Charging the AC. Advice needed

[RandallNeighbour]

The Pelican Parts DIY is so good there's no reason to duplicate their efforts:

Porsche Boxster Air Conditioning Recharge - 986 / 987 (1997-08) - Pelican Parts Technical Article

Took all of 10 minutes to do this. Best DIY to date!

[KB944]

Originally Posted by RandallNeighbour

The Pelican Parts DIY is so good there's no reason to duplicate their efforts:

Porsche Boxster Air Conditioning Recharge - 986 / 987 (1997-08) - Pelican Parts Technical Article

Took all of 10 minutes to do this. Best DIY to date!

Resurrecting this old thread since I'm getting neck-deep into my Boxster's AC system.
My AC stopped working probably 3 years ago, so I just stopped using it. Now that I'm getting things restored & back to stock -- I've checked through enough fuses & relays to be an expert at the electrical side of the AC system, and I bought a gauge set to check pressures while I hotwire the compressor. The only electrical issue is that the pressure switch is open, but I'm trying to focus first on the compressor and just making sure there's enough refrigerant in there.

With the pressure sensor open, depriving the compressor of its voltage, to check the pressures I've hotwired the compressor relay with 12V from the nearby spoiler relay. When I look at the pressure gauges, hi & lo are both 30 psi. I expected the high side to pop up in pressure, at least a little. This led me to fear my compressor is shot, or the expansion valve is wide open. (?) But I can't help but wonder if it's been so long since the AC was used, I have a "bootstrap" problem with unusually low refrigerant...

I already tried buying a can of "AC Pro" and brought the system up to 30psi from about 20. So it was definitely low before. I'm afraid to go any higher in pressure since instructions say not to, but...

What's the static AC pressure supposed to be after not driving for a while? Mine is at 30psi now, first thing in the morning, or car on, or compressor running, both high & low sides of the expansion valve. I'm wondering if the pressure is sooo low, the compressor isn't "grabbing" any of it, the high side isn't going high, and the low side isn't getting sucked down to where it ought to be, given the (possibly) low amount of refrigerant. I'm wondering if I should just "give it hell" and put a bunch of R134a into the system, the compressor will finally start gulping enough refrigerant to work, high will go high, low will possibly get sucked *down* to ~20psi in pressure once the compressor starts operating properly, and then I can continue topping off to ~30 psi.

The fact that the pressure sensor is open (indicating a fault) would seem to back this up, although I have no documentation about what pressures the sensor/switch is looking for. However I'm afraid if I go too high on low-side pressure, in defiance of instructions (even though they all assume the compressor is actually functioning) that I'll damage something with over-pressure.

Has anyone had experience with this particular corner-case?

Thanks,
-Kevin.

[Gilles]

KB944

Please note that I am in not in any way an expert on AC systems..

However, we happen to sell PCA Systems (AC ground support systems) for aircraft, and I know that for an AC system to work properly there has to be a difference between the Hi pressure and Low pressure systems to allow the heat exchange process to occur.

.

[KB944]

For future reference for other folks, I did some more digging and found that, indeed, I was so low in my refrigerant after not looking into it for 3 years that following all the instructions to recharge it did not immediately work. Looking at this rennlist thread:

https://rennlist.com/forums/924-931-944-951-968-forum/509239-so-you-want-to-re-charge-your-a-c.html

I see this informative mention of the static pressure of our refrigerant, R134a:
________
"In my case they [high- and low-side pressures] are both showing about 80 PSI, and this explains why I'm not cooling much and why the compressor cycles on and off. Here's why: at 83 degrees, liquid R134a has a static pressure [AC not running] of about 88 PSI. If I have less than that for a static pressure, it means that there is NO LIQUID REFRIGERANT in the system. Just enough to be in there as a gas, which is NOT enough to do its job.

...[snip]...

- If the static pressure was 88 PSI: All that tells me is that there is SOME amount of liquid refrigerant in the system. Not how much. A 3000 gallon tanker truck full of R134a will read 88 PSI at 83 degrees, as will a 14 ounce can that you've taken all but 1/2 an ounce or so out of. As long as there is still LIQUID refrigerant, the PSI will have a linear relationship with its temperature."
__________________

I had tried following the refrigerant instructions to fill the low side to 25-35 PSI with the engine & AC running full-blast. As I mentioned, I came to discover that the high & low side pressures were the same, indicating that the compressor wasn't doing its job. I stopped everything, thought about the above post, and figured I was safe in filling the AC system up to ~80 PSI with engine & AC *off*. I didn't want to go much past that, for fear the gauge wasn't calibrated well and I didn't want to go all the way to 88 PSI where the refrigerant would start liquifying -- not until I ran the AC again to follow the instructions properly.

When I started the engine again & ran the AC, lo and behold!
The compressor now had enough refrigerant to "grab" so it could start pumping. Now, the low side gauge read something like 20 PSI -- still low but at least now I could follow the instructions. I'll spare you the physics, but it suffices to say since it was a hot day I leaned towards the high end of the pressure recommendation and filled to 35 PSI. Now I have cold air.

Mind you -- that doesn't mean I have exactly the right amount of refrigerant. You need to evacuate the system & use a scale to really be correct. But it's better than it was, and that's all I was going for this time around.