1972 Duster Build with my Daughter

Members Restorations

  1. MaxPF

    MaxPF Active Member

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    Not surprising. The original hoses were made of nitrile. It was determined back in the early 90's, when systems were being studied to determine what steps would be necessary to retrofit an R-12 system to R-134a, that nitrile hoses which had seen at least a season of use would have absorbed enough mineral oil in their interior surface to act as a barrier against R-134a leakage. The same goes for the nitrile o-rings commonly used in R-12 systems back then. It was recommended that used hoses could be re-used if they were otherwise in good condition. It was also recommended that, if a hose fitting were disconnected, to replace the o-ring with HNBR (or neoprene). Any hose fittings that were not disconnected and had used but leak-free nitrile o-rings did not need to have the o-rings replaced. The same was true for used nitrile o-rings used in compressor body and shaft seals. Also, although nitrile o-rings are supposedly a bit more permeable to R-134a vs HNBR or neoprene, it was said to be acceptable to use nitrile o-rings during a compressor rebuild or shaft seal replacement if HNBR or nitrile were not available.

    As a side note, it was during this period that many R-12 "retrofit" refrigerant blends were developed. They were intended to be near-drop-in replacements for R-12, with the main feature that they would work with the original mineral oil. A couple of these were HFC based, with the addition of small amounts of propane and/or isobutane to help carry the mineral oil, and those refrigerants carried the same hose and seal recommendations as R-134a. Most of the retrofit blends, however, were based on R-22. While they worked extremely well, R-22 severely swells nitrile and HNBR. For applications using such blends, the hoses had to be replaced with 134a-type barrier hose which has a neoprene-coated nylon inner lining. All o-rings had to be replaced with neoprene o-rings, including those in the compressor. Some people tried to use the original hose and nitrile or HNBR o-rings and quickly found that their system leaked like the proverbial sieve. Because of the requirements to change hoses and all o-rings, the R-22 based blends ended up not being used much, even though most worked better than 134a. Particularly on systems with stock tube-and-fin condensers. Some of the blends even gave the system more capacity than the original R-12 did, although they did run at head pressures similar to 134a rather than the lower pressures of R-12.

    You are going by absolute pressures, as you would with the EPR valve. Expansion valves work on differential pressures. In the temp range where evaporators work for AC and medium temp refrigeration, the delta P for a given delta T is nearly identical for both refrigerants. For example, at an evap temp of 30° (your daughter's system is probably running around 25°-28° saturated in order to give a 32° duct temp), the pressure of R-134a is 26.1psi. At 40°, it is 35psi, which is a difference of 8.9psi. With R-12, 30° is 28.4psi and 40° is 36.9psi, for a difference of 8.5psi. The difference between them over that span is only 0.4psi. The difference in temperature per psi is only about 0.05°F, so over a 10psi range the superheat would only change by about a half a degree.

    All that said, you should still check the superheat to make sure you have enough. In refrigeration, 10° is typical. However, the specs I have seen for automotive valves is usually around 4°-6°. IMO, 4° is a bit close, and can risk a bit of flood-back if the valve hunts or evap pressure suddenly changes (i.e. with engine RPM). I'd like to see at least 8°, and ideally 10°, even if it causes vent temps to go up a degree or two. If the superheat is at least 8° under steady-state operation, I would leave it alone. Even if it is 12° or 15°, I'd leave it alone. You're getting excellent vent temps, so you're better off to protect the compressor even if it means warming up the discharge air slightly.
     
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    • MaxPF

      MaxPF Active Member

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      I would highly recommend running the OE RV2 compressor. The typically retrofitted Sanden not only looks out of place, but it doesn't have the refrigerating capacity of the RV2. Granted, NJ isn't the hottest place in the US, but I still wouldn't want to downgrade the AC from OE.
       
    • Jim Kueneman

      Jim Kueneman FABO Gold Member FABO Gold Member

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      Thanks for the details! The problem is measuring super heat is not really accurate. The only tap I have for getting the low side pressure is all the way back at the compressor in the head of one of the pistons. I would have no idea how to handle going through the muffler then through the ports where the EPR valve use be then into the head low side chamber.
       
      Last edited: Apr 17, 2021
    • sireland67

      sireland67 FABO Gold Member FABO Gold Member

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      Sorry no for Duster Daddy
       
    • MaxPF

      MaxPF Active Member

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      The pressure drop probably isn't too bad. I'm surprised that you don't have a Schrader port on the suction manifold where it bolts to the compressor body? You must be using a line set from a non-EPR vehicle?

      I'd get the best measurements that you can. A slightly inaccurate measurement is better than no measurements at all.
       
    • Jim Kueneman

      Jim Kueneman FABO Gold Member FABO Gold Member

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      Nope original hoses and I have a NOS one as well. No Schrader valves on the line set.
       
    • MaxPF

      MaxPF Active Member

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      Ahh. OK. That means the Duster was originally a cycling clutch system, not an EPR system. I don't recall off the top of my head which models used which systems, but I do recall that the higher-end and/or large sedans used the EPR while more compact and/or mid-range vehicles used clutch cycling to control evaporator temp.
       
    • Jim Kueneman

      Jim Kueneman FABO Gold Member FABO Gold Member

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      Yes the '73 system was the only year with a cycling sensor in the evaporator.
       
    • DusterDaddy

      DusterDaddy sledgehammer mechanic FABO Gold Member

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      I gotta get around to getting it from you Shawn
       
    • MaxPF

      MaxPF Active Member

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      I thought all systems had an evap temp switch, including those using an EPR. EPR's have a setpoint of around 24psi at low load, give or take a psi or two. At that pressure, the saturated temp of R-12 is well below freezing. The reason the evaporator usually doesn't freeze is the temperature drop between the refrigerant on the interior of the evaporator tubes and the exterior surface. That temp gradient is dependent on the amount of airflow across the coil as well as the temperature and humidity of the air. It is possible for the evap coil to freeze on an EPR system if the return (inlet) air temp is sufficiently low and the air if fairly humid. Condensation is an insulator, and it tends to restrict airflow, so if the airflow is low (blower on low speed), the cabin is fairly cool (cool return air flowing into the coil), and the coil is good and wet, the clinging condensate can start to freeze around the tubes. Once it starts, it spreads because the developing frost further restricts airflow.

      A properly set evap temp switch on an EPR system won't normally open. Only if conditions are right to allow frost to form and the exterior coil temp drops to a few degrees below freezing would the switch open, cutting out the compressor until the coil defrosts and warms up to a few degrees above freezing. In essence, the switch operates just as a freeze 'stat in an EPR system, rather than actually being the primary evap temp control.
       
    • Jim Kueneman

      Jim Kueneman FABO Gold Member FABO Gold Member

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      Nope the '73 was the only year. All rest from that era (like my '68 Coronet) only had the EPR and a low pressure cut out at the dryer.
       
    • MaxPF

      MaxPF Active Member

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      That is odd. With the EPR, a low pressure cutout won't protect against evaporator freeze. It will shut down the system if you lose most or all of the charge, but that's all.

      I will have to go back and watch the relevant Chrysler Master Tech videos. They talk about which vehicles used the EPR and which used cycling, but I don't recall if they mention what pressure and/or temp controls each system also had.

      Did you delete the EPR valve on your 68 in order to run R-134a?
       
    • zeebeeone

      zeebeeone Well-Known Member

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      so did you get the cn3330pf condenser for your daughters duster & will it fit a 1970 duster
      I had my 70 duster in the shop to get it charged & was told i now need the 134a tip condenser
       
    • Jim Kueneman

      Jim Kueneman FABO Gold Member FABO Gold Member

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      Long story but I did not (remember this is a '73 system on a '72) and there is no _need_ for a condenser. What is the long story of your problem? I have spent a lot of time on this one and now get it. Listen to @MaxPF as well.
       
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