: R134A conversion: 1970 SDV

07-06-06, 08:07 AM
Hey guys. Now that it's summer again, I have started looking into reviving the AC system on my Caddy. Since the cost of R12 is insane and I hate the thought of taking my car to a shop anyways, I want to switch it over to the R134a and do the work myself.

The part I'm not sure on is what needs to be done with the controll part of the system? The '70 system uses an expansion valve and a STV to regulate pressures in the evaporator core. No cycling of the compressor - it's on 100% unless I turn the climate controll off... (although I am thinking about installing a high pressure cut-off switch)

Do I need to switch the expansion valve to let the correct amount of refrigerant in? I've read something about switching the fluid in the temp probe so it works right? Is this nessacary? Are there R134a-designed expansion valves that would bolt in?

I've also read that you want lower evaporator core pressures with R134a, something like 18-23psi. The FSM however states that my STV is designed to regulate the evaporator to 29psi and makes no mention of adjustment. I want this car to have frostbite-inducing AC if possible, and it seems like the evaporator core pressure would get in the way there.

Im also looking around for a set of manifold gauges and a vacuum pump - any sugestions on low-priced sources? NODIH - if you're reading this, you mentioned a homemade, hand-powered pump for the AC system, could you give some more details? It sounds like something right up my alley there.

07-06-06, 12:26 PM
Check my thread on this forum on AC Flushing (search for ACETONE and FREON, I started the thread also, that should find it), for a R12 to R134a, it is a good idea, but I have done several without flushing. But the one (non converson, already was R134a) that I flushed is now a frostbite inducing AC system now, and before it wasn't.

Rule of thumb is 85% of the freon that was in R12 system for R134a conversion. Get dual gauges and gauge it honestly. It works best. As for what your pressures should be, well, hard to say, but I did post the chart for the 94 Fleetwood to another poster recently, the 70 is larger, so it might take a little more freon that mine. My car uses 1lb 12 oz I believe R134a. So likely 2 lbs would be close if not dead on. Run it by the 85% rule of what R12 should be in there.

My cost effective gauges were the low pressure gauge on the cans that you can get. And a Low/High gauge from Miejers (Walmart competition from Michican that is spralling to Chicagoland and Springfield, IL) that was only $20. Autozone has a cheap set for $50 or so that is to mimic a good manifold pressure set, just much smaller.

I use the engine to vacuum me down to around 18 inches or so, whatever idle vacuum is. Give it 20-30 min on it. Then go with the hand pump to 25 inches. That is got to be very close to 100% water free. The pump is similar to this: http://www.mityvac.com/pages/products_hvpo.asp and exactly like this: http://www.actron.com/product_detail.php?pid=16292. I hacked up an old R134a filller hose for my R134a cars and another (my last, sob...) R12 filler hose for R12 converted cars, as it is hard to vacuum through the R12 to R134a fittings. I have often found the R134a fittings you need to press them in to get to them to fully open. Espectially through the adapters. Some leak, so sometimes you need to pull the adapaters off to get the valves to seal.

Now, an issue on older cars, the molocules in R134a are smaller than R12, so the later cars got nytril barrier lined hoses to prevent leaks. R12 cars generally don't, at least for sure not that old. So expect it to be somewhat leaky and lose its R134a faster than R12. R12 systems generally were pretty good on leaks, R134a generally pretty bad. And Conversions worse. IMHO at least...

Remember, the pressures are based on humidity and temp, doors open, fans on high and if possible fans pushing air through the radiator in addition to the mech fan. Only then are the pressures accurate. Less air, higher pressures and less AC performance.

The argument on water pump pulleys I am sure goes on, but the real reason to have smaller water pump pulleys is bring additional air through evaporator to dissappate the heat (especially at idle to maintain acceptable AC performance), not to improve any cooling through the engine radiator. R134a is not good at low airflow, R12 always will blow it away for performance. So expect it to get warmer at idle, but ok once you are moving. Check into an electric fan that is triggered to run with AC on, that would help.

07-06-06, 03:00 PM
Yup, my FSM has all the charts on compressor pressure and outlet temp as a function of ambient temp and humidty. The system takes 4# of R12 though :D So, likely over 3# of R134a! Can't imagine shelling out the fortune for that much R12! Haha 1970 is so long ago that the service procedures don't even mention capturing the R12 when depressurizing the system for service. It actually says that the referigerant should escape as a gas and to use a special tool to avoid any oil being let out along with it...

I'm definately going to get a set of manifold gauges. Since my low side is fixed, I won't know a thing without a high side gauge. I suppose I'll have to see if I can find an old R12 hose somewhere to make a redneck vacuum pump. Come to think about it, just hooking the AC to the engine vacuum and going for a drive on a hot day might just do it. All the heat under the hood, plus the vacuum ought to more than vaporize any water... Of course, I'll need to buy an air compressor to flush the system, so might just spend the extra $10 on an air powered vac pump.

Yeah, I've read about the hoses, but the big auto parts stores don't even list them for my car. Maybe I'll see what I can find in the junkyard, or just keep a can or two of R134a from Wal-Mart on stock. Some of the old car conversion sites I've found on Google say tha the older hoses are thicker so don't leak as much as one might think. Who knows, guess I'll find out.

My biggest concern performance-wise though is the low-side pressure. The stuff you posted has a bunch of different values for the low side pressure, while my system is designed to hold the low side constant at 29 psi. Some of the things I have read sound like this pressure won't give me as good performance as I could be getting if I could drop that pressure by about 10#. What kind of low side pressures do you usually see on a nice cold R134a conversion?

07-06-06, 03:08 PM
4#! Wow! My Suburban has around that! I'll have to find you the Suburban AC charts, they would be better for you.

Johnny Bravo
07-06-06, 08:14 PM

Johnny Bravo
07-06-06, 08:14 PM
the real reason to have smaller water pump pulleys is bring additional air through evaporator to dissappate the heat (especially at idle to maintain acceptable AC performance), not to improve any cooling through the engine radiator. .
You mean condensor, not evaporator.:tisk:

07-06-06, 08:32 PM

You got me! You are correct sir!

I have it right in my head, just doesn't mean my head is on straight all the time!

You mean condensor, not evaporator.:tisk:

07-07-06, 12:16 PM
oh, yeah. It has a big flex fan up front, no clutch, and of course the factory shroud. Even at idle in drive you can easiy feel the suction with your hand several inches in front of the condenser.

07-07-06, 12:18 PM
I have one of those killer phans from my 1970 Pontiac 455 that came out of a hearse. It is nice, those phans save you gas mileage!

Why? If you rev it up it will suck you to the car in front of you!!!