AC High Side and Low Side Pressures
 

I am fortunate in having a 928 with working AC and a wife who loves the car and loves to go to 928 events. She is, however, rather demanding of AC system performance and in expectation of my trip to the PVGP next weekend, I hooked up my gauge set to check pressures to make sure the system is doing all that it can in case next weekend is a scorcher. I am getting cold air from the vents and at 70 degrees and 40 percent humidity my high side is about 190 psi and my low side is about 40 psi with the condenser fan running. Using a generic R12 table, I should be getting around 215 on the high side and 32 on the low side.

I am reluctant to add any more R12 since the low side pressure seems a bit high already and I have no desire to waste a precious can of R12 just to add a few ounces. BTW, I have no idea what refrigerant or how much was added by previous owners. I have had to top off the system once or twice in the 5 years I have owned the car and think it is miraculous that the AC works at all and is virtually leak free after 32 years.

All input and opinions are welcome.

Thanks in advance.

Hey Otto,

Can't say on the high side, but when I recharged mine with Pete and Bilal at Camp 928, I remember that the low side was at about 25-28 psi with one can added.
As we were close to 30 psi, we decided not to add some more.
When I got home I checked and I was getting cold air from the vents at about 8 degree C. 46 degree F. Quite good to me.

Is the temperature is going up from last year or quite stable?
If it's stable, maybe the system is a bit overcharged.
I think I read that an overcharged system would not be as cold as a system with the right pressure.

Wally's got an excellent writeup on the 928 Specialist page for HVAC troubleshooting. I can't link to it directly, but below is where it's located:

I'm where Bertrand is in the mid-40s and I thought that was pretty good. That was right up until I saw what Worf928 (Dave C.) said it should be in the 928s for sale thread and realized I still have some work to do. And then someone posted a pic of them getting 17F at the vent and my head exploded.

https://www.928gt.com/default5.htm

https://cimg0.ibsrv.net/gimg/rennlis...0b26795739.jpg

NoVector: Your picture of that nubile young thing makes it impossible for my personal temperature to get anywhere near cooling. Damn! That is a great looking hat she's wearing!

Otto: I would want to see readings from a dash thermometer, as a snapshot so to speak corresponding to the pressure readings you just took. If it is 40F or below, I think in humid weather that may be the best you can do. OTH, I'm not subject to "She Who Must Be Obeyed". I saw that character subtitle in a terrible old movie the other night, the title of which I thankfully, immediately forgot.

There are no "set pressures". With a proper fill there is a broad range so to speak.

The all of the vacuum stuff working, and no bubbles in the glass, youre in the right neighborhood.

The only absolute is weight of refrigerant in the system. Somneone with knowleege can get kinda close tho...but its hard to explain.

I checked the WSM for my 85 and did not find the charts for Hi and Low settings, can anyone assist ? The reason i want to check is when I start the car from the garage or early in the morning the AC is cold, not sure how much , I will have to check with my temp gun. But if the car sits in the sun for a long period of time like 2 to 4 hours, if I start the AC it never gets as cold as when I start the car under cool conditions, it's not even close !

Is there air in your sight glass with AC on?

If so, pressures are irrelevant, and you need to 1) Add some* and 2) Find the leak, 3) Surely you have behind the dash vacuum pod leaks as well.

*Add until theres only a faint suggestion of bubbles in the glass first, IMHO. But note pressures then too, as they may indicate other issues in the system.


Seems that 50% ot the internet is porn, and the other half is AC how-tos...and people are still asking about AC how-to. ;)

Is this chart any good ? Found this on web, not in WSM.

https://cimg7.ibsrv.net/gimg/rennlis...fcd073ea08.jpg

I remember checking that last year and the liquid was creamy with no bubbles, haven't checked this year. It's doing the same thing as last year and the year before.
I will have to check for leaks when I store it, right now enjoying the drives. I do have one of my vents (comb flap I think) which is not operating like it should but the rest are fine. I also have to check my recirc vent, which I have no idea what to look for and where ?

That is what made me go there in the first place your post, I did a search for R12, and saw no chart. I will check that page without doing a search again. I am using the PDF version.
Thank you

Are you referring to page 87-116 ?