I have made some progress on the A/C learning curve but have hit another road block. I have some R12 in cans. I have a set of manifold gauges and can connect to the 928. When I put in about half a can the manifold gauge on the low side reads about 70 Lbs. This was enough to trigger the low side cut off switch and get the compressor turning. The sight glass shows some Freon going through the system, but I am unable to add any more Freon from the can. Needless to say with about 6 oz. in the system it is not producing much cold air. So the questions are:

1. When I started to charge the system I pulled down a vacuum of about 22 inches. It held for a couple of days no problem. I then added some Freon from a big tank I had that was almost empty. This gave me about 10 lbs. On the low side, even though there was about 60 ozs. in the big can. Is it possible I corrupted the system by using the big tank? The system held the pressure for several days.

2. I then added some Freon from a new 12 Oz. can of R12. The system took about half a can. Since the pump was not turning I jumped the clutch to get the pump to draw in more Freon. No luck.

3. I ran through an electrical check and found a loose connection on the temperature sensor on the evaporator. Cleaning the connection caused the pump to kick on normally.

4. After that I tried to add more Freon with the system running. Not much luck. Maybe after some running time I got in another couple of ounces.

The system seems to check out and work electrically. The A/C comes on when it is called for and the climate control and flaps all work.

How do I get the system to accept more Freon?

Do I have to bleed the high side as I add Freon?

There should be no air in the system at this point. If the big tank put in air should I purge the system, evacuate it and start over? Would air in the system prevent it form taking a charge?

Since the compressor has two connections, where do I hook in the high side?
Is there a place other than the compressor to hook in the high side.
Remember this is an 85.

How do I know when the evaporator valve is working correctly? I hear the typical hiss in the evaporator when the A/C is turned on so I am assuming the evaporator value is working correctly and calling for Freon.

The system has sat for five years with the engine out of the car. Over this time most of the Freon leaked out, but the system was still holding some pressure. It could have been low before I got the car since I have little history on it.

When the A/C is turned on should the auxiliary fan kick on? This is an 85.
I do not see the connection on the wiring diagram, but I could have missed something.

Finally the top of the condenser gets a bit warm - not so hot that you cannot put your hand there, but warmer than the lower part. The expansion valve is cold, but not freezing.

Thanks much.

Dan the Pod Guy
Portia's Parts


I no little about AC systems (Not for much longer as I will have to learn as summer is here and fuba AC is not an option).

My WAG is that neither of the two cans had sufficient pressure to deliver the full load of R12. When the compressor is not running presumably there will equal pressure all around the system. I am guessing that when you added
R12 on the suction low pressure side you got a bit more in because the LP side goes to a lower pressure running than at rest. Although after some time of operation the gas temp in the system will presumably increase above the at rest temp this will cause an overall pressure increase. Maybe try again when the system is cold - first thing in the morning? having kept the R12 in a warm room overnight.

The other thing that occurred to me is that while the system held a decent vacuum for a few days a vacuum will certainly assist keeping a dodgy crimped pipe connection sealed, but once said pipe connection sees internal pressure may well leak as the pressure is opening up the joint.

Just the ramblings of a novice shadetree AC tech.



Thanks for the support. Turns out I made a totally wrong assumption. The port by the dryer with the sight glass is the high side. Oops! I was attempting to force Freon in on the high side. Once the system got to the pressure in the can no more would go in - and for good reason. Thank God I was working with an empty system.

The low side port is on the pipe coming out of the top of the compressor and running along side the right hand (might be different on your car - I don't know how much they changed for right hand drive cars) firewall has the low side port. I switched ports and the Freon goes in no problem.

Now we will see if the damn thing can hold a charge. Back to tracking done the last oil leak!

Dan the Pod Guy
Portia's Parts


Warm room. Mmmm.
There goes one of those stray neurons.

Last year at SITM when they were doing the AC fillups, one of the extra "tools" the AC gang carried around was a bucket of HOT water, along with the big ole R-12 can. Think they placed the AC fluid in the hot wasser to increase input pressure.

Now I know why they were carrying around that steaming white bucket.

Did you guys add in a fresh drier. It's just an aluminum can with drygel packets inside. The drier is supposed to hold some of the R-12 in fluid form as a AC charge reserve. AC lines never leak, right. 8-P One of my compressor line has a gentle leak.

1. When I started to charge the system I pulled down a vacuum of about 22 inches. It held for a couple of days no problem.

Why only 22? I thought it should pull down and hold at least at 28 if not 28.9.

DanT 85 928


For those of you who are interested here is what it took to get set up to do my own A/C work. I will put this up on my web site later.

1. A used Robinair R12 recovery system. Used and in good working condition.
It is illegal to vent R12 into the atmosphere. $375.00 including shipping.

2. A manifold gauge set from Harbor freight. The set is sold for R134a only, but the hoses fit R12 too. $40.00 new.

3. A conversion nozzle for the Robinair Recovery unit. The Harbor Freight input hose is threaded for 134a only. I wanted to use it in both environments. $26.00 including shipping.

4. Robinair 3 CFM vacuum pump. Buy it now new on eBay $170.00 including shipping.

5. 12 cans of R12 refrigerant. $200 for the case including shipping. It takes three cans to fill the 928 without rear A/C.

6. Leak detector from Harbor Freight - new and on back order. $70 including shipping.

7. Digital scale. I have not bought this yet but will need it for the big tank when it is full. About $50.

I will eventually have to buy an assortment of o-rings and seals. Right now the systems I am working on are all sealed.

I will have to recover 18 lbs of Freon to justify the cost of the recovery system. However if you are searching for leaks you first have to put Freon in the system and then draw it back out to fix the leaks. There is no practical way to do this without a recovery station. I bought one that is able to both recover and recycle so eventually it will pay for itself.
There are a lot of people getting out of R12 work so there are many reasonably priced used recovery systems around.

So it took roughly $1000.00 to get started. I have enough Freon to do five cars. At $200 a service station visit am even when I have used up the supply of Freon.

It was a lot of fun and very satisfying to feel cold air coming out of the
system for the first time.

Dan the Pod Guy
Portia's Parts

I have the climate control system working as one would expect. Select a temperature and the car will moderate between heat and fresh air to get close to that temperature. However, when I push the A/C button the pump always runs. The system still adds in heat as I slide over the heat control, but it does not shut down the pump clutch.

Is this the way the system was designed to work?

Anyone have a fix for this or do I dig into the wiring diagrams. The car is an 85.

Dan the Pod Guy
Portia's Parts


> The system still adds in heat as I slide over the heat control, but it
> does not shut down the pump clutch.

Hi Dan,

Good job on the A/C work! I think mine does mostly work as you describe. The A/C pump does cycle some, but for the most part the temperature is regulated by mixing cooled air from the A/C with heated air from the heater core. I would have expected it to just vary the duty cycle on the A/C pump and mix with outside air.

88 S4

That has also been my experience. The system has a sensor that shuts off the compressor only when the evaporator temperature is low enough to risk trashing the compressor with liquid refrigerant. The normal mode of operation is to run the compressor any time you close the AC switch on the console, and regulate the temperature by heating the chilled air.



The compressor should *not* run all the time unless it is working as hard as it can and cannot bring the evaporator close to freezing temps where the freeze switch will cut it off. If your compressor clutch is engaged all the time when the button is pressed at fan speed 2 or lower and it is not that hot outside, you have a problem (unless you have selected the defrost setting).

Did you hook up your gauges to the system and check high/low pressures? I noticed in a previous post that you pulled a vacuum of 22. That really is very low, and it should be pulled to above at least 28 for an hour or so.
This is very important because it allows all the water vapor to boil away inside the system. The lower the pressure inside, the faster the water will boil. The pressure readings from your gauges (along with ambient temp at the time) will tell you if there is a problem with the charge.

'86 928S 5-spd w/LSD



Thanks for the info. I understand how the freeze switch works. However, that was not my question. I was curious if the climate controller would shut down the compressor when it calls for heat. Looking at the wiring and from input from others it appears the A/C compressor will remain engaged no matter what the climate control is thinking. At least this is how it works on an 85. Kind of an oversight from Porsche.

I suspect the freeze switch will never engage is the heater core is putting some heat into the system. At the moment the heater control valve on the engine is not cutting off 100%. I feel very cold air from the top of the center vent and warm air on the bottom. The same symptom is observed when the A/C is cut off. The heat will increase as I move the slide towards hot and cold down as I move it to cold. It never completely cuts off some of the heat. I have a new control valve on order.

Dan the Pod Guy
Portia's Parts