Quick Questions About A/C System Pressure (Charge) and the Compressor
 

Trying to get the a/c sorted and the compressor spinning.

My first question is, does my A/C system hold pressure?
Here's the situation: I disconnected the lines at the compressor today. I have not been able to use the a/c for about 10 years because the low pressure switch has probably prevented the compressor from working. When I disconnected the first line on the compressor, which happened to be the upper connection, there was a loud hissing for several seconds. Does this hissing sound mean the system completely holds pressure? (For future readers: Be careful and wear eye protection.)

Second question (The compressor won't engage):
Is the wiring to my A/C compressor toast, or is it the relay? I attempted to hot-wire the low pressure switch, but the compressor clutch did not engage! Is it the relay? The relay I was able to use today in the G17 position (titled, "a/c compressor") had an alarm on it and appears to be meant for the catalytic converter "gong" position. It has a tiny speaker that sounded like a bell. Are these relays interchangeable?

Thanks for your efforts.

Wrong relay in wrong socket. Look under the cover for correct relay placement.

On one of my cars the system would leak down pretty fast to the point where the AC would not work, but it would hold a little pressure for months, maybe years. It had a small crack in the condenser and I suppose it could hod 30 psi but not 50 or 75. So it'd leak down to the point of uselessness but I never found it dead empty. So maybe it'll hold pressure but maybe not enough.

Sounds like the wrong relay, the AC relay is specific and putting the wrong relay in there can blow the fuse. However the compressor won't engage if the low pressure cutout is not seeing enough pressure. From dead empty you have to gas up the system a bit before it'll kick in.

The question is will the system hold the correct pressure potential, meaning a full system charge.
When the system is fully charged and it has sat for a while, a while meaning the low and high side pressures are equal. The pressure in the system at a "static" or non-operating state.
The static pressure of a fully charged system will vary based on the ambient temperature (temperature outside around the vehicle).
When the system is fully charged and operating the low side pressure and high side pressure will vary based again on the ambient temperature.
To know if the system will hold a full charge you would need to
1) Properly evacuate (pull a vacuum), charge the system to the full amount and sniff the system components and connections with an electronic refrigerant leak detector to check for leaks, and then
2) Document the static pressure and ambient temperature on a given day, and weeks or months later repeat that same process and reference a refrigerant temperature and pressure chart in the event
that the ambient is different to see if the pressure has dropped below the expected.

1) You can disconnect the compressor power wire, use a 12 volt source (such as battery) which shares the same common ground (ie battery) and apply 12v to the compressor side of the power wire
and see if the compressor clutch 'clicks' in. If you do not use the vehicle battery for the 12 v source you could use a battery charger however you'd need to ensure the ground on the battery charger is connected to the vehicle.
2) You can test the resistance of the compressor clutch coil which is nominally 3 to 3.5 ohms between the compressor power wire going to the clutch and the compressor's body.
3) With a min of 35 psi nominal pressure in the system or the low side of the system pressure switch jumped, you can turn on the AC system and test for 12 v from the power wire to the compressor

Ok, thank you all for the great responses! I will get the correct relay and connect the compressor to the battery positive lead.

I am basically the 2nd owner of this vehicle. The previous owner only ever brought the car to a mechanic, so I have reason to believe the system was cared for properly. After replacing some o-rings and the receiver drier (griffiths appears to be a top-notch resource for this), and evacuating the system, I think I'll be ready to fill up the system with refrigerant and the lubricating oil and test for leaks.

How much refrigerant and oil is needed to fill up an empty 944 a/c system?

"Officially", the Porsche Technical Service Bulletin 8717-9501 describes a process where any residual R12 refrigerant and R12 refrigerant oil (mineral based) is flushed out of the system. The guidance in the document was written many years back and assumed the dealerships had a specific piece of equipment which could perform the magic. Unfortunately this is not so.


OIL
The first issue is not how much oil can be added but rather what kind of oil. Extracting the residual mineral based oil and injecting an R134a compatible PAG (Porsche's suggestion) type oil is difficult without having a dedicated machine that flush out all the mineral oil.
Your other option to 'flushing' out the oil without the magical machine is flush individual components (hoses, condenser, evaporator with the expansion valve removed; you do not flush compressors or driers; evaporators can be challenging to flush out) using an approved dedicated AC flush solution in either an aerosol can or a can you can attach an airline to.
Mineral oil and PAG oil do not always get together that well. So rather than use PAG oil you can use Ester oil which is a bit more friendly.

The second issue with oil is how much. If you could have flushed all of the old mineral oil out you would simply need to add to a 1986 model year is 4.5 to 5 ounces of R134a compatible oil; here we will suggest Ester for this discussion.
If you could not flush out all of the oil then you know you need an unknown amount of oil (you got the unknowns, knowns, knowns unknowns, unknowns unknowns, and the known knowns). It is the refrigerant that carries
the oil through the system in a continuous cycle. If you have too little of either the compressor, which does not have a sump, will starve, there will be excessive friction, heat and a 'grenade' of parts tossed into the system which can become messy and expensive to repair.
So the suggestion when dealing with the unknown is playing it safe and simply put into the system a minimum of 5 fluid ounces of Ester oil. Worse case is you have too much oil and the evaporator will loose a bit of performance however you will not grenade your compressor.

REFRIGERANT
Officially the TSB, for 1986 is 860 grams by weight or 30.33 ounces of R134a. However, you want to check your pressures to ensure they are not too high in the event you have residual air in the system or your measuring method is not accurate.

Personally I have used POE in the past and it picked up enough moisture to rot the condenser. POE plus any water=acid. PAG also would react with water to make acid, however they now have double-end-capped PAG which is a lot more stable and doesn't form acids with water. So I now recommend flushing the system to get as much of the residual oil out as possible and going with PAG150 double end capped oil.

R134a usually requires about 80% of the R12 rating of refrigerant. Evacuate the system, hold a vacuum overnight, then evacuate again to get any residual water out, then fill with 80% of the amount of R134a.

You *can* also just fix the leaks and use R12, there's still a lot of it around. To be honest that might be the easiest and best way to go.

Thank you very much for all the help so far. I have a general charging question. Many of the instructions I have read don't mention anything about the air in the lines of the equipment. In other words, when you are ready to charge, and you open your valves, doesn't the air in the service line enter the system? One guide I read mentioned to very slowly open the valve until a drip of refrigerant appears. This is done to purge the air. One thing to remember here is to not spill the refrigerant. Thoughts on this?

I am planning on staying with R12. I tested the compressor for a moment today and the clutch grabbed. I am ordering all the o-rings (including the 4 on the compressor: 944.126.935.01) and a new drier. I haven't decided if I should also go with Griffith's compressor seal kit so I can replace the nose seal race and nose seal o-ring, among other things.

After replacing the parts, I will evacuate the system and charge it with refrigerant and oil.

My understanding is that if I stick with R12, flushing the system can do more harm than good. I read that in this particularly good write-up: So you want to re-charge your A/C... - Rennlist - Porsche Discussion Forums
Any thoughts on this?

My understanding is that if I stick with R12, flushing the system can do more harm than good.


No.
If you improperly flush a system you can have issues regardless of the refrigerant or oil type.

Moisture and POE (Ester) oil.

We have successfully helped DIY's, independent Porsche repair shops, and authorized Porsche dealers to convert approximately 2,581 various Porsche models from R12 to R134a using POE (Ester) oil since 1999.
The issue is not how much greater Ester can absorb moisture vs. PAG or 'double end caps'. The issue with moisture in systems relates more to:
1) Evacuation process.
2) Drier.
3) Past car repair history.
4) And something as simple as leaving a can of refrigerant oil with the top off the can for quite some time.

Acid formation and its damage in the original R12 refrigerant system starts way back in time, when the system was running R12. R12 and moisture tends to form more acid than R134a and moisture.

Many 944's have had more owners as compared to other Porsche models. Unfortunately, based on the vehicle valuation, some of the repairs or lack of proper repairs leads up to critical issues which become quite expensive to repair in terms of parts and labor.

It is a bit difficult for an ac system to literally "pick up moisture". If a system is properly evacuated to remove moisture, has a good drier with desiccant that is not saturated with moisture, and is under positive pressure greater than the ambient pressure, it is not going to pull in moisture.
The moisture was there from past improper practices or lack of care. ; unless you are the original owner, then again you got to wonder.

In this case, I am the original owner. Your mileage may vary, and I totally respect your extensive experience. I simply prefer to use DEC PAG instead of POE in my cars.