R134a in an R12 system

[Warren Seale]

Two years ago I replaced the compressor in my 79 and used expensive R12.  Over the past 2 years it lost some of the Freon and needed a charge.  The slow leak was not detectable with one of those electronic testers.  I no longer want to spend over $35 a can for the R12 especially if it will need an occasional charge.  So I gave in and put R134a into the stock system.  A lot of people said it won't work as well unless modifications are made.  Guess what.  It appears that it works just as good as using R12.  I don't know if it depends on the type of system but with the late 70s style using a pressure sensor and orifice tube it's fine.  Now I can save some $$$$!

[81Blackbird]

To help the cooling at idle I replaced the Orifice tube with a VOT or variable orifice tube.  At idle it will restrict the flow of the R134a so that it stays in the condenser a little longer helping it cool before hitting the evaporator.  When the pressure is up, at higher RPM, the valve will open because now there is more air flowing over the condenser.  I didn't change the old tube and fin condenser.  It works well but the parallel condenser will extract more heat.  What also helps the cooling is a hood to radiator core support seal.  This seal will push more air across the condenser rather than up and over it.  Also, a shut off values on one the the heater hoses will also reduce the air temperature by 3 or 4 degrees.

Below is an article taken from Griffiths:

R134a is NOT less efficient than R12 – Actually R134a is more efficient.

Pound for pound R134a is a more efficient refrigerant than R12, however it runs at higher pressures in some aspects and therefore requires more effective condensing. Whether R134a performs as well as R12 in any given a/c system depends upon system components and the amount of R134a used.

Given two identical vehicles, each with the same weighted amount of refrigerant, the vehicle with the R134a has the “capability” to remove more heat (measured in btu’s) from the vehicle than the same type of vehicle using the same amount of R12.

The most common influences which effect the capability of R134a to perform well are the condenser, in some cases the the superheat setting of the expansion valve or the amount of R134a. Condensers designed to release greater amounts of heat help to expel the greater amount of heat which R134a removes from the car’s interior. And by “matching” the correct amount of R134a to use in a given vehicle, correcting the superheat of the expansion valve (if necessary), you can in some manner nearly balance or match the amount of heat drawn out by the evaporator and released by the condenser.

These efforts to “balance” the system can not be realized if there are problems within the a/c system, such as: poor performing compressor, dirty condenser or poor air flow through the condenser, malfunctioning expansion valve, water or air in the system, improperly operating fresh air or heat input in the climate-air mixing system.

[b_hill_86]

Did you notice a difference with the VOT? That’s one thing when I replace my A/C components that I didn’t try and kind of wish I had. My A/C is nice and cold when moving but at idle it can tend to warm slightly.

[Wallington]

Also need to adjust the pressure switch settings for the potential of the R134a. This can be done if system is already pressurised if it uses the high/low switch with schrader valve under it, 78-81 style. Some are fixed and sealed settings, others have the adjusting screw but never gets checked, regardless of system.

[81Blackbird]

I can't tell you if there was a difference with the VOT since it was all changed over at one time.  Not sure if they adjusted the pressure switch setting, but the compressor runs all the time.  Unless it's cold outside. 

[Warren Seale]

I don't think the pressure switch setting needs to change unless you find that the evaporator is freezing up.  That's the purpose of the switch.  I haven't noticed this problem.  But I only operate the A/C when it is hot outside.

[Wallington]

That's when you back it off a touch.

[Jur5664]

From R12 to 134A is frequently done in France, and more generaly speaking in Europe