134a Freon in 4230/4440 tractor.in 50 degree weather and in 80 degree weather, what pressure should I run to make it cool the best. Dale
 
At 80 degrees ambient temp, if the system is in good shape...40-50 low side... 175-210 high side. As for 50 degrees ambient none of my charts go that low. But trying to run a a/c system at that temp would more than likely cause a freeze up situation unless you blend some warm air from the heater, basically defrost mode.
 
I just changed my heavy truck over to 134a, and put in 3#. I have a new high pressure side switch coming which controls the fan clutch. That should take care of my too high hi side pressure. But the low side is running 15# or less. Does that mean more refrigerant is needed?
 
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If the low side is too low and the high side is too high, this usually indicates a partial blockage. Don't know which system you have but it could be a sticky or clogged expansion valve or partially clogged orifice tube.
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I am back------ 4440 was r12 system so I filled it to 230 psi "cooling was not good" "80 degrees= 175-210psi" will it cool better at 170 or 185 or 220 psi? that is my question. Dale PS it is hot when it gets to 75 for us northerners. the end
 
You CAN'T fill it by the high-side pressure, as that is dependent on the ambient temp at that time and airflow through the condenser, (combination of engine fan speed and how clean the air flow path through the condenser and radiator is) NOT so much by the amount of charge ('til you get it TOO full)!

You best bet is to fill it to about 80% (by weight) of the original R-12 charge petrifaction.

My belief is that in these converted systems, it is better to be a tad SHORT on charge (and accept the cooling you get) vs. putting all you can in it and over-stressing the compressor and hoses 'til something blows and you have NO cooling!
 
(quoted from post at 08:36:36 07/20/14) If the low side is too low and the high side is too high, this usually indicates a partial blockage. Don't know which system you have but it could be a sticky or clogged expansion valve or partially clogged orifice tube.
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JD 30/40 series AC's have expansion valves
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Answering this way so you get Bob's comments first, your answer can only come from your seat of the pants experience in the first place. Since you have an expansion valve controlling the freon flow thru your evaporator (thanks, TX Jim), you can no longer go by pressures per se, they only serve as troubleshooting tools at this point as a rule of thumb. When it's hot, your high side will be higher and what you get is what you got. In any event, one does not use the high side pressure for indications of how good the cooling is going to be, that's what the low side gauge is for.

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According to this chart which is actual freon temp and not ambient air temp, I would want a low side pressure of 30 to 40 PSI, the expansion valve may prevent you from going lower than 40 normally, but one can cheat the system by shorting it on freon by some set amount such as the 80% already suggested. This is almost forcing it to behave as an orfice tube system where low side pressure is what to go by entirely.

My seat of the pants advice would be to go by the frost on the outlet tube coming from the evaporator, you don't want it to ice up clear to the compressor, but I do like to see at least a few inches of frost going in that direction before I'll call it as cool as it can get.
 

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