A/c pressure readings

I cannot remember the equation for figuring the pressure range. Will be pulling the system down tonight and refilling out of a 30lb can.

Just Google a/c pressure charts. Mastercool has some good charts and troubleshooting tips.

get a digital bathroom scale to put the 30lb can on , that will help you get close. Its pretty easy to overcharge a system just using gauges and a 30lb jug.....

(quoted from post at 16:49:15 07/06/14) Its pretty easy to overcharge a system just using gauges and a 30lb jug.....

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Please explain?
I just replaced an AC line on my JD 4255,evacuated the system & put R134A in system from a 30# jug without the aid of a scale and it blows COLD AIR. I thought that was the reason for using gauges. I'm aware it's easy to overfill with R134A. 45# low side & 250# high side on a 90 degree day
Thanks,Jim

Nope,

The gauges are to tell you if everything is working right. There's no way gauges can measure the amount of Freon.

Allan

So if the gauges can not tell you when a system is full; you have a VERY slow leak that leaks about 1/2 a small can a year; and you just need to add some more Freon because enough years have passed that it has leaked enough to not cool properly.......

How do you know how much to add????

Actually your pressures sound high, like an overcharge, to me. A lot of things come into play when recharging with just gauges : engine rpm, ambient temperature,cab temperature,windows up or down, cab fan setting speed, airflow thru evaporator(dirty or clogged cabin air filter),airflow thru condenser-engine fan on/off (if it is cycling type or constant on), and what type system- expansion valve or cycling clutch orifice tube. Then you want to measure the outlet vent temperature with a good thermometer.
If I charge just using the gauges,only charge on the low side,engine rpm at least 1000-1200(close to operating condition rpm) all fans on max(engine fan locked on),with strong air flow thru condenser and evaporator , thermometer in dash vent. Charge til high side reads 175-220 and low side 30-35 and 50' duct temp if it's an expansion valve system. Now if it's an orifice tube system watch the on/off time of the clutch-should stay on longer than off=10-15 secs on then off.
An a/c system is engineered to run with a measured amount of refrigerant and run a rpm that you normally run when operating.Too much reduces efficiency and strains the compressor, too little can cause oil starvation to compressor as it circulates with the freon thru the system. Also you should add a little pag oil if you replace a component :1/2 oz for a hose 1-2 oz for drier or condenser.
Allan in Ne is correct:the gauges really just tell you if everything is working properly along with feeling lines (to be cold or warm where they should).Weigh in the charge for best performance and longevity. Mark

(quoted from post at 19:04:05 07/06/14) Actually your pressures sound high, like an overcharge, to me. A lot of things come into play when recharging with just gauges : engine rpm, ambient temperature,cab temperature,windows up or down, cab fan setting speed, airflow thru evaporator(dirty or clogged cabin air filter),airflow thru condenser-engine fan on/off (if it is cycling type or constant on), and what type system- expansion valve or cycling clutch orifice tube. Mark

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If my AC blows cold air then it has to close to correct. I understand that all the things you listed affect AC operation and if I implied gauges measure coolant it was unintended.

Guess it won't happen tonight. It is a system that has been retrofitted to 134 and my guage couplers are to big for both to fit on the fittings. Oh well. Thanks for the help it is really appreciated

Oh there are a couple of ways, but I use a thermometer in the (center) outlet with the fan on low.

Ideal is 42 degrees and I just add just enough Freon to get it there.

Another way is to hold your hand on the receiver (if possible). When the system is at optimum, it will be too cold to comfortably keep your hand there.

Can't beat the scale tho.

Allan

Since you are evacuating the system and starting over, WHY not just install the factory-recommend amount of refrigerant?

No sticker with info? It's still better to take the effort find out what factory specs are than to muck it up.

NOT sure what you are working with, but if it is a never R134A system with an orifice tube (rather than a TXV) charge is critical.

If it is an R12 to R134A conversion, about 80% ( by weight) of the original R12 amount will do the trick.

If it is an older ex-R12 system, and you want some cooling without blowing hoses and NOT being cool and spending $$$, slightly UNDERCHARGED is better than a bit overcharged.

(quoted from post at 16:58:00 07/06/14)

(quoted from post at 16:49:15 07/06/14) Its pretty easy to overcharge a system just using gauges and a 30lb jug.....

Click to expand...

Please explain?
I just replaced an AC line on my JD 4255,evacuated the system & put R134A in system from a 30# jug without the aid of a scale and it blows COLD AIR. I thought that was the reason for using gauges. I'm aware it's easy to overfill with R134A. 45# low side & 250# high side on a 90 degree day
Thanks,Jim

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Yep and with the engine running at least 1200 RPM or better.

(quoted from post at 19:52:27 07/06/14) Guess it won't happen tonight. It is a system that has been retrofitted to 134 and my guage couplers are to big for both to fit on the fittings. Oh well. Thanks for the help it is really appreciated

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If retrofitted it should have the R134 adapters on the old R12 fittings.