[Jim Sims]

Typically receiver/driers should be replaced whenever the system is opened for servicing. An old R/D designed for R-12 is not compatible with R-134a.

Cans of R-12 are not available "over the counter" to consumers. Do not mix R-12 and R-134a. When converting to R-134a the refrigerant oil must be changed. The system, if still holding pressure, should be taken to shop and any remaining R-12 extracted and recycled. Have them leak check the system. Then begin the conversion process:

Recommendations for good performance and value when converting a 911 from R-12 to R-134a:

- Extract any remaining R-12 refrigerant and recycle.
- Open all lines at low points and drain out mineral oil.
- Flush all condensers (this can be done on the car) and then allow them to fully dry out; again this is to remove the R-12 mineral oil.
- Do not flush the evaporator unless you remove it from the car and remove the expansion valve first. If the evaporator is out, clean the air side of dirt and lint and when reinstalling ensure the the foam air side seals are in good condition.
- Do not attempt to flush a compressor or a receiver/drier.
- Drain the compressor of mineral oil and replace with the appropriate amount of ester oil (POE). Use a fresh unopened bottle of ester oil; buy it in the 8 fluid ounce size bottles (NAPA and AutoZone sell this oil). If no other information is available regarding the amount of oil to add then replace what was drained from the compressor plus a ounce for every other component flushed or changed. For example, if two condensers were flushed and the receiver/drier replaced then add an additional three ounces over what was drained out of the compressor. Excess oil will impede performance as will R-12 system mineral oil left in the condensers.
- Install a new receiver/drier with XH-7 desiccant prior to evacuating the system. Most receiver/driers sold today contain the XH-7 desiccant. Inexpensive receiver/driers (less than $30) may be purchased from NAPA or AutoZone and can be used with some "hose flipping" if your wheel assemblies are not too big. Otherwise one will need the use the slender OEM Behr receiver/drier ($90+).
- Hoses if in good condition do not need to be changed to barrier style unless there is some other problem like a leaking fitting. If one is adding new fittings or components (such as a condenser or compressor) and can change both sides of the fitting convert to the o-ring style whenever possible.
- Replace all fitting o-rings using new HNBR rubber compound (usually colored a light green or blue) o-rings; coat the o-rings with Nylog (HFC compatible, part no. RT201B from Refigeration Technologies) gasket thread sealant and assembly lube. Also use Nylog on the metal to metal sealing surfaces of the flare type hose fittings.
- Install R-134a servicing fittings or adapters to both the high and low side; on some R-12 fittings it is necessary to remove the old Schraeder type valve core.
- Install a high pressure shutoff switch (wired into the compressor) in the high pressure line between the compressor and the first condenser. EPA requires that pro A/C shops do this when making the conversion to R-134a along with a sticker noting the conversion.
- Evacuate the system and leak check; if leak tight then evacuate for three hours using a good two-stage vacuum pump that can at least reach 40 microns of pressure; new dry vacuum pump oil in the pump is helpful as is having the car sit in the hot sun during evacuation.
- Charge the system using refrigeration pressure gages and thermometers) one at dash air vent closest to evaporator and one on the air inlet side of the main condenser) - the sight glass method doesn't work with R-134a. Air must be flowing over the condensers for proper charging - on a 911 have the engine deck lid down and the engine running at 2000 rpm. Do not over charge; converted systems will use less R-134a as compared to the original R-12 charge. Leak check the system at pressure.

If you do not understand these systems nor have the proper equipment seek experienced assistance and the right gear. A/C systems can blind, maim or kill you. Always wear eye protection.

A subcooler may be installed either before or after the receiver/drier in a 911. After the receiver/drier and as close to the evaporator as possible is better but not always feasible.

For improvements in performance I recommend (in order):
Proper conversion, charging and maintenance.
Better and leak tight air flow in dash and evaporator area.
Clean condensers with air moving over them (fans for slow traffic conditions- triplex pressure switched is the ideal).
Subcooler.
Better and more condensers.
Improved evaporators.