces damaged, replace them or try when the pipe sealing surface is not to deeply damaged then use 600 or 1000 waterproof sand paper, rotate radially, and then polish surface. When surfaces are clean, without scratches, burrs, crannies, put a drop of compressor oil on the new O-ring, fit it with your fingers and not with finger nails nor screw driver, tighten to recommended torque.

Don't use any kind of household cleaners, use only the recommended or a fussy free cloth.

Using compressed air:
Should you use oil free, dried pressed air, probably with a (micron) amount of oil, water vapour in it, or should you not? Compressor mineral oil combines with mineral compressor oil Pag, mineral oil doesn't combine with ester oils, for instance, Polyalkyleneglycol. As the first refrigerant system changes took place from R12 to R132a and after changing several components on the system, there was still a considerable amount of mineral oil contained, witch, would have meant, extra flushing $$$, draining or even rinsing the compressor which would have made it even more expensive, as it already was. So, ester oils couldn't be used on these systems, they had to use a mineral oil to mineral oil based lubricant (Pag), together with the refrigerant R132a. The mineral oil left behind didn't do any harm to the refrigerant, on the other-hand, R132a AC system with an ester oil as the meant lubricant, (I'm writing here about micron amounts of mineral oil), these micro amounts of mineral oil will be rolled through the moving refrigerant and stay at the lowest point of the system, where it want take part in the necessary lubricating process, so, never mix these oils, compressor depth!

We have two different diameters of piping on an A/C, the one with a small diameter is our high pressure side, which carries liquefied refrigerant, the other diameter carries gassed refrigerant. High pressure side, compressor outlet - condenser (liquefaction) - dryer bottle - inlet expansion valve - low pressure side (gassed)- evaporator inlet - evaporator- outlet expansion valve compressor inlet.

Possible failures:
Mostly, not enough refrigerant, due to diffusion (normal). AC compressor: leaks oil (radial seal, seals)=leaks refrigerant, compressor: worn-out or seized, compressor clutch: winding open, (slips, no power, not enough power (voltage drop, bad connection), relay, module, .... ). Condenser: Oily spots=leaks refrigerant, a very old one probably with bad heat dissipation, cloggy (ware, corrosion residues)=bad cooling. Condenser cooling fan: Out of order=limited heat transfer, first stage (low) on = normal cooling, second stage (high) defective (heavy cooling) = lack of cooling at high temperatures, fan motor, pressure sensor, voltage drop, module, ... Dryer bottle: Opened pipe-lines for to long, dyer bottle saturated itself with condensed water (this happens fast, 80% saturation in minutes), now, with the dryer bottle saturated you have ice-crystals circulating, being pump through the system, which leads to, after a while, to clogging of the expansion-valve measuring orifice, damages to the compressor (ice is not compressible) and to a lack of cooling. Older dryer bottle: Filter clogging, due to compressor ware, corrosion residues, which ultimately leads to compressor death.

Pipe/hose:
This Jaguar with a thoroughly corroded high pressure line, system had been open for years, cost-cutting measures had to be introduced. With the use of the internet, no reasonably priced AC line could be found. So, I decided to repair the line myself. with the use of a pipe cutter I replaced the corroded length, turned on the lath, made two sleeves, soldered these with AC aluminum solder, filled the pipe with a solution (warm-hot (faster dissolving)) household drain pipe cleaner to dissolve the aluminum oxide layers that occurred whilst soldering, flushed it thoroughly.

A/C compressor (7):
The Jag's compressor hasn't be used for a long time, the compressor is still dry, no compressor oil leaking, though while the compressor was exposed to atmospheric moisture the compressor had to be dismounted and oil had to be changed (turn compressor shaft and fill into measuring cylinder, rinse, with the same amount of new compressor oil again) my hope is that, the compressor's casing seal, its radial shaft seal, shaft running surface haven't dried out or been corrosively damaged through the lack of lubrication.

Some newer compressor models (to 2011, around about 7 years now), don't use a magnetic clutch, these compressors are constantly engaged, this means, by the lose of refrigerant (partially or thoroughly), lubrication diminishes, friction increases, ware, damage occurs. For example, an automobile with a Denso compressor, one without a magnetic clutch, shouldn't be driven further than 100km when cooling has halted. (see manufacturer's specification!).

Newer (models to 2011) A/C systems that are constantly engaged, have to be monitored closely, than replacing a defective compressor, with internally fragmented remains scattered through out the systems, means replacing condenser, dryer bottle, flushing lines thoroughly, and installing an extra compressor suction line filter, is thus time consuming and an expensive task. Recommendations from vehicle manufactures like Mercedes, VW concerning the torque security mechanism of the compressor, Mercedes suggests, if torque security mechanism sets in, all components of the A/C should be changed. VW on the other-hand recommends in a compressor guaranty case that the expansion valve, condenser be replaced, too. Always follow manufacturer recommendations! Problems will come sooner or later. Down below Q/A.

Why has the torque protection mechanism set in? Compressor torque exceeds manufactures specification.
What could have caused this problem? Compressor internally damaged, compressor oil dirty (old), compressor leaks, seized pressure switch (pressure to high).
Defective generator (free wheel), an overseen problem.

Condenser (9):
Check for damages (oily spot, oily connections, this includes all system line connections, are always obvious signs of leakage, where oil leaks, refrigerant leaks too. This Jag had a decaying, deformed, leaking condenser, installed a rebuild.
Carefully clean the condensers cooling fins, if needed straighten (special comm tool), check that the condensers cooling fan rotates freely, if possible, check the fan's function of low and high speed, by disconnecting and bridging its connection. Mostly controlled through a transducer on the high pressure line, whereby higher levelled pressure indicates maximum cooling, the reciprocal, normal cooling condition.
Most of the compressor ware-residues lands here on the inner wall tubing, these residue plaques change the heat dissipation (insulates), however, first after years/miles of service. So, if its an old one don't flush it, replace it.
Newer condenser models, do not only have a coil line but also parallel cooling lines, that can't be flushed thoroughly.

Note! Changing any pipes/hoses or other parts, add compressor oil, use your JTIS to find the proper amount to add for each part.

Dryer bottle (receiver, filter) (12):
Not serviceable, no dryer agent rejuvenation under normal means possible, if its a new bottle that saturated, here's what I found, is it worth rejuvenating?
R132a:
Silica gel 120°C,
3a Molecular sieve 130°C-250°C,
Active Aluminum Oxide 177°C-316°C all at atmosphere, time? 2h?, vacuum? bringing only water to evaporate at a lower temperature, haven't tried it yet, internal package holding the substrate, will probably not withstand high temperatures.
If it's a Zeolith A mole sieve 3a:
>80°C over the weekend,
drying-pistol >100°C,
200°C 2h,
200°C 0.7mbar over night,
100°C 0.35mbar),
an open system or one that had been open (ends that haven't been closed immediately), drying systems will have saturated, and therefor can not absorb any remaining moisture (<3%), acid residues, bringing the system into a vulnerable state with excessive internal ice crystallization, clogging, a not compressible substance, leading to compressor damaging, corrosion.

How to check if a dryer bottle has a restriction, with the system properly filled, AC turned to maximum cooling conditions.

See, feel or measure the inlet(e) and outlet(f) temperature of the dryer bottle, there should be no temperature difference between the two, an obvious sign of a restriction is an icing of the the outlet (line) and icing at the bottom of the bottle. Here the refrigerant starts to expand, becomes gaseous before it passes through the expansion valve.

If your system has just lost refrigerant through normal diffusion, though, still with the remaining pressure above atmospheric pressure no atmosphere moisture has crept in and system only needs a refill, the dryer bottle doesn't necessarily have to be changed (age?).
Note! The last thing you change on your AC is the dryer bottle! Never buy a used dryer bottle they have saturated. New dryer bottles when purchased should always have their inlet and outlet plugged, air tight!

Evaporator:
Most of the time hard to reach, one thing you can do, is, look at the condensed water outlet, probably underneath the car, if you can see (feel) a drip-let or even smears of an oily substance in or around the outlet that is a typical sign of refrigerant lose, either from a bad o-rings, expansion valve, corroded pipes or the evaporator itself (corrosion through decaying organics acids).

Expansion_valve (Thermostatic-valve)(2):
It's not serviceable, (6) liquid (high pressure) refrigerant entrance port, (5) measuring orifice, (8) gassed (low pressure) refrigerant entrance to the evaporator, (10) gassed (low pressure + heat added (Hella, opening temperature 6.3°C) refrigerant exit.

Probable problems: If new, jamming (5) can occur through metal debris, left from manufacturing. Ice crystals jamming measuring orifice (5) (plugged and not plugged cycle, leading to a warm/cold cycle, is a case of dryer bottle saturation, seizing (9) of the measuring orifice's mechanics or evaporation (1,3) of its own internal filling of R132a, either through default or corrosion after years of serving a cool climate, and probably years of service neglect itself.

Pressure Gauge (Fast Test), Air Conditioning system with an Expansion valve

Important: Don't forget to change the Schrader-type valves, they are found in the Gauge ports (5,10).

A/C Vacuum (pump):
If the A/C system was thoroughly de-pressurized, atmosphere (condensation build up) has occurred internally, A long evacuation session has to pursue (>120min), if possible warm-up engine, turn heater, re-circulation vent on to heat up the evaporator, A/C linings. When A/C components warm-up, water evaporates (20°C 23mbar, 50°C 123mbar, 70°C 311mbar) at a less lower vacuum, pump until vacuum doesn't fall any more (vacuum stays constant), close vacuum valves, turn off pump, read vacuum-meter, vacuum should be constant for 3min. (Trucks 5min.) if it rises a bit, then stops, keep on vacuuming than the system still contains moisture, if not there is leakage. You can also let the system be flush dried through the use of dry nitrogen gas.

Leak tracing methods:
If no refrigerant sniffer at hand, in many air-conditioning systems installed in vehicles use a florescent dye (highlight yellow) where leakage can be traced through the use of an UV lamp and special goggles, whereby a dye capsule in the dryer bottle dissolves when refrigerant is added.
Soap lye or special leak detection spray makes leakage visible thanks to bubbles.

Tips:
Use the A/C in all seasons or once a week (30min), to keep your car, car windows dry (free of condensed-water (wet foot mats)/frost), and last but not least, to keep your A/C's O-rings, compressor seals oil moist, otherwise they will shrink, harden, become leaky, no rejuvenation possible when dried. Be aware gasoline consumption will rise.

An auto-mobile AC isn't a closed confinement as in terms of a refrigerator, freezer, fridges where pipes are soldered, compressors are capsuled.

Refrigerant loss of about 10% per year, + an age dependency, is to be considered normal (older car models). Cause, diffusion between refrigerant and the flexible hosing, O-rings. Through the loss of refrigerant, internal pressure falls accordingly. With an out of a pressure/temperature parameter the PCM breaks the circuit to the magnetic clutch, system shuts off.

Lose of refrigerant also means, your compressor has to do more work (on/off cycles), ware and gasoline consumption rise.

Turn the AC temperature to about 6°C under the outer temperature, thus avoiding a temperature shock, especially with elderly on-broad! Not to blow cold air directly towards head, or body ligaments, is a good cold prevention.

Turn off your AC a few minutes (to 10min) before ending your trip, this would give the evaporator and its concealment time to dry, therefore helping to reduce unwanted odours, corrosion.

If you don't have a pollen-filter, if possible install one. Stopping pollen, dust, dirt from entering your heating/cooling system, further stops unwanted odours. Change in recommended or area specific intervals.