1st Generation SHO and Cobria,
To convert an A/C system that is using R-12 to R-134a without flushing out the R12 oil, and replacing the filter/dryer can be done, BUT it is risky. If you flush the system, DO NOT do anything to the compressor, other than drain the R-12 oil out of it.
The risk you run is that your system may end up being plugged on either the high or low side, and/or the compressor being destroyed by moisture and/or the microscopic bits of metal (compressor ware)that do not get trapped in the filter.
You could get lucky and go for several years before any of this bad stuff happens.
Most auto parts stores sell a "flushing kit" for about $50, and a good conversion kit should have the R-134a oil, springs, "O" rings, and R-134a fittings with caps. The conversion kit should be specific to the make of car, in this case Ford.
The system, once it is put back together, should then be vacuumed at about 30 inches of vacuum (depending on altitude - subtract 1 inch of vacuum for each 1000 feet above sea level)for no less than 30 minutes. I like to vacuum for 30 minutes, then let it set for 30 minutes to check for any leakdown. Then I vacuum for 30 more minutes.
Moisture (water) has a boiling temperature of 212 degrees F, at normal atmospheric pressure (approx 14lbs). Increase pressure, and the boiling point rises, for example your radiator cap at 16 lbs.
The boiling point is lowered under a vacuum. At approximately 30 inches of vacuum, water will boil at room temperature. The vacuum process is very important for the removal of moisture AND any remaining R-12.
Keep in mind that an A/C system is NOT an R-12 or An R134a "SYSTEM" any more than you gas tank is a Sunoco, or a Texaco gas tank. The exception to this statement is the seals and/or "o" rings have been changed to be compatable with the properties of R134a and R-134a oil. The oil in a conversion kit is a synthetic (ester) oil. Use R-134a oil made for a conversion (should be in the "kit".
Recharge the system with R-134a refrigerant. Use 80 to 85 percent of the amount of R-12 specified for the System. A 36 ounce charge of R-12, would be approximately 30 ounces of R-134a. This difference is due to the properties of the two different Fluids. ( I'll leave it at that, rather than go into a discussion of chemesty, physics, and thermodynamics)
I like to add approximately 24 ounces of R-134a, and then add enough additional R-134a to achieve 50 psi on the low (suction) side, and approximately 250 psi on the high side, depending on the outside temperature and the underhood temperature (ambient temp). In addition I shoot for a 35 to 40 degree temperature of the air coming out of the center of the dash, with the A/C on Max, and the blower on high, with the windows up.
The mechanics of the conversion are simple nuts and bolts. A set of R-134a guages, a vacuum pump, a thermometer, and a set of the tools used to spread the springs at the hose couplings (available at Auto Zone and other parts stores), are the tools needed (plus normal hand tools).
This is not a very difficult job. However, you must keep in mind that you are dealing with a liquid that has a boiling point of something below -200 degrees (-200+). If you get this liquid in your eyes the damage can be permanent and substantial. Instant frostbite! Wear safety equiptment. It remains a liquid because it is under pressure (raising the boiling point) in the can.
If you hook up the can to the high pressure side, which is supposed to be impossible but I have heard of folks doing it, you have an explosive situation on your hands (maybe literally if your holding the can). I am not trying to frighten anyone away from this job, but rather giving a warning that it cannot be taken lightly.
SDPATT, and others, please add anything that might make this post clearer, more correct, and/or more helpful. Thanks rangerj
