Thought these comments might help resolve questions regarding compressor problems noted on the list:
There are numerous reasons for compressors bumpin belts, locking up or making internal noise.
If the belt has jumped the track it is more often the result of the compressor locking up (either for good or for the moment, and in either case you might as well write off the old compressor). Lock-ups are a result of either (1) poor lubrication or (2) high system pressures.
(1) Lack of lubrication can be caused by a few scenarios:
(a) Low refrigerant levels - caused
by system leaks or restrictions. The compressor needs a constant
flow of oil. The oil is carried by the refrigerant, so if you
have low refrigerant flow you have a low oil flow. If the
system is constantly running with a "border-line" low pressure,
the compressor life will not be long. Repair all system leaks,
such as hoses, expansion valves, fittings, condenser, evaporator
or the compressor itself.
(b) Oil traps - on systems with dual evaporators there
is solenoid valve at the rear evaporator
system. We have seen solenoid valves that are not completely
closed when the rear system is turned off. If the solenoid
valve is partially open by a hair, refrigerant will pass through
the small opening and oil will collect in the rear section of the
system. In time this collection of oil will reduce the total
system oil level to the point that the compressor will "lock up".
(c) Failure to replenish the oil supply over the years when
"topping off" a system - typically this happens when an owner
notices bubbles in the site glass and attempts to add more refrigerant
to the system without adding more oil. If you are wondering how much
oil you need in the system and you need to guess, here is a good rule
of thumb that works (on the 1980+ systems), when replacing the compressor
have at least 6 ounces of oil in the compressor, if you replacing
another major component (condenser, evaporators, drier) add 1 additional
ounce per major component. R12 refrigerants use "refrigerant mineral
and R134a refrigerants use either ester or pag type refrigerant oil.
Ester can be used for either R12 or R134a refrigerants and we have found it has an excellent performance record. Keep in mind that if you have too much oil in a system you will have poor performance as the extra oil will coat the condenser and evaporator walls which reduces the transfer of heat.
(2) High system pressures or high "head" pressures can be caused by a few problems:
(a) overcharging the system, usually happens when we try to
accomplish the charging task by purchasing those economical cans of
"drop in" refrigerants from the local parts store and we fail to use pressure gauges to monitor the system.
(b) condenser fan failure, dirty condenser fins, internally contaminated condensers,
(c) expansion valve failure, or (d) not common but it happens - mixing refrigerants.
(a) Most clutch bearing failures, other than simple high mileage, are caused by heat. So if you have either low oil flow or high head pressures the heat moves quickly from the aluminum compressor to the bearing on the nose of the compressor. The grease in the bearing melts down and the bearings wear out.
(b)Clutch coils typically fail because of heat rather than indirect electrical shorts. As the heat builds up in the aluminum compressor it is transferred to the steel can and copper windings making up the clutch coil. The windings are covered with a thin layer of insulation and are packed in the steel can with epoxy. The insulation breaks down quickly with the high heat levels and sooner than expected the coil windings short out. You can imagine as well the transfer of this excessive heat to the bearing and visa da versa.