Low pressure switch - cutoff pressure for R134a

[Michael]

I was going off the tech manual pressures but now I realize that is for r12.

Both high and low side pressures drop during throttle, when the compressor cuts out both rise to 65 and 200 before the compressor re engages..

I checked and all the pipes that are supposed to be hot, warm, cold are just that.

[82DMC12]

I was going off the tech manual pressures but now I realize that is for r12.

Both high and low side pressures drop during throttle, when the compressor cuts out both rise to 65 and 200 before the compressor re engages..

I checked and all the pipes that are supposed to be hot, warm, cold are just that.

When you open the throttle, the high pressure is supposed to go up while the low pressure drops (assuming the compressor is engaged and running). I'm not sure I can explain why the high pressure would also drop. Can you confirm that the adapter valves for your recovery system's hoses AT THE COMPRESSOR are both open and the "fill valves" for the gauges on the recovery unit are both CLOSED while monitoring the pressures? If both are fill valves are open, or leaking internally, you would probably see some mingling of the high and low side within the recovery unit itself. Just spit-balling here since I haven't seen high side drop when giving throttle before.

[82DMC12]

By the way, when the car is shut off and the A/C system cools back down to ambient temp, the high and low will equalize and you can check the gauge's temperature scale to see if the system is relatively close to being charged correctly.

See the photos below (this is my car with engine cold and the ambient temp was about 80F, compressor service valve knobs open, valves on gauges CLOSED), you'll see the inside scales of the gauges show various refrigerants including R134a. If the gauges are hooked up to the service ports and the engine is cold , the needles should both point to the same PRESSURE on both high and low AND the needles will point to approximately the current ambient temperature.

So, if it is 80F outside and the system is "cold", both needles will point to approximately 90 PSI because that is the pressure that corresponds to 80 F on the inner R134a scale.

It's not an exact science but a good ballpark reading so you know if you are way off. If the system is undercharged, both gauges will LOW and if the system is overcharged both will read high.

PXL_20240520_150111826.jpg

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[Michael]

I know the fill ports were closed...otherwise I would know real quick but my compressor adaptors are known to loosen while in use...I should have double checked to make sure they were still engaging the valve.

I will crank it cold tomorrow morning or evening and double check my readings.

[Bitsyncmaster]

By the way, when the car is shut off and the A/C system cools back down to ambient temp, the high and low will equalize and you can check the gauge's temperature scale to see if the system is relatively close to being charged correctly.

See the photos below (this is my car with engine cold and the ambient temp was about 80F, compressor service valve knobs open, valves on gauges CLOSED), you'll see the inside scales of the gauges show various refrigerants including R134a. If the gauges are hooked up to the service ports and the engine is cold , the needles should both point to the same PRESSURE on both high and low AND the needles will point to approximately the current ambient temperature.

So, if it is 80F outside and the system is "cold", both needles will point to approximately 90 PSI because that is the pressure that corresponds to 80 F on the inner R134a scale.

It's not an exact science but a good ballpark reading so you know if you are way off. If the system is undercharged, both gauges will LOW and if the system is overcharged both will read high.

PXL_20240520_150111826.jpg


PXL_20240520_150108238.jpg

Static pressure does not show a full charge. If you have one drop of liquid coolant it will show the same static pressure if you have 2 lbs of liquid.

[82DMC12]

Static pressure does not show a full charge. If you have one drop of liquid coolant it will show the same static pressure if you have 2 lbs of liquid.

That's why I said it's a reference point only. It remains true that in general, if both gauges read low, the system is undercharged. In general, if both gauges read high, the system is overcharged. That would be one clue along with other points of evidence to make a diagnosis.

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[Michael]

I know I have the right charge. What I will do is tomorrow evening, I will make a quick video of the gauges. I was thinking the high side dropped with acceleration but on another video I took (before I confirmed my freon charge), it barely moved.

[Michael]

...then again, I think that was one of those tines when my valve knob backed off.

[DMC-Ron]

Static pressure does not show a full charge. If you have one drop of liquid coolant it will show the same static pressure if you have 2 lbs of liquid.

That's why I said it's a reference point only. It remains true that in general, if both gauges read low, the system is undercharged. In general, if both gauges read high, the system is overcharged. That would be one clue along with other points of evidence to make a diagnosis.

Dave is correct. The static pressure does not indicate the level of charge. You could have 1oz or 2lbs of refrigerant and the pressure indication would be identical. In an extreme case, if your gauges read a static pressure below ambient temperature then you have lost your charge and don't have any liquid refrigerant left. There would be no way to have the system static pressure read higher than ambient temperature.

Ron

[82DMC12]

All right, sounds like I have a misunderstanding of the static pressure. I sometimes have to work with equipment that has industrial HVAC built into it and while I do not do repairs, I sometimes have to give a rough diagnosis of why it's not working. The distributors of the equipment have explained to me that if a machine is found with both gauges reading surprisingly low, when the machine is not running, it is "almost surely" low on refrigerant. Same goes for both gauges reading high (over charged). Typically I would only be seeing these machines in an ambient temperature of 60 to 90 F , and I see a hell of a lot of them. If I see four in a row and three of them are all reading the same and the fourth one the gauges are low, I guarantee you the machine is not working right.

Maybe that does not translate over to automotive very well.

That being said, has anyone tried to charge the system using the superheat method? Since some of us are using different compressors and different types and sizes of condensers along with different types of fans, it seems like the optimal way to charge the system is not necessarily by weight but by hitting the optimal superheat change at the evaporator. You would need temperature probes to do this.

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