GregBBRD

Nothing to add to this. Perfection in theory and in tracing the problem.

FredR

Dave,
I will be checking gas pressure over the weekend- the problem has noting to do with heater valves as my heater is totally disconnected from the cooling circuit. I do have a heater valve fitted with the disc wired closed but it is only looped from the block back to the header tank and does not flow other than I ocassionally open the valve to flush the system and hopefully avoid any localised corrosion due to the dead leg.

Regards

Fred

James Bailey

I too have had similar experiences of after a large Mexican food brunch thankfully not including the run....Just note holding the gas in is an unnatural act much like trying to keep oil in an old Jaguar motorcar...

j.kenzie@sbcglobal.net

I am sure you are on top of it. I meant that I have heard of valves that are locked closed developing internal leaks that allow fluid into the heater core. It sounds like that doesn't apply to you.
Hope you figure it out.
Dave

The Forgotten On

I operate my 81 with the heater core completely open with the internal shutters closed over the heater core and I still get cold air. And that is on a hot SoCal day.

It sounds like you might have overfilled the system activating the shut off or your expansion valve is getting clogged. Just hope it is the earlier of those two

FredR

I have not added any refrigerant for over a year and the a/c system has been working better than ever since we rebuilt the compressor before the last charge so overfill is not on my radar unless of course a pressure switch is not working correctly and triggers when everything gets hot thus breaking the circuit to the clutch solenoid.

Regards

Fred

dr bob

Fred-

Was your car originally supplied with R-134a in the AC system? On factory-fitted cars, the pressure safety switch does double duty. Actually some earlier cars have a similar dual function but the switch terminals are different. Anyway, later cars for sure have the low-pressure safety function that protects the system from operating under-charged. That same housing holds an over-pressure cutoff that is intended to protect the system and compressor from high head pressure when a fan fails or there's some other reason that the heat isn't being removed from the condenser. In my experience, safety switches almost always only cause problems when they are bypassed for "testing". Yours might be that 'almost' exception though. If you have the gauges on it as your reproduce the cutout scenario, you'll be able to see if there's a pressure rise there that's opening the over-pressure section of the switch.

WallyP

Fred,

One thing missing in this post is the answer to one question:
When the AC stops working, is the compressor working? That is, is the compressor clutch still engaged so that the center of the compressor pulley is turning?

If the clutch is not engaging, the testing of the electrical string as suggested by dr bob is the next step.

Alan

Fred - do both cooling fans run together with AC on?

Alan

FredR

When it started this behaviour I immediately tried to switch the a/c on/off and could not detect any difference so I was assuming it was not a solenoid trigger type of thing.

Now the mystery moved a little differently today- I took the car for a run at 4.00 this afternoon- initally it seemed to show the same behaviour and then settled down to a nice cool temperature even though the outside temperature was some 36C-indeed when I stopped the key fob was very cold. The only thing I did was play around with the a/c controls a bit before I started- normally I never touch them.

So it seems the refrigerant is OK and something is/was causing the solenoid to trip. Anyway it ran OK for 30 minutes until I returned home. This tells me one of the limiting elements is falsely tripping or a cable was grounding- distinctly possible- but no idea why ti should run OK today.

Rgds

Fred

FredR

Alan,

Last time I checked they both ran OK [a couple of weeks ago that is] when the a/c was switched on. I have often wondered why they programmed the system this way as air blast at speed should render that need superfluous- I could understand them being switched at low speed as in slow moving traffic.

Rgds

Fred

FredR

Bob,

My a/c system being one of the earliest GTS models was originally kitted with R12. As I understand, just like my late 90 S4, this system has a low pressure cut off. I had to convert to 134a a couple of years ago and we changed the hoses, blew out the system including draining the old oil out of the compressor, fitted a new filter thingy and it seemed to work OK. Initally I thought the performance was a bit less than with R12 but recently it seems to be working fine [until now]. Then today it seeemed to kick back in after looking as though it had cut out again. Was running well for half an hour until I stopped the car upon return.

I believe it has a high pessure cut out feature as well.

Regards

Fred

FredR

I think I have eliminated gas fill from the issue with a good cool run yesterday and another one today when it was even hotter outside [about 39C this afternoon]. The car was cooling very nicely today but then as I was near home it seemed to cut out after about 20 minutes of a "spirited" drive so I need to know what sensor caused this. Why Porsche could not create a "first up" alarm is beyond me.

As I understand the system the triggers that can cause cut out are the high and low pressure switches on the a/c circuit, the freeze switch/capilary, high engine temperature and high gearbox oil temperature- Have I missed anything? I presume each of these items normally has a closed circuit and opens if the condition is not healthy- correct?

The only way I can think to test these is to individually take them out of the circuit- loop them in the health condition and measure the signal as the car is driving with an idiot light in the cockpit- any better ideas anyone?

dr bob

High engine and oil temp shouldn't interrupt the AC system. At least it doesn't on my car.

I fondly remember a Sharktoberfest discussion with an owner who was going on about how his AC was working so well, and that it cut out under hard accelleration so all the AC ponies could be diverted to the wheels. We quickly diagnosed his vacuum issues, leaks were causing the heater valve and defrost vents to open when hard on the pedal.

I remember mention of an AC compressor cutoff xomeplace in the very early cars, but there's none on mine that I'm aware of.

Alan

No

AC Triggers On: AC switch, DEF mode, max low temp switch, recirculate (All via Head unit relay)

AC Disable: Freeze switch, Low refrigerant pressure, Absurd refrigerant pressure (if equipped)*

* You will only have a high pressure clutch cutout if you added a late model GTS 3 way pressure switch. R12 systems come with just a 2 way switch with Low & High sections: Low is a cutout for the compressor clutch when freon pressure is inadequate (<29 PSI), High causes the fans to run on high mode with high refrigerant pressure (~260 PSI)

Factory R134a cars have a low/high/'absurd' switch. Low & high do ~the same as before. Absurd is a second cutout for the compressor clutch for when the pressure exceeds 464PSI - without this switch and with inadequate condenser cooling (e.g. one or both fans dead) the pressure spike can be huge - you may blow out lines or at least leak out much of your refrigerant though the seals.

If when you did the R134a conversion you didn't fit this switch you should add it now. If you did add it it could be the cause of your cutouts after time... Root cause really will be something else: likely the fans (possibly a faulty expansion valve)

Alan