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Converting A/C from R12 to R134

 
Old 08-20-2010, 03:30 AM
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928man
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Default Converting A/C from R12 to R134

What should be done to convert an R12 system to R134? The system is not blowing cold air and I am replacing the compressor, dryer, and expansion valves.
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Old 08-20-2010, 04:44 AM
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Flush the condenser, evaporator and lines. You may want to consider replacing the hoses with new ones, or have them rebuilt using barrier hose. Replace EVERY o-ring with the green ones. If your compressor comes with oil in it, dump that **** out and put in ester oil. I think it takes 8 ounces, maybe more if have rear air. Get new service ports. Don't get the ****ty ones that just have an extension for the old Schrader valves, get the ones that have their own Schrader valves (you'll need to remove the old ones. Don't rush. Take your time and be thorough about it. It will make a difference. Vacuum and recharge.
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Old 08-20-2010, 06:03 AM
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The search function is your friend here.

There has been numerous write ups on this conversion as well as a Porsche Technical Bulletin on the subject.

Tails 1990 928 S4 Auto
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Old 08-20-2010, 10:41 AM
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VehiGAZ
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I say don't do it - it will run lower pressures and you will get colder air on R12 than on 134a, and it will be less work than making the conversion, too. I am dealing with getting mine working after changing back to R12, and it's not pretty.

+1 on the suggestion to remove the oil the compressor comes with and replace with a known type and quantity of oil.

Btw, you are replacing an awful lot of parts there - do you know exactly what was wrong with the system in the first place? Don't do all that work for nothing...
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Old 08-20-2010, 10:44 AM
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Originally Posted by VehiGAZ View Post
I say don't do it
+1

What's wrong with R12? Don't tell me the ozone layer either.
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Old 08-20-2010, 10:54 AM
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Was at a local HVAC supply store (for a $15 switch for my home's central air); R12=$1200/30 lb. tank; R22=$200/30 lbs (for my central air). UGH!
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Old 08-20-2010, 12:02 PM
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Originally Posted by mickster View Post
Was at a local HVAC supply store (for a $15 switch for my home's central air); R12=$1200/30 lb. tank; R22=$200/30 lbs (for my central air). UGH!
That's a little out of line.

R-12 at $675/30 lb.

R-12 at $699/30 lb.

There are a couple of 30 lb. cylinders on eBay right now for under $500.

When you consider that the refrigerant cost (which should be well under $100 for R-12) on a major A/C service job is usually a small portion of the overall cost (especially considering labor), you might as well get the what your system was designed for: R-12.
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Old 08-20-2010, 12:15 PM
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Originally Posted by mickster View Post
Was at a local HVAC supply store (for a $15 switch for my home's central air); R12=$1200/30 lb. tank; R22=$200/30 lbs (for my central air). UGH!
BTW, R-22 (the most widely used refrigerant in the world) is being phased out, too. Hello, R-410a.
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Old 08-20-2010, 12:15 PM
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Old 08-20-2010, 12:19 PM
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How many o-rings are there to replace on these cars?
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Old 08-20-2010, 12:23 PM
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Originally Posted by Mongo View Post
How many o-rings are there to replace on these cars?
Lots (24 if I counted correctly); I did all of them a year ago. Some of them are a pain to get to (like the ones on the rear A/C valve under the passenger seat). Front expansion valve isn't exactly lots of fun, either.
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Old 08-20-2010, 01:19 PM
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+1 on keeping R12. Readily available on EBAY and fairly cheap these days now that most cars in the world are R134. 609 certification easily obtained on line so you can order it.
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Old 08-20-2010, 02:02 PM
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Andy E.
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Originally Posted by VehiGAZ View Post
I say don't do it - it will run lower pressures and you will get colder air on R12 than on 134a, and it will be less work than making the conversion, too.
+100

With a backround of over 30 years experience in the HVAC & R industry, I'm strictly against converting an existing R-12 system using mineral oil to an R-134a system using fully synthetic polyolester oil. PAG recommends it only so that they'll be in compliance.

Compressor manufacturers demand a <5% residual mineral oil content when converting over to 134a. If not, the remaining mineral oil will not mix well with the synthetic oil and a sludgy mixture will take place. The sludge will then lodge itself everywhere causing havoc with your system pressures. Remember, the old oil does not only reside inside the compressor crankcase but is also logged in the receiver, condenser and evaporator coils as well. Can you be 100% assured that you've removed over 95% of the old oil without replacing major components?

Furthermore, 134a is not as efficient as 12. I've read all 'experts' claims that the two are identical. They are not and can never be on a 928 with an undersized condenser. Yes, all new R-134a systems utilize a larger condenser coil to make up for the added inefficiency of this refrigerant.

Save your money on conversions and invest it in some HVAC tools and a refresher course on system diagnosis & repair. A bottle of nitrogen with a regulator with service hoses is a good place to start. After replacing the old seals to the new green ones, you can use the nitrogen to pressurize the system to a few hundred pounds pressure and then use an ultrasonic leak detector to find leaks. This procedure will eliminate the need to use expensive R-12 to pressurize and leak test the system.
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Old 08-20-2010, 03:16 PM
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I am wondering where the thought that the 928 condenser is undersized came about? The ratio of cabin space to condenser size on the 928 is much better than many other cars. I don't currently have the measurements handy, but I did the calculations a couple of years back when I was messing with custom parallel-flow condensers. It was big enough that Porsche cut its size in the later years to add an ATF cooler.

And to the question about being sure 95% of the old oil was flushed out, I can certainly say yes to that. It is very easy to set up a flush apparatus with mineral spirits and you can tell when all the crap is out. Then when you blow it out with compressed air, you can tell when there are no mineral spirits left in the component.

Yes, R12 cools better than R134a and runs lower pressures, but when the whole climate control system is working properly in a 928, R134a is more than good enough to cool the car down unless you live in a place where the heat index (ambient temps + humidity) is regularly higher than 110 deg F. I could care less how high the pressure is in my system, as long as it's freezing me out of the car and the system works and doesn't leak, who cares?

My R134a converted 928 is black/black and we just came back from Savannah. On the way there we hit stop-go traffic for an hour or so. The dash read 104 deg F (heat index that day was 104 - 108 deg F) and I wasn't even using the rear A/C at all and my passenger started to get uncomfortably cold with the fan speed at '4'. I bet my high-side was ~300psi... yeah so what?

Oh, and if there ever springs a leak (and don't fool yourselves, an auto refrigerant system eventually will), fix the leak and $10 in refrigerant is all it costs me.

Dan
'91 928GT S/C 475hp/460lb.ft

Last edited by dprantl; 08-20-2010 at 06:11 PM.
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Old 08-20-2010, 05:28 PM
  #15  
Alan
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Originally Posted by 928man View Post
What should be done to convert an R12 system to R134? The system is not blowing cold air and I am replacing the compressor, dryer, and expansion valves.
Another perspective here. if you don't know why your current R12 system isn't cold - you may find simlar issues after the conversion too. It will be especially difficult to determine if it was a pre-existing condition or soemthing related to the conversion...

Many other things can cause a lack of cold air - stuck hot water valve, mixing flaps, temp sensors, flap position actuators.

Alan
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