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Hi,
Something you could try:
Go get your can of R134 with the low pressure gauge. Hook it up to the low side port. If there is any refrigerant left the pressure will real like 85 psi. Start the engine and turn on the a/c. Watch the gauge. If the pressure was 85 before, the low pressure switch should allow the compressor to run momentarily. If the pressure drops to 15 psi, you are low on refrigerant, the compressor will stop, but you will be able to inject refrigerant to charge your system. Eventually you will have enough pressure to keep the compressor running, and maintain a low side pressure of 30psi or so. That should get you home with some a/c.
It will confirm a leak, and you then need leak diagnosis, repair, evacuate and proper recharge by weight.
That's what I'd try.
No tools required. If pressure doesn't drop, meaning no compressor function, then you're not going to fix it on the road anyway.
Good luck,
Dave
my god, was every 928 built differently? I expected to pull back the rubber from the low pressure switch and need to build a male-male jumper. in this case, the lead is male-male and I would need a female-female jumper....assuming of course I'm actually looking at the low pressure switch in these pics....I could be wrong ...
In your situation with no tools and very modest experience the best thing you can do is take the car to an ac shop and in a matter of 5 minutes they should be able to diagnose the general nature of the problem. A complete analysis will take longer depending on what the issue is but they should be able to define whether the problem is immediately fixable [compressor seal] or something more complex.
As has been well covered by the chaps either the compressor is not driving or you have lost the gas. To understand the gas situation you need a gauge set and see what the pressures are at 2k rpms. The gauge set will immediately tell the operator whether there is enough gas in the system to engage the LP switch. If the system is low on gas you need to know whether it is a major leak or long term [slow] fugitive emissions. If the latter it can be topped up, if something has failed putting more gas in is pointless. A good ac shop will also have sniffers that can pin point any leaks.
its working !!! I did the final check to make sure there was power to the low pressure switch, but I did not go as far as jumping the switch. instead I connected a can of Avalanche( the only parts store within 50 miles only carried that brand). within 30 seconds of adding refringent, I saw the compressor begin to spin, actually I heard it first. it took most the can. the avalanche does not have a gauge, its uses color tabs which are not great. I was suspect at first because the temp was only coolish at idle. so then I took the car for a run with some rpms. front and rear are blowing cold now. the rear seems a bit colder. I would not call the temp "ice cold", but its cold, and adequate...on par with what I deemed acceptable over the past few weeks. I don't know if the system will hold pressure, or for how long, but today its cold. I will monitor and have it looked at by a pro when I get home.
thanks for all the newbie questions and patience. I have learned a huge amount of the past 2 days from this forum!
If you're really going to fiddle with AC systems I really recommend buying a proper manifold rig/gauge set. Heck you can buy a cheap-o one for $35 bucks on Amazon. That will tell you a lot about what's going on and should be considered the bare minimum for working on AC.
I am in the middle of the mountains in Montana on a road trip. my local shops are Ace hardware and Napa. next closest is 60 miles away. sometimes you have to make do with what is available. Napa was sold out of AC gauges. Amazon would be 10 days to deliver tools.
I also "fixed" both cold air intakes with Gorilla tape. all of these items will be addressed properly when I get home.
Some thoughts for those who end up in a situation like the OP found himself in and may be interested.
The ac system in our car requires a certain amount of refrigerant to work correctly. In round terms our cars take 1kg of R134a [a bit more if R12]. Under normal circumstances when the car is stood still the static pressure in the system is ostensibly controlled by the ambient temperature that it is in equilibrium with. In the summer season if we assume the ambient temperature for purposes of discussion is 30C the system will sit at a pressure of 6.7 barg [97 psig]. I know from practical measurement that to bring the system to equilibrium pressure with the filling cylinder at this temperature that it takes about 200 gms [0.2Kg] of gas. Thus when filled correctly to 1 Kg some 800gms of R134a resides in the liquid phase.
For the ac system to work the low pressure switch must be activated and as I can tell this happens at about 25 psig. So, when gas leaks from the system the static pressure remains at the value supported by the ambient temperature until such time as all the liquid has evaporated. So as per the above, once this happens some 800gms of gas has escaped. To get to the point where the compressor will not activate due to the low pressure switch the system will lose around half of the remaining 200gms of gas so at this pont there will be about 100gms of gas left in the system. As gas is lost the performance of the system drops off but unless the ambient conditions are really high [like in my location] the performance drop will not be too noticeable until such time as the inventory drops to about 200 gms and there is no liquid in the system. From that point onwards the loss of performance is very noticeable until it reaches the point where the system will not engage at all.
Take one of those cans and fill with 200 gms [7 ounces] and the system will work but somewhat weakly. The above gives some idea of what is actually going on but that is not the full story. The compressor oil in the system is suspended in the gas stream when the system is working. It does not dissolve into the gas and when the system is static it will tend to drop out into the various parts of the system and the WSM gives some indications of what proportion of the oil sits where in the system. The big problem is that as the gas escapes it takes some of the oil with it. About half of the oil sits in the compressor and the most likely source of leakage is the seal face. I have seen leaks from the compressor seal, the compressor body and from some some joints and every time one sees oil at the source of the leak. Thus when the charge has been lost one really has no clue as to how much of the oil has escaped. Thus I find it reasonable to assume that when 90% of the gas has escaped a reasonable amount of the oil has quite possibly gone with it.
When the amount of gas is low the amount of oil circulating [i.e. carried in the gas] is low- not good. At some point too little oil will see temperatures inside the compressor increase due to friction and at some point the compressor will fail. As to how much gas leakage one can get away with without topping up the oil I have no idea. Too much oil is not good either but it is better to have too much than too little. Different compressor models take significantly different amounts of oil but if I was to take a SWAG [scientific wild *** guess] I rather suspect that if one loses gas to the point where the LP switch does not trigger the system probably needs somewhere in the region of half the full oil charge to be added.
I understand it is possible to get hold of kit to determine whether the oil charge in the system is sufficient - use of such devices after a major gas loss would seem very prudent if one wants to avoid grenading the compressor. I am currently trying to get hold of such items- no idea if they are any good [or not] but worth considering.
excellent explanation. In hindsight, I can't help wonder: why didn't I simply add refrigerant on Day 1? instead I checked fuses, electrical to freeze switch, low pressure switch, etc. I certainly learned a lot, and I feel less intimidated under the hood( my first Porsche, I'm used to muscle cars). would it not have been a more direct troubleshooting path to have simply added refrigerant first? or would that have put other things at risk if low pressure had not been the root cause?
My personal view is adding refrigerant is not the first step but making sure the compressor is working. If the compressor is not on and you jump the low pressure switch and the compressor kicks on, then I would add refrigerant and see if things are now working.
excellent explanation. In hindsight, I can't help wonder: why didn't I simply add refrigerant on Day 1? instead I checked fuses, electrical to freeze switch, low pressure switch, etc. I certainly learned a lot, and I feel less intimidated under the hood( my first Porsche, I'm used to muscle cars). would it not have been a more direct troubleshooting path to have simply added refrigerant first? or would that have put other things at risk if low pressure had not been the root cause?
Good first step would be connect gauge from recharge can and check pressure at least on low side. Start engine and a/c, see if compressor runs. Add a little refrigerant if needed, see if compressor starts.
Now you know you have a leak, maybe slow, maybe fast, and you need to diagnose, and repair then evacuate and recharge by weight.
I think you did well for battlefield repair, and now you can get home and figure out the actual details.
Good luck,
Dave
I think no one here will be surprised when I report that the freon I added on Monday had all disappeared by yesterday when I drove the car for the first time since charging. yup, there is a leak somewhere.
I had a little bit of freon left and added it yesterday. I noticed a faint hissing sound that seemed to be coming from the general area of the low pressure valve. I noticed that the valve quick connector was swiveling freely on its post. I did tighten up the post nut, but hissing remained. no idea yet what the source of the leak is, or if the hissing is a leak or evaporative etc
Oddly enough I chased a leak on my 944S for months with dye. Could never find any except around the valves (which of course there would be because you fill it there!) Guess what was leaking? Yep, schrader valves.
I'd recommend buying one of those $20 R134a detectors. That will help you find the leak, but don't be surprised if you need to replace the valves.
08-24-2020, 04:37 PM
