Rob Edwards

Thank you for catching my error this morning Bill- I just went back to the thread where I'd done this based on Dr. Bob's advice and it wasn't the low side, it was the high side:

https://rennlist.com/forums/7725928-post3.html

dr bob

The choice of low or high side for liquid filling isn't that critical. I suggest the high side so the liquid has to pass through the drier/evaporator or the condenser before it gets back to the compressor. Lots of places to absorb heat and flash to vapor on the way. Reality thouh is that if you charge through the low side and fill the suction line with liquid, it will still flash to vapor in a not very long while. If you spend 10-15 mins disconnecting stuff and tidying your workspace before you cycle the compressor, it should be really safe on any warm day. Rotate the compressor by hand a few times if you want to be sure, no matter what.

Bill Ball

In my understanding, no matter which method you start with (vapor or liquid) at some point in the charge you will have to use the low side WITH the engine running and hence vapor method. Just hooking up to the high side (or low side - no difference since they communicate and equalize) with the engine off cannot produce a full charge. The resting pressure in a fully charged system is 90+ PSI. Cans of Freon only put out about 50-60 PSI unless you do some extreme heating. So, once the system pressure exceeds that, you need to use the compressor to reduce the low side pressure to get the rest of the charge into the system via the low side port. That's my experience, so I may run in one can or so in the high side, but as the system pressure rises and the high side charge rate slows, I switch over to low side vapor fill, engine running, AC on full. That could be what Sean was describing.

Lizard928

What system pressures is everyone looking to obtain?

I have filled a few systems by weight and have always found that the pressures tend to be too low......

Bill Ball

Hmm...I see about 90+ fully charged in a resting system, but I haven't made a careful study of it. I could go out and measure mine tomorrow morning.

Bilal928S4

Wally,
What would you pressure test your system at?
Right now with a leaking compressor I am holding 120psi.
Colin,
If after filling by weight the pressures are low, what do you do to get the system correctly operating?
Roger,
Not all of us are lucky enough to have Sean next door.
What is the sight glass for? I have used that successfully for my Japanese cars AC fill up and also for my residential units. Why can it not be used for the German cars?
Thanks for all the input.

dr bob

The resting pressure in the system varies based on temperature, so there's no reliable way to say "charge to this pressure and you'll always be happy". That's why charging by weight is used; The refrigerant weight doesn't change with temperature or pressure.

The sight glass is a yardstick indicator of how a system is operating. In a perfect system, operating in balance, the condensing pressure and temperature are such that the refrigerant is just getting cooled enough to condense to liquid as it exits the condenser. That liquid is filtered and dried in the receiver/drier cannister, and stays barely liquid as it passes to the expansion valve. There, the expansion valve meters liquid into the evaporator. So long as liquid is presented at the expansion valve, the system will be fine.

Meanwhile, the compressor is a pump, with a defined ratio of inlet vs. outlet pressures. Higher discharge (condensing) pressure means higher condensing temperature. At the same time, lower suction-side pressure means lower evaporator temperatures. The capability of the compressor is defined by displacement and RPM, and affected by how well the seals and valves are working, and you want the lowest possible suction pressure to get the coldest evaporator. So with the constant compresor capability, you really want the lowest possible discharge pressure that will give you condensed liquid at the outlet of the condenser. Finding the optimum balance in the system therefore requires enough pressure to condense at the outlet of the condenser, given the air flow and ambient temperature, the cleanliness and efficiency of the condeser, yet low enough pressure in the system to boil all the liquid at the lowest possible pressure and temperature in the evaporator, robbing heat from the air passing through the evaporator to do the boiling.

So what are the best pressures? It depends... Barely enough to condense to liquid at the sight glass. As low as possible at the suction side. In your gauges, there are temperature values that relate to pressure, so looking at the suction side with R12 you'd love to see 20º or about 20-25 PSI with the system running at speed with proper condenser airflow. At the same time, you want to see just that occasional bubble passing in the sight glass. Any more pressure than that (no bubbles) is wasted and means that the evaporator pressure could be lowered and therefore colder if there was less total system charge.


If you are serious about working on your own AC, I strongly recommend you buy a Chiltons or Haynes AC manual, and study it before you plunge off into the system on your 928. There's nothing at all exotic about the system in the 928. With care, extreme cleanliness, and the right techniques and practices, you can make ice cubes with a well-balanced system. A little dirt, a little moisture, even just a little air, over- or under-charging, dirty condenser or evaporator, failed or weak condenser fan, and system performance tanks precipitously.

dr bob

With the engine off and cold, refrigerant will flow from the can to the system if the can is only the tiniest bit warmer than the system. Pushing liquid from the cannister means the refrigerant is boiling outside the can, someplace in the system where heat is available to do the boiling. If you charge with vapor and the compressor is running, the refrigearnt is boiling in the can, until it draws so much heat from the can that the pressure drops to the same as what's in the system. At that pressure equilibrium, the gas no longer flows. Add heat to the cannister, pressure goes up a little and you have flow again.

So, with engine off and system cold, you can charge liquid with just the heat from your hands holding the can, so long as system temp is about 90º F or so. Once the liquid starts to flow, it flashes and cools the system since it's sucking heat from the system rather than from the liqud in the can. That lowers the pressure in the system slightly, and eventually you reach the point where the heat of evaporation of the refrigerant balances with the heat available to be absorbed from the system.


For those fortunate enough to have the Morehouse CD manual set, spend a little time with the factory AC system manual that's included. It's an interesting read, and although it doesn't get deep into the reasons why this technique works, it does describe the method nicely.

Dozman

The sight glass is a deceptive indicator tool on how the system is operating, hence bubbles (won't get into saturation temperature of the liquid ref. since in laymans terms not many care). It is very useful if everything in the system is operating correctly and clean of debris. IF the evaporator and condenser is clean no air blockages, the condenser and evaporator fans are operating and at oem rec. speeds, and no problems with the expansion valve. Then the sight glass is useful tool.

One way to charge the system without running the compressor at all, is by charge via a 30lb cylinder of refrigerant. Charging by liquid would easily over come the pressure in the system. During the charging process the charging vessel being a small can or larger has little refrigerant in it and pressure in the system equals the refrigerant charging vessel. Simply heat the refrigerant charging vessel either by setting the vessel in hot water or heating the vessel by heat gun. This will raise the pressure of the charging refrigerant above the pressure of the system charging.

Don Carter

I can say from first hand experience it's not a given that the can WILL explode if connected to the high side. I took my car to a local non-Porsche shop to have it evacuated and charged a few years ago. Since our low side and high side are reversed from where they are on many cars, the "mechanic" connected to the high side and tried for about 45 minutes to put R12 in the system, with two different cans. The freon would not enter the system, but the cans did not explode either.

I had never done much AC work at the time, so didn't catch his mistake until afterwards. Obviously, it's not safe to connect to the high side when the compressor is running, but just adding some real experiences to the discussion.

dr bob

The full-sized cans are test rated to something like 225 PSI, and I'm sure there's some safety margin between that rating and actual frag pressure. The little cans can be grenades, though. Those crimped-on lids are probably the first failure point. I don't want to be that Darwin Award winner for something simple like this. Or merely blind from the spraying liquid.

Bill Ball

Well, of course, if the system is not fully charged, the pressure on the high side will not get very high. A high side pressure of only 60 PSI will stop flow, and once flow stops, the pressure will go no higher.

WallyP

It can be difficult to buy just plain R-134a at the parts stores - most of the stuff on sale there will have some kind of sealant or super-mouse-milk added. The cost is usually $12-15 per can.

If you have a Big Lots store, you can buy standard R-134a for $8 a can. You will need to ask at the register in most cases - apparently the stuff is very popular with shoplifters...

Bilal928S4

I will be going back with R12.
What pressure should I be using to check for leaks in the system. I found the leak at the compressor with 200 psi and the system leaks down to 125 psi and holds.