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, Jonnys.1991 alias the Tarkster.

 

The final post, by troger, in your previous thread under your tarkster heading, seemed like an excellent starting diagnosis.

The low pressure control will shut off the compressor at 20psi.

His assertion, that you may have a restriction at your orifice, at the inlet to your evaporator, sounds like an excellent place to start.

If refrigerant cannot circulate readily to the low-pressure side of the system, pressure will rise on the high-pressure side, while the low-pressure side, seeing little-to-no refrigerant flow , will start to attempt to pull into a vacuum, being stopped by the low- pressure safety cuout on it's way down.

 

Did you change the orifice tube to the appropriate red on for R134? Did you vacuum down the system before you filled 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.

 

I have a ’91 f150 that I converted from R12 to 134A Freon. I did all the usual stuff, all NEW Compressor, O-rings, Hose Manifold, Drier (R12/134A type) and VOV Orifice Tube. I also flushed the Evaporator Core and ’94 Condensing Core. The compressor was drained of its factory oil and replaced with 7 oz. of PAG 46. I evacuated the system for 2 hours at -29 and did a 30 minutes test and had no leaks. Then charged with 32 oz. of 134A. At 85°F the gauges read goes up to 150 psi on the high and the low side continuously fluctuates between 45 and 22 on the low pressure side, as the pressure cut out/clutch switch cuts on and off. This cycle happens every 30-60 seconds. If I bypass (wire jumper) the pressure switch and let the compressor run the evaporator core freezes up at the orifice tube area. Is the thing cycling because it is under charged? A ’91 (R12) complete charge is 44 oz. and a ’94 is 32 oz. The ‘94 was the first year for 134A. The only thing on my ’91 that has changed to a 134A system is the ’94 condensing core. Is there more cubic inch volume in the old R12 system, therefore it needs more Freon?

 

You now have three separate threads active on this same subject. You'll get better advice if you use only one thread. It will also save forum members from reading through the same info over and over.