GMC Western States

 Tech Center Number 16 - November 1996

Is It Cold Enough For You ?

Here is a good winter time project for
those of you that travel less during the
winter months. Check your dash heater and air conditioning system and perform any needed repairs and up grades. Zay Brand and Duane Simmons wrote and presented an excellent seminar in 1994 on the design
and operation of the four air flow system configurations (see Zay, 310-598-5028, or Duane, 714-633-4731, for reprints). Tips included:
1) Checking all vacuum controls and lines. 2) Replace deteriorated foam seals on the temperature and re-circulation doors.
3) Check the vacuum water valve to be sure it cuts off the flow or add a manual ball valve in series.
4) Modify the air flow on 75 an 76 systems to provide 80% re-circulation rather than 100% outside air.
5) Add a second evaporator unit in the drivers area in parallel with the under dash unit.
If your air conditioning system needs recharging it probably needs repair. In fact, in Southern California and some other states, certified technicians will not recharge a system without performing a leak check and repairing any leaks.
With the ban on manufacturing R12 Freon (CFC-12), what are the current alternatives for the do-it-yourselfer (DIY or the shop technicians? First, it is illegal for any one to discharge any of the current refrigerants into the atmosphere. A recovery system must be used. It is possible for the DIY to make a simple recovery system, but that is a subject for another Tech Center.
The most straight forward solution is to have the system repaired and recharged with R-12 at a trustworthy shop. Even at the current price of around $25-$30 per pound this is not a bad deal. R-12 will be around for some time, only the manufacturing has been banned. That, coupled with an increasing excise tax will cause the price to go higher, but supplies are expected to be available for some time.

Another alternative is the use of one of the EPA approved so called "drop in" substitutes that are shown in Table 1. The EPA approval simply means they are
safe and does not speak to performance. These are not really "drop in" and require fitting changes and labeling. These also require recovery of any residual R-12. In addition, the availability may be regional or even specific to certain shops, creating a potential service problem.

Acceptable Substitutes for Air Conditioning under the Significant New Alternatives Policy (SNAP) Program as of May 22, 1996:

 (Name Used in Federal Register)

 Trade Name

Motor Vehicle AC uses

 Non-Automotive Motor Vehicle AC uses



 R, N*



 R, N*



 R, N*



 (HCFC Blend Beta)


 R, N*

 (HCFC Blend Delta)


 R, N*

 Blend Za


 R, N*
Key: R = Retrofit Uses, N = New Uses
*These refrigerants are actually "acceptable subject to use conditions." The conditions include 1)the use of unique fittings, 2)the use of descriptive labels, and 3)a prohibition against topping off one refrigerant with another. Details may be found in EPA's fact sheet on automotive refrigerants.

If your system needs major repairs such as a new compressor, than an excellent approach is the conversion to R-134a. The performance is slightly reduced, say 3-4 degrees because of the higher vapor pressure, but if one can make some of the changes outlined by Zay Brand to improve overall performance. Conversion to R-134a involves the following considerations as outlined by the EPA Significant New Alternative Program (SNAP): According to EPA regulations, the use of any alternative refrigerant to replace R-12 requires at a minimum that:
-- unique service fittings be used to minimize the risk of cross-condition of the air-conditioning system or the service facility's recycling equipment;
--the new refrigerant be identified by a uniquely-colored label to identify the refrigerant in the system;
--all R-12 be properly removed from the system before filling the system with an alternative refrigerant; and
-- separate, dedicated EPA-approved equipment be used to recover the R-12 from the system.
In addition, alternative refrigerant blends that contain R22 must be used with barrier hoses [these are high density, extremely low permeability hoses].

PAGs vs Esters.

The mineral oil used with R-12 cannot be sufficiently transported throughout the a/c system by R-134a. Automobile manufacturers tested both PAGs and esters for refrigerant/lubricant miscibility, lubricity, chemical stability and materials compatibility. In the process of developing recommendations, they also considered the additives and conditioners present in the oils.
Most - but not all -chose to use PAG lubricants in new vehicles equipped with R- 134a, and are also recommending PAG lubricants for retrofits. Some compressor manufacturers are shipping new compressors with PACTs, some with esters, and some are shipping them empty.
The amount of mineral oil that can safely remain in a system after retrofitting, without affecting performance, is still being debated. It was originally thought that any mineral oil left in the system might cause system failure. As long as the tech has removed as much of the old mineral oil as possible, any residual R-12 left in the system should not have a significant effect on the performance of the system. Removing the mineral oil may require draining certain components. Unless the vehicle manufacturer recommends flushing the system during the retrofit procedure, a service tech can assume that flushing is not necessary. (Although the SAE J1661 procedure for retrofit includes flushing, SAE no longer believes that flushing is critical to a successful retrofit.)
When R-134a was first introduced, it was thought that all non-barrier / nitrile hoses would have to be replaced during an a/c retrofit. Early laboratory tests showed that the small R-134a molecules leaked through the walls of non-barrier hoses more readily than the larger R- 12 molecules did. In the lab, this caused unacceptably high leakage rates. More recent testing, however, has shown that oil used in automotive a/c systems is absorbed into the hose to create a natural barrier to R-134a permeation. In most cases, the R-12 system hoses will perform well, provided they are in good condition. Cracked or damaged hoses should always be replaced with barrier hoses.
Unless a fitting has been disturbed during the retrofit process, replacement should not be necessary. Most retrofit instructions call for lubricating replaced O-tings with mineral oil to provide this protection.


Industry experts once thought that a retrofit would require compressor replacement. This belief helped create some of the horror stories about the expense of retrofitting. Now it is routinely accepted that most compressors that are functioning well in R-12 systems will continue to function after the systems have been retrofitted.
When a compressor is first run with R-12, a thin film of metal chloride forms on beating surfaces and acts as an excellent anti-wear agent. This film continues to protect after the system has been converted to R-134a. This helps explain why a new R-12 compressor may fail more quickly if it is installed in an R-134a system without the benefit of a break-in period on R-12.
A few older compressors use seals that are not compatible with either R-134a or the new lubricants. Any compressor that has Viton seals should not be used with R-134a because the refrigerant will cause the seals to swell excessively.
Of course, any compressor that is not in good shape should be replaced during the retrofit procedure. Service techs should make sure that any replacement compressor is approved for R-134a by its supplier.

Desiccants, Accumulators, Receiver/Dryers

R-12 systems use an XH-5 desiccant, while R-134a systems use either XH-7 or XH-9 desiccant. Some manufacturers recommend routine replacement of the accumulator or receiver-drier with one containing XH-7 or XH-9 during the retrofit procedure. (Any systems with silica gel should also be switched to XH-7 or -9 desiccant.) Others recommend leaving it alone. Manufacturers generally agree, however, that the accumulator or receiver-drier should be replaced if the vehicle has over 70,000 miles or is older than five years, and is opened up for major repair. In that case, the only recommendation is to use the R-134a-compatible desiccants.

Condensers and Pressure Cutout Switches

When retrofits were first studied several years ago, it was thought that the condenser and perhaps the evaporator would have to be replaced to maintain an acceptable level of cooling performance on a retro-fitted system. Now, it is generally accepted that if an R-12 system is operating within the manufacturer's specifications, there may be no need to replace either part.
It is true, however, that the higher vapor pressures associated with R-134a may result in lost condenser capacity. When retrofitting, service techs should consider how the air flow and condenser design on the particular vehicle will affect the success of the retrofit. In some cases, the installation of pusher-type engine/condenser cooling fans mounted in front of the condenser have improved the performance of retrofitted a/c systems.
If you haven't replaced any hoses in recent years, Cinnabar Engineering Inc. is offering barrier hoses and compressors with barrier seals fitted for R-12 refrigerant, or you can visit your local Hose Man.


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