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Where do I find the Low Point for recharging on the 1981 280zx? Is it on the compressor? The Dryer? The system has been converted to r134.

Thanks.

If the compressor is stock, it's on the back top of the compressor. Stock, the suction side service port would just have a brass flare cap covering it. There are two lines coming off the compressor, the high pressure (liquid) line goes through the radiator core support on the left side and connects to the condenser ("A/C radiator"). The low pressure line (suction) goes across the radiator core support (on the back side IIRC) over to the right side fender well. The Receiver/Dryer is on the liquid line after the condenser. Both service ports are in relatively close proximity to each other on the compressor.

Thank you for your response. The compressor has been rebuilt and the two caps on the compressor have been color coded (red and blue..Don't know if that means anything) either way the low point appears to be on the back point valve. Just want to validate. I appreciate your response.

Obviously this is my first BBQ when it comes to this problem. I have a couple more questions if you would. What about pressure? Can I rely on the fill up gauge? Thanks again.

The 134a convention is Red for High Pressure (liquid) and Blue for Low Pressure (suction).

Stick a thermometer in the vent to monitor the air temperature (or use a temperature gun). Turn the A/C on high with the windows down. At 90-100 degrees ambient get the suction side into the green on the gauge (around 25ish PSI) while you're holding the engine at 1500 - 2000 RPM. If there's a sight gauge (window) on top of your receiver/dryer check to see if there are air bubbles in the liquid. You want them gone as much as possible. You should be getting temps out the vent below 50 degrees (hopefully in the 40's).

I have room with my charging gauge set and hoses to put the refrigerant can in a small bucket of water - that helps put some heat into the can so the "freon" gases out faster. Remember that refrigeration is physics - it's all about "changing state" (from a gas to a liquid and back again and again). It requires energy to change state.

I'll take this opportunity to explain the physics of refrigeration to the entire group (this is a simplified description):

Refrigeration works on the physics principle of "changing state" like I said above. Normally substances like to remain in their "native" state (condition). Let's take water: three states - liquid, solid, and gas(eous). Water mostly likes it's liquid state. It requires a good deal of energy (heat) to change liquid water into steam (gas). You know this bringing water to a boil on the stove. Liquid water must lose a great deal of energy (heat) to enter a solid state (ice). You know this from the ice cube tray in the freezer or being up North in the Winter. When steam condenses to water (from gas to liquid state) it releases the energy that turned it to steam - very quickly. That's why steam will burn your skin so easily - it releases a great deal of energy very quickly!

On to the A/C system:
Refrigerant wants to be a gas. It only goes to it's liquid state under pressure. It requires energy from the engine to turn the compressor. When a gas is compressed its molecules start releasing energy as they are pressed together. They don't like to be that close to each other - just like your brothers. The resulting compressed refrigerant is hot. (energy and heat are the same thing in this example. "cold" is not an actual thing, it's just the absence of heat(energy)).

The refrigeration system is a "closed loop". From the compressor the compressed refrigerant goes to the condenser (the A/C "radiator" that sits in front of your engine radiator). The condenser is a "heat exchanger" - air moving through its fins captures and removes a good deal of the energy (heat) that the compression process introduced. The refrigerant condenses into a liquid (still held under high pressure). The liquid leaves the condenser with much less energy than when it entered. This is important for the next step.

The liquid then goes inside your car to an "expansion valve" located on or near the "evaporator coil" - which is another "heat exchanger". At the expansion valve the liquid (compressed gas) is allowed to return to its gaseous state. (The refrigerant "evaporates" into it's gaseous state). It order to change state, it requires ALL the energy (heat) that was removed as it traveled through the condenser core.

Where does it get that energy?? From the interior of your car! The blower fan moves warm air from inside the cabin over the evaporator coil in the dash and the refrigerant uses the energy (heat) to complete it's change of state from a liquid to a gas. Does your vent blow cold air? Yes, because the energy has been removed from the cabin air by the process. (if there isn't energy inside the cabin, the whole process is thwarted!)

Once it's absorbed the energy it required to be gaseous again, the refrigerant travels back to the compressor to undergo the entire process over and over again. It just keeps getting squashed and released, squashed and released. It's the transfer of energy that makes it all work.

In its simplest terms the A/C in your car removes energy (heat) from the inside of your car and deposits that energy into the outside air.

And that's my physics lecture for today...

EDIT: Corrected the above to correct as per Carl's comment below (but there is NO strikethrough so I couldn't leave my original text in place - you'll just have to guess how stupid it was...)

Good stuff, Chuck, now could you explain how to balance my checkbook?

Duffman wrote: ↑Tue Jun 25, 2019 7:35 am Good stuff, Chuck, now could you explain how to balance my checkbook?

Only if there's a green chili burro when it's done!

CGSHEEN, Thanks again for your time and effort. I appreciate the explanation as it helps with the big picture. This is part of the reason why I joined this club and left the other.

Great explanation Chuck. The only correction I have is that the refrigerant leaves the compressor as a hot, high pressure gas. As it moves through the condenser it turns to a high pressure liquid.