A/C Help Needed

vhato · March 27, 2005

I am doing my yearly checkups and decided to check the A/C on both my new 2500HD and my Fiancee's 2000 Mustang.

I am doing this because through the years her A/C would freeze during long trips.

Anyway the temp outside is 63 degress F. I let the engine with A/C on full blast run for 5 minutes. My trucks A/C never cycled off. It stayed on.

Low Pressure reading was 35 PSI. Continuous.

The Mustang however cycled on and off. 30 seconds on then 30 seconds off.

Low Pressure reading was 20 PSI while on and 40 PSI when the compressor cycled off. Each reading was constant, no bouncing so I am sure the reed valves are fine.

I am thinking she is low on 134a.

1. Should I just add a can and see how it does?

2. If so, Should I hold the can upside down when adding or standing straight up?

MAGNUM · March 29, 2005

from the pressures you provided, I don't think I would add any refrigerant. If the system is low, it will not cool very well, in the form of warmer than you'd like air. As far as the cycling between the two outfits, they have different means of controlling the cycle and they will act differently, so I wouldn't be concerned about that to much. the freezing up problem can be dealt with by keeping your fan on a higher setting and setting your temp control to a little bit warmer to offset the fan speed. The theory being that having more warm air pass thru the evaporator, so it won't freeze. Hope that helps.

snoman · April 3, 2005

I am doing my yearly checkups and decided to check the A/C on both my new 2500HD and my Fiancee's 2000 Mustang.

I am doing this because through the years her A/C would freeze during long trips.

The freezing in mostlikely cause by a bad low pressure cutout that stops compressor to keep it from getting to cold and freezing or a slight overcharge which prevents cutout on low side while running. Do not add anymore R134a here as that is not the problem

Winchester · April 3, 2005

1. Should I just add a can and see how it does?

63F ambient temperature is to cold to be testing your AC system. I would suggest waiting for temperatures to increase (normal operating temps) before making any diagnoses on the AC system.

2. If so, Should I hold the can upside down when adding or standing straight up?

You don't want to slug the compressor. Slugging is when the compressor trys to compress liquid, it can damage the valves. So becareful when tipping the can not to let to much liquid into the low side at once. The liquid refrigerant need to flash into a gas by the time it reaches the compressor. It would be safer if you could just hold the can upright so only gas goes into the system.

The most common cause for the evap to freeze over is refrigerant shortage.

The theory being that having more warm air pass through the evaporator, so it won't freeze.

The moisture in the air going over a below freezing evap is going to make ice. The warmer the air, the faster the ice build up. Warm air carries more moisture than cold air. The only cure is to raise the temperature of evap coils above freezing. This means letting more refrigerant into the evap.

The freezing in most likely cause by a bad low pressure cutout that stops compressor to keep it from getting to cold and freezing or a slight overcharge which prevents cutout on low side while running.

A low pressure switch has only one function to protect the compressor. The compressor relies on cool gas and oil to be returned from evaporator to keep it from burning up. When the pressure goes below the cutout point the AC system shuts down. The low pressure switch is not design to control the system. Thats the job for the thermostat inside the vehicle.

snoman · April 3, 2005

A low pressure switch has only one function to protect the compressor. The compressor relies on cool gas and oil to be returned from evaporator to keep it from burning up. When the pressure goes below the cutout point the AC system shuts down. The low pressure switch is not design to control the system. Thats the job for the thermostat inside the vehicle.

Boy are you off base with that one. The temp setting in car only controls the air mixture valve to control air temp. On both of my GM trucks the low pressure cut performs two functions, one is to prevent stetup of system without enough charge in it to cool compressor and the other is to limit evaporator lower temp limit and prevent icing because as the evaporator gets colder the pressure drops and without it your A/C system would freeze solid in cool weather when using the defroster. THere is even a adjustment on this switch to fine tune it on some systems.

Winchester · April 4, 2005

Boy are you off base with that one. The temp setting in car only controls the air mixture valve to control air temp. On both of my GM trucks the low pressure cut performs two functions, one is to prevent stetup of system without enough charge in it to cool compressor and the other is to limit evaporator lower temp limit and prevent icing because as the evaporator gets colder the pressure drops and without it your A/C system would freeze solid in cool weather when using the defroster. THere is even a adjustment on this switch to fine tune it on some systems.

I fail to see how I'm off base at all. The low pressure switch is simple, yes sometimes it's pressure cut out or cut in may be adjustable. Electrically it's either open or closed. Low pressure will open it, stopping the A/C system by opening the line to the compressor clutch. The TXV or some other metering type device controls the amount of refrigerant allowed into the evaporator coil.

I'm always willing to learn a new way a thing can work or be used but you failed to explain how a low pressure switch does anymore then I already described. The only time the low pressure switch comes into play is when you have little to no refrigerant in the system. The low pressure cutoff offers no system freeze protection.

BBSilverado · April 4, 2005

The low pressure switch has nothing to do with freeze-up protection. the purpose of the LPS is to control the compressor since it is alway running at different RPM's (in relation to the engine), same as the high pressure switch but on the low side.

The dash mounted A/C only turns the system on and does not control the output temp. The dash mounted temp. control only controls the air damper in the fan box to mix the evaporater air with outside air or even heated air on some systems. The temp control does not turn the compressor on or off as it does in home AC systems.

The only reasons the evaporated would freeze up is either low charge or restricted/low air flow through it.

snoman · April 4, 2005

The low pressure switch has nothing to do with freeze-up protection. the purpose of the LPS is to control the compressor since it is alway running at different RPM's (in relation to the engine), same as the high pressure switch but on the low side.
The dash mounted A/C only turns the system on and does not control the output temp. The dash mounted temp. control only controls the air damper in the fan box to mix the evaporater air with outside air or even heated air on some systems. The temp control does not turn the compressor on or off as it does in home AC systems.

The only reasons the evaporated would freeze up is either low charge or restricted/low air flow through it.

<{POST_SNAPBACK}>

It has everything to do with it as I learned for experiance with my 89 burb when it was new and I had A/C problems. It is adjustable on it and if set to low the A/C would freeze solid on humid days at lower blower setting and if set too high it would not cool properly. 30 years ago they used to use a varible and thermostatically controlled expandsion valve to control evaporators temps but the dumped those long ago to control costs and simplify installations. While it does provide low coolent protection, it does also by design limit lower temps on evaporator because as it get colder (below freezing) the pressure will drop as the gas expands less so switch will kick the compressor out until pressure rises a bit. I wish that they would go back to thermostatically control expansion valves mounted on evaporator but that is not likely to happen either.

Winchester · April 4, 2005

it does also by design limit lower temps on evaporator because as it get colder (below freezing) the pressure will drop as the gas expands less so switch will kick the compressor out until pressure rises a bit.

Sort of. The thing is the evap should not ever go below freezing. If it is does, there is a problem. If the pressure drops and the low pressure switch shuts the compressor off (protecting it), the system pressure on the low side equalizes pressure with the high sides. This will reset the low pressure switch allowing the compressor to run again. It will also thaw out the evap. If your system is cycling on the low pressure switch it's because you do not have enough refrigerant in the system. If the evap is icing over it's because you don't have enough refrigerant in the system, regardless off the ambient temperature, or the metering device is clogged. I clogged metering device will show it self as being excessive high side pressure.

I wish that they would go back to thermostatically control expansion valves mounted on evaporator but that is not likely to happen either.

The TXV is only one type of metering device. In the biz, TXV is misused to generically refer to a metering device. Today an orifice is most commonly used in automobiles to flash the high pressure, high temperature gas at the evap to a low temperature, low pressure gas. The metering device has to let the right amount of liquid and gas into the evaporator coil. In the evaporator coil the liquid refrigerant will boil picking up heat. To little refrigerant and the evap turns into a ice block. To much and you have poor to no cooling.

snoman · April 4, 2005

it does also by design limit lower temps on evaporator because as it get colder (below freezing) the pressure will drop as the gas expands less so switch will kick the compressor out until pressure rises a bit.

Sort of. The thing is the evap should not ever go below freezing. If it is does, there is a problem. If the pressure drops and the low pressure switch shuts the compressor off (protecting it), the system pressure on the low side equalizes pressure with the high sides. This will reset the low pressure switch allowing the compressor to run again. It will also thaw out the evap. If your system is cycling on the low pressure switch it's because you do not have enough refrigerant in the system. If the evap is icing over it's because you don't have enough refrigerant in the system, regardless off the ambient temperature, or the metering device is clogged. I clogged metering device will show it self as being excessive high side pressure.

I wish that they would go back to thermostatically control expansion valves mounted on evaporator but that is not likely to happen either.

The TXV is only one type of metering device. In the biz, TXV is misused to generically refer to a metering device. Today an orifice is most commonly used in automobiles to flash the high pressure, high temperature gas at the evap to a low temperature, low pressure gas. The metering device has to let the right amount of liquid and gas into the evaporator coil. In the evaporator coil the liquid refrigerant will boil picking up heat. To little refrigerant and the evap turns into a ice block. To much and you have poor to no cooling.

<{POST_SNAPBACK}>

A varible and thermostatically controlled expansion valve controlls freon flow to limit evaporator temps but todays cars use fixed valve for a quick, cheap fix that is a compromise most of the time in operation. The freeze is not because of overcharge it is because the auto A/C must cope with 100 degree air and 60 degree air and it will freeze in cooler temps if a low pressure limit is not used since there is no varible expansion valve. Even a 99 buck window A/C at wal mart has a tempature controlled expansion valve in it while auto makers gave them up years ago to simplify things and cut costs. You can try to fine tune the freeze with charge level but it is not the real cause only a bandaid fix on a cheap design.