1986 Dodge Truck R12 to R134a Refrigerant Conversions

JYBUSBIN

MEMBER
1986 DODGE TRUCK
V8
4WD
AUTOMATIC
168,000 MILES

Air conditioning systems in older cars and trucks use R12 refrigerant. If charging an evacuated system there is usually a specifed weight of (R12) freon to be added to the system. My questions concern changing over to the R134a refrigerant. Once seals and new oil have been added to a "cleaned" system, is the weight specification for the R134a refrigerant the same as specified for the R12? And if not, is there a translation table table avaailable for this purpose? Also, it seems that systems using the R134a refrigerant must utilize higher pressures to achieve the same level of cooling that was attained by the R12. Since an R12 to R134a conversion is using the same components, how less efficient is a system that uses R134a with R12 era components?

Thanks,

Jim

1 Reply

CARADIODOC

MECHANIC
33,871 POSTS

All common questions that have become non-issues. When we first started talking about doing these conversions, it was suggested that the compressor needed to be changed to a higher pressure pump to achieve the same efficiency. No longer true. R134 molecules are smaller so they seep through the porous rubber hoses easily. The hoses needed to have a nylon liner. No one changes hoses. You won't even find them unless they're custom made. The evaporator and condenser needed to be changed to handle the higher pressures. Not necessary if you don't replace the compressor. Higher operating pressures means changing the pressure cutout switches. Not now.

It's true the R134 is not as efficient as R12, but if you think about this backwards, efficiency is not the same thing as temperature. You already know that you have to remove all traces of R12 oil, but that's it. The "H" valve and sensing bulb still will try to maintain the evaporator temperature at around 40 degrees which R134 can easily achieve. As the heat load increases, so does the required amount of refrigerant. Would you agree the H-valve restricts the amount of flow? In other words, there's more refrigerant available than is needed. That's why the compressors in some systems cycle on and off. R134 still gets plenty cold when it vaporizes but it doesn't absorb as much heat, therefore, you need more of it to maintain the evaporator at 40 degrees. There's plenty of refrigerant in reserve, and the compressor can cycle on longer. Self-regulating compressors with variable-pitch wobble plates will pump more vapor to meet the cooling demand.

What it all boils down to is you must drain out the old oil, install the adapter fittings, replace the receiver / drier, and recharge with the normal amount of R134 and correct oil. The pounds of refrigerant are volume, not weight, and the volume hasn't changed. Poof! Done! Did this on my 1988 Grand Caravan many years ago and froze myself out of the vehicle.

With the lower heat absorbtion, there will simply be more flow and less refrigerant in reserve, but the temperature out of the ducts will be normal. It's possible there may be a few bubbles in the sight glass when the system is fully charged. I added 2 ounces extra to my van but there was still the occasional bubble.

Caradiodoc