A/C question: how much 134??

[curiousone940]

Question to our resident a/c gurus and/or those of you who have successfully converted to 134 (I would consider 45 degree vent temps on a 95 degree ambient to be "successful"), I know I've seen this info somewhere, but despite a search, I can't find it now. I have a stock '89 , stock condensers, new Sanden 507, barrier hoses, new Rennaire evap core/expansion valve, new Griffith's thermostat switch, new r/d, added electric cooling fans to rear condenser....question is, approximately how much 134 should be in the system?? I've seen the figures of 80-85% of the r-12 amount being mentioned, which would put the amount of 134 somewhere between 37-39 oz., but seems like I've also heard figures on the order of 32-36 ozs. I know this is not a precise figure for all cars, but considering the possibility that too much or conversely, too little 134 can adversely affect the system, I'm curious as to whether there is a consensus on a more specific range than 32 to 39 ozs.? This is my first venture into a 134 conversion project....the reason I ask is that I'm currently charged with about 32 ozs. (high side pressure at about 220 on a 90 degree day; low pressure at about 20, medium fan speed), but still getting some occasional evap icing (yes, thermostat tube is correctly seated in the evap core and compressor is cycling). Vent temps will start out as low as 40 degrees in 90 degree ambient, but will start creeping up to mid 50's vent temps (symptoms of evap icing). Maybe it's just the high humidity/temps on some occasions, but I'm now wondering whether 32 ozs. of 134 might be a little on the low side?? Is this primarily a "trial and error" procedure?? Can a low freon charge, in and of itself, be responsible for evap icing, assuming everything else is in order?? Comments/suggestions would be appreciated.

[mthomas58]

Marc, at 32oz, I think you may be a little low. We just converted tomphot's 85 RoW 911 with the same approx capacity as yours (new barrier hoses, stock rear condenser, Nippo compressor, Rennair evap and upgraded Kuehl front serpentine condenser plus Rennair's ProCooler) and charged using 30lb tank of 134a and measured charge with a scale at 36oz. We got high 30 vent temps while charging. He has since gotten vent temps on the road in low 30's! Ambient temps while charging were 90 and we took high side pressures up to about 270 - 280. I've got two addl rear fender condensers plus a ProCooler on my 87 and my total charge is 39oz.

Odly enough, I called the local dealer to see if they had a 134a capacity spec for R-12 47.6 oz conversions. They did not stating that they rely on their charging machine to measure the correct charge.

[Tigerrat]

I monitor the vent temp while charging and stop when the vent temp stops falling. That's with fans on the rear condenser, high idle, etc. I think I ended up with about 30 - 32 oz.
Is the temp creep caused by icing? The thought that symptom was reduced airflow? Ken

[curiousone940]

Gentlemen...thanks for your responses. I think at 32 ozs. my system might be on the marginally under charged side (meaning the system will operate OK in not so hot/humid conditions, but when hit with high temps and/or humidity it starts icing up), SO I'm thinking another shot of 134 might be in order. When it was initially recharged, we were primarily monitoring high side pressure and not so much the vent temps; will monitor both this next time and see if more optimal performance can be achieved.