I am a new member that is converting my 1991 F150 AC system from R12 to 134A. I changed out everything, but the evaporator core, which I flushed. I put a 134A type clutch cycling/pressure switch in. I evacuated and charged the system with 32 oz. of 134. The gauges cycle CONTINIOUSLY from 22 to 50 psi (low side). And at the same time the high side goes from approx. 150 to 300 psi as the compressor cycles on and off. Yes, it doesn’t cool worth beans either. Also, if I put in a small amount more of Freon, let’s say…boost it up to approx. 36 oz. the clutch burns, as if it has too much pressure? Anyone ever see this type of thing before? Help, Tarkster.

 

That sounds like it still has air in the system. How long did you pull a vacuum, how low did it go, and how long did it hold for?

 

Since you evacuated the system it sound like you did bring the gauges into some sort of a vacuum reading lower than atmospheric.

If you somehow reintroduced air into the the system when you weighed in the charge, that could account for the pressure reading, as 300 psi sound high for 134 even in an automotive application.

You may have to consider that whatever your metering device is, that is modulating refrigerant flow into the evaporator, may be restricted, or is "hunting", a condition that would accout for an unstalbe reading on the gauges.

 

You might not be moving enough air over the condensor and it's overheating. That's what it sounds like. Turn the water hose on and let it run over the condensor and then check your pressures and see what you got.

 

I would think somewhere you have atmospheric levels in the system. It needs a vacuum. Also you said yiu flushed the system and added r134a. Did you add PAG oil? Usual sign of undercooling is not enough PAG oil. If you have a vacuum and no leaks.

 

Did you replace the orifice tube in the evaporator inlet tube. if it is plugged it will act this way. Also jump the cycling switch and see what happens to the pressures. If the low side goes way down like Zero or into a vacuum. The the orifice tube or evaporator is plugged. I hope you replaced the receiver / drier. The R-12 driers desiccant bag will come a part with 134-a and required oil and it is like sand being dumped into the system.

 

Yes I vacuumed it down to 29 for 2 hours and let it set overnight.

 

I will try it. Thanks

 

Ok. I am extremely confused on the PAG 46 Oil amounts.
I installed a new DENSO compressor. On the compressor port cover it had a label that stated it had “15 oz. of PAG 46 in it”. It also said it was a “complete system charge”. Now I hand turned the compressor 10 times as the instructions stated to disperse the oil. I later discovered online that a ‘94 F150 (first year for 134A) complete charge was 7 oz. (although my truck is a ’91 / retro R12 system with a ’94 condenser core). I decided to carefully remove the compressor and new Ford hose manifold and drain out 8 oz. of PAG. Making sure I did not lose a drop of oil, I removed the manifold and noticed there was absolutely no oil in the manifold. I then turn the compressor over to pour the oil in a clean and accurate measuring cup, but only 2 oz. came out. Now I am really confused. I decided I had best pour the oil back in the compressor. Does the oil remain in the compressor? It seems that in the past I had to put a measured amount of 46 in a new compressor. I thought that DENSO had made it easy. Now I am not sure if the compressor had 15oz. in it as it stated. What are your thoughts on this matter?

 

You never know how much oil will be in the compressor. You should always drain it, and also make sure the lines, and evaporator and condenser are flushed clean.

The whole system holds 7 oz max. It does get distributed to throughout the system so only draining the compressor will only get some. Some will puddle up in the evap unit and condenser.

Too much oil will affect cooling. Not enough also affects cooling and will dry up and burn out the compressor.

Sucks, but you might need to start over. On the upside, if the accumulator hasnt been exposed to atmosphere for more than an hour or so once the system is open, you won't need to replace it.

 

I installed a new Four Seasons VOV Orifice tube for under 105 degree. Yes I vacuumed it down to 29 for 2 hours and let it set overnight. Now I am worried. I did put a new R12 Drier (that fits my '91) on this 134A conversion system. I never heard about the drier issues. I guess I had best pull the orifice tube out to see if it is got sandy material in it. I pray not. I am wondering how I am going to get a ’94 Drier (134A) to fit in a ’91 R12 set up?

 

Do you think the oil was till in the compressor other than the 2 oz. that I drained out, because I did the hand turning on it. Not a drop of oil was in the hose manifold yet, so it had not gone into the other AC components either. I had not connected the whole system up. I had installed all new parts, except for the now 2 year old '91 Evaporator and '94 Condensing Core. They where flushed with AC flush very good.

 

if you ever turned the system on once and the compressor kicked in and engaged, then the oil was distributed throughout the system simply by the pressure in the lines.

I would assume that the desiccant material inside the accumulator is different between the two refrigerants, so you would need to replace with a newer one. As far as I know, they have the same mounts

 

Most of the new driers are for both refrigerants. So you can get one that will fit your truck and be good with 134A.