Thanks to the awesome experts on this forum, there has been lots of guidance over the years about converting (1970"s era) OEM Mopar air conditioning systems using R-12 Freon to upgrade to R-134A. Has anyone out there completed the conversion using the original Mopar compressor and system? Can you offer feedback on your success (or not)? (I understand that using a newer dual flow condenser is heavily recommended.) I'm looking to do the R-134A conversion on 2 Darts, 1974 & 1975, when I get the elusive "Round Tuit". Thx. P

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Must remove mineral oil and replace it with PAG or POE oil this would include draining the compressor sump. I would flush the whole system except the compressor you will need a new dryer with the modern desiccant so don't bother trying to flush it. A modern condenser is highly recommended. Back to the refrigerant oil I would use POE because supposably PAG will react with the chlorine from the R12 and cause corrosion in the system. I would also replace the expansion valve. All the hoses that are in good condition can be reused; many people think you need to replace them all with the newer barrier hoses if you feel rich go ahead but the old hoses have long been permeated with mineral oil which seals them up quite well. All O rings need to be replaced with HBNR green O rings. Fill the compressor sump with the correct amount of oil (see your service manual) and add the remainder to the high side of the system as to not slug the compressor with oil and possibly damage it. Pull a vacuum on the system and see if it holds it before you waste freon on it. Most of the specialty tools can be rented from your local parts stores (AutoZone, O Reillys, Advance). I hope I have covered most of the details.

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63 Plymouth Valiant Wagon

Good stuff. Much appreciated. P.

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

The type of condenser to get is a parallel-flow item, the bigger the better.

As has been covered already, O-rings and compressor oil need to be R134a-compatible. It is a good idea to flush the evaporator (that's the part inside the box under the dash) to clean out as much of the R12 mineral oil as possible.

The TXV (expansion valve) needn't be replaced on spec; they are not different for R12 versus R134a. They tend to last a long time and give minimal trouble; if your system works well with R12, the TXV is probably fine.

Remove the EPR valve from the compressor and discard it. It is in the rear port of the compressor, where the large low-pressure line bolts up to the compressor. That valve is calibrated for R12 and will put a severe crimp in the system's cooling performance if you leave it in with R134a. There are no R134a-calibrated EPR valves. To replace the function of the EPR valve, revert to a clutch-cycling setup as used on pre-1974 A-bodies. That calls for adding a thermostatic clutch-cycling switch, as described here (almost two decades ago, yikes!).

Speaking of the compressor: real good idea to put a new shaft seal in it. New shaft seals are R134a-proof, and old shaft seals are old.

Having the lines re-hosed is not necessary to prevent R134a permeation; as has been said, they've been effectively sealed over the years by the mineral oil. But having the lines re-hosed is a very good idea anyhow because old hoses are old, and rubber doesn't last forever.

You will, of course, need a new filter-dryer. It's just about impossible to buy one that isn't R134a-compatible these days, so as long as it's shaped, sized, and configured compatibly, you're fine.

The system will need to be vacuumed hard and held at a vacuum for a long enough period to pull off all moisture; that's just proper A/C service procedure regardless of what refrigerant is used. It will have to be charged by a person who knows what they're doing; you can look at the tag on the compressor to get the original amount of refrigerant and multiply that by 0.75 to get an approximate correct amount of R134a, but changing the condenser will alter that number a bit. Do not rely on the sight glass; they don't work with R134a the way they did with R12.

Above all, though, the best reason to let an experienced A/C tech handle the actual charge-up is that it is dangerous to people and parts if one doesn't really know what one's doing (not just 'look in the book/read on the forum/follow someone else's instructions'). If you get away with it with no accident or injury, great. If you don't…not great. Aside from that, there are a bunch of important things you have to know to avoid messing up the job. For example (just one of many): you don't unseal (remove the caps from) the new filter-dryer and install it until immediately before you vacuum the system. If you take off the caps and leave it open to atmosphere, unless you live somewhere dry as old bones in the desert, it will quickly saturate from ambient moisture and be rendered useless.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Very good specifics! Thank you, Dan, for spending the time to share your knowledge for myself and all our forum denizens.

Also another nod to bcschief for likewise sharing your experience.

I'm feeling much more confident at attacking these two conversion projects.

Pat P.

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Also, it would be very-very wise to add a relay to the compressor clutch circuit. Not because you're converting to R134a, but because that's an awful lot of current to be pulling through the firewall (twice) and through the not-available selector switch.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I wouldn't be concerned about the clutch currant draw. It will/would be no different than it was before the retrofit. Plus the clutch draw is very small. Between 3 and 5 amps depending on whether copper or aluminum wire wound, and temperature. I would be more concerned about the current for the blower circuit, at as much as 25-30 amps (which will also not change) due to the conversion. I would also install a binary switch on the drier. That will protect the compressor in the event of loss of charge, or excessive high pressure.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

I've bolded that part up ↑there↑.
The contacts in the selector switch are very small, and they cook to death gradually. I think it's a good idea to turn off the oven, thus likely extending the selector's useful life considerably.
That sounds high to me—in '73 the factory put the compressor clutch and blower on separate circuits, each with a 20A fuse. An improvement in how blower current is handled also falls, for me, under "not strictly necessary but a good idea while you're at it".
Good idea!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I can't recall ever have a switch failure in the clutch circuit (a lot of years doing AC work). However many failures in the blower circuit. Both the PB switch but more often the speed switch. Also poor connections at these switches.
The current readings I referenced are from the factory service manual for the clutch circuit, and personal experience for the blower circuit.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Huh. Wonder why they would put a 20A fuse on a circuit carrying 30 amps. Must've been the new math!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

"and personal experience for the blower circuit." Possibly aftermarket replacement parts used. Just changing the blower wheel could cause additional load. Just stating I would be more concerned about the draw on the blower circuit, than on the clutch circuit. I personally have never had to repair/replace components in the clutch circuit except for the clutch itself. I have had to replace blower motors, resistors, speed switches, and push button switches, along with overheated connectors/wires in the blower circuit.
EDIT: I have had to replace pressure switches, and/or "T" stat switches, in the clutch circuit, but not from over current, from contact arcing due to EMF kickback from the clutch coil, if the circuit did not have a diode or the diode was open.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225