A/C refrigerant amount

Put your hand on the outlet pipe of the evaporator (larger). Charge liquid until the pipe suddenly becomes very cold. Stop... you have enough. High side pressure 175-200 with the fan running (unplug it to make it stay on while you charge).

Sight glass is not reliable with 134a. There will usually still be bubbles at the drier. If you use a sight glass, it should be in the liquid line, right at the expansion valve.

 

Yes this is an old thread.

But what's the sense in asking how much to put in if you don't have a scale to weigh it?
I understand you all go by cans. But how do you determine the remaining oz. Needed in the last can?

My 800 I'm driving is a daycab. The label says 4 lbs. But at 1.5 lbs. It starts warming up.

Hi pressure is 135 with fan running. 1400 rpm


To be honest. I think the truck has the wrong condenser to hold such a little amount.

The w900 i used to drive was 2 lbs. Also daycab. Hi pressure was 140.

Yes I have a scale. I also use 30 lb. Cylinders.

I've found the best way is to use a thermometer in the vent. Get it to it's coldest point. If it starts to go warm. Call it good.

That's the basic and easy way. For the inexperienced.

 

Depends on the amount required. My Truck requires 3 lb. 7 Oz. So 55 Oz. I always put in 5 cans. 60oz. I purge the air from hose each time I use a new can. Works perfect. Worse case it’s a little bit over. Won’t hurt it. If not using gauges. The low side gauge that comes on most hoses works. Can’t get high side, but if everything’s good low sides between 35 and 50 depending on outside heat and humidity. The other shade tree technique I’ve always used before getting proper gauges, is to monitor the compressor/fan cycling. Ideally the compressor should run at least twice as much than not, at an idle, on the hottest most humid day. If it’s cycling more often, probably overcharged. Fan should come on around the same time compressor cycles off, then go off around same time compressor cycles on.

 

Sounds like good advise but applies to your particular truck. Might share what that is.
Last A/C I worked on was in a field machine. Owner said " Well it might need a little more freon " . Then asked: What are you doing? I am letting freon out. It has to much. I asked him the next day: How is your A/C? Better. Probably still over charged. I had no thermometer but agree that is the best method.
That is especially true on R12 machines running 134A. 4 ounces one way or the other can be 10 degrees on some vehicles.

 

Mines a Freightliner Classic. A friend who’s a Licensed HVAC helped Me on my first Truck. Brought some R12 and gauges from work, on his lunch break. He noted what most cars take, stood back, looked at my Bunk, and put 8 lbs in it. Next day, hose started leaking, I tried to tighten it and it blew out!!! He meant well. I couldn’t complain. Lol.

 

I charge off pressure, not weight. Lot's of variables that affect those pressures too.

On a day like today. 100* and 90% humidity. Fan on at 900 RPM. High side roughly 170-190ish, low side 36-40.

@snowwy I would assume your system would freeze up due to being undercharged, with a low side pressure at 20 or less.

 

No. I've never seen 134 freeze up.

if you use the cans. They barely get cold when draining. The evaporator tubes barely sweat if the conditions are right..

Now if you wanna talk r12. That freezes.

I.use high side pressure also. Semi's can be tough sometimes when that fan kicks.on. Thermostat comes.in handy for me just to be safe.

I play with cars alot. Not much semis. I don't want to risk damage somehow. That's why I bought the scale and it sure comes in handy for the problem cars and the semis.

I've been buying the 30 lb cylinders since 134 came out in 94.

P.S. It should also be noted that I live in dry air. I have no idea how humidity effects things. Other then Florida. lol

 

So where does your low side pressure run? I've seen 134 freeze up hundreds of times, same as 12, when run in undercharged conditions.

 

Mines 1 can low currently. It’s working great, but it’s trying to freeze up a bit, near expansion valve on cab. I just haven’t put the last can in yet. Running them low on Freon puts extra wear on compressor. As does putting too much oil. Oil displaces the Freon. Makes compressor run hot.