wwest

In my opinion the early 911 A/C system design parallels the one in my MH, or even a typical window A/C.

The only control point is the temperature of the evaporator vane surfaces or the airflow through those surfaces. So what happens if that control point fails in a manner wherein the compressor runs continuously...?

In my MH or the typical window A/C the house circuit breaker might open, or is more typical the CB inside the compressor drive motor opens due to heating of the windings. Now the CB cycles every 5-7 minutes until the refrigerant pressure declines to the point that the motor will restart without tripping the CB.

So, what happens when the control system in these early Porsche A/C fail in the same manner, how did the Porsche factory design prevent subsequence damage to the compressor due to "slugging", liquid refrigerant reaching, entering the compressor inlet. Or did they just ignore the issue..?

Is the clutch designed to begin slipping with too much refrigerant pressure? Is the compressor itself of somehow a design that limits downstream pressure? A spring loaded relief valve, Pressure CB, that ports downstream pressure back into the inlet if pressure rises too high...? Or is the factory recommended refrigerant charge intentionally low enough that the compressor running continuously will not, will NEVER result in slugging?

My advice to anyone is that BEFORE you begin upgrading your factory A/C you should either find a positive answer to one of the above questions or add a hi/lo pressure switch downstream of the compressor so the (old/ new) compressor doesn't inadvertently destroy itself.

Hladun1

To get slugging you would need to have a low pressure side that is cold enough and high pressure enough to have liquid formation. Over filling the system might allow that to happen. Fortunately early 911's didn't have enough condensor to extract enough heat to probably allow this to happen. When I built the A/C system for my 911 I used a high pressure shut off switch (300 psi I think) mainly to protect the miles of hoses. I noticed that it would occasionaly shut off the compressor.

griffiths

From our experience in servicing stock systems and designing our improvements and upgrades, liquid slugging seldom ever happens for various reasons in the 911 or 930: either the stock system cannot achieve low enough temperatures, expansion valves seldom fail in the open position, and there is a significant distance between the evaporator outlet and the compressor inlet for liquid to change state back to gas. And compressor failure due to either lack of enough oil flow, refrigerant flow or excessive high pressures, although it happens on rare occasions in these models is more common in 924s,928,944,951,964,968, and 993.

What the 911 and 930 had been missing, as you pointed out, is a low and high pressure switch feature:
If the system is low on refrigerant the factory design, lacking a low side pressure switch, will allow the compressor clutch to engage and the compressor can continue to pump which results in compressor wear (pistons, bores and wobble plate).
If the system pressures get too high, either due to a front condenser fan motor failure, over charging or running the system with engine deck lid up, and the system lacks a high side pressure cut off feature you could blow a compressor nose seal, have alike excessive compressor wear or blow out a weak hose.
We typically encourage our customers to use our binary (low and high) pressure switch when upgrading or repairing their systems.

The most common problem encountered with the 911 or 930 resides with the thermostat's capillary sensing tube inserted in the evaporator core; if the sensor is not properly placed in the evaporator core the thermostat will not turn off the system when the evaporator's core temperature nears freezing temperatures. The typical symptom is a vent temperature that starts off cold and gradually warms up over time. After shutting off the system for several minutes and turning it back the system blows cold but warms up once again.
Details on the thermostat can be found in the following hyperlink:
http://www.griffiths.com/achelp/achelp2.html
The AC Temperature Switch - An example in detail with a typical 911 Porsche

wwest

But my point was, like you point out, that the factory system is so poorly designed, inefficient, that there was/is no need for a hi/lo pressure cutoff sensor. Make any improvements whatsoever and you will likely require a hi/lo sensor.

Just what is the failure rate, time to fail, once the refrigerant is gone, leaked out??

The reason I ask is that I don't seem to see any complaints of compressor failure due to refrigerant loss even knowing that refrigerant loss is VERY common. I've had to replenish the refrigerant in our '88 Carrera just about every year since purchase (...'96??) and so far the compressor seems to be holding up.

Hladun1

The (40 feet of) old hoses were not barrier hoses so the refrigerant leaks out but the oil does not. The compressor then runs with no load but is lubricated and so doesn't fail.

rusnak

Wow, there is a TON of info there in Griffiths post. Now I see the reson to have a hi low switch.

wwest

The only thing I notice that is misleading, terribly WRONG, is the procedure for checking the refrigerant level via the "site(sight??) glass", bubble glass/window. It take s two people. You need to have someone turn on the A/C, activate the compressor, while you watch the "window" for the transition from gas to bubbles to liquid refrigerant.

Absent a deeply colored refrigerant it is usually not practical to observe the difference between gas flowing past the window vs a "solid" flow of liquid refrigerant.

griffiths

"that there was/is no need for a hi/lo pressure cutoff sensor. Make any improvements whatsoever and you will likely require a hi/lo sensor." No. There is a need for a low and high pressure switch or a binary switch in all brands and models of cars. The reason the early 911, with a York brand compressor, did not see many compressor failures due to lack of oil flow was because the design of that model compressor is different than later Nippondenso compressors.

"Just what is the failure rate, time to fail, once the refrigerant is gone, leaked out??" I don't know of any documented history of failure rates or a publicized MTBF (mean time between failures), however I'm sure if you chatted with an old timer he might say 'not many York's having worn pistons or cranks, leaking manifold valves though, maybe 'a few Nippon's with worn bores and wobble plates'. Based on our experience of rebuilding the models noted earlier I can tell you I have seen signs of wear on low mileage cars with refrigerant system leakage problems.

"The reason I ask is that I don't seem to see any complaints of compressor failure due to refrigerant loss even knowing that refrigerant loss is VERY common. I've had to replenish the refrigerant in our '88 Carrera just about every year since purchase (...'96??) and so far the compressor seems to be holding up.?" The compressor gets its lubrication by way of the refrigerant moving the oil through the compressor. If you have either lack of refrigerant flow or a low oil to refrigerant value you will have premature wear. Whether you subjectively feel your compressor has or has not seen wear is not important. What is important is that you are aware of the need for oil flow, what can happen without out, and what price you could pay earlier than you have to because you ignored it.

"The (40 feet of) old hoses were not barrier hoses so the refrigerant leaks out but the oil does not. The compressor then runs with no load but is lubricated and so doesn't fail." Yes, 40 feet of factory stock non-barrier hose does leak the day it came out of the factory, hence why we replace all the hose with barrier hose. And NO, the oil leaks out of the system when refrigerant pushes it out of the leak path (for example, hoses with cracks or breaks, blown compressor nose or shaft seals.... the black line under your deck lid). If the compressor clutch is engaged, and it will engage if you turn on a system low on refrigerant that does not have a low pressure switch, the compressor does NOT get lubricated because there is no refrigerant carrying the oil through it (the compressor does not have a sump).

FYI, some states, independent of federal mandates, require that when a vehicle is converted to an approved alternative refrigerant that a pressure switch is installed into the system.

"The only thing I notice that is misleading, terribly WRONG, is the procedure for checking the refrigerant level via the "site(sight??) glass", bubble glass/window. It take s two people. You need to have someone turn on the A/C, activate the compressor, while you watch the "window" for the transition from gas to bubbles to liquid refrigerant. Absent a deeply colored refrigerant it is usually not practical to observe the difference between gas flowing past the window vs a "solid" flow of liquid refrigerant." Yes and No. If you turn on the thermostat to max cold, turn on the fan speed switch, have the engine idling, tranny in neutral, parking brake on, you can look for yourself. However the newer style site glass without the old fashion floating ball does make it difficult for a novice to see whether there is refrigerant flow or sufficient refrigerant. You could add a refrigerant dye the next time you evacuate and charge the system (orange or green) which would be visible. What is preferred or more a more accurate method of checking the refrigerant charge is to have a technician check the pressures using gauges; the drier's site glass won't tell you if the system is overcharged.

Mazinger

At what pressures does the A/C system become overcharged, also are we talking about R12 or R134 in this context?

Makmov

You have to look at the graph. It is dependant on ambient temp but it's around 19 bar.

griffiths

Quote:

Originally Posted by Mazinger

At what pressures does the A/C system become overcharged, also are we talking about R12 or R134 in this context?

In a perfect world what you do is use a refrigerant Pressures & Temperatures (P&T) chart, for the specific refrigerant you are using, and try to match the high side pressure of the system to the ambient temperature around the vehicle as shown on the P&T chart; Assuming you have a perfect evacuation (there are no other gases or moisture in the system), your system components are operating correctly (the compressor is pumping correctly), the condenser fan is operating, the expansion valve is metering correctly, etc... the system would be considered overcharged if your high side system pressure exceeded that on P&T chart.

In the context of your question it would be any refrigerant.

Mazinger

I suppose that taking a reading while the compressor is not in operation (and has not been in some time) is irrelevant to the amount of gas charged?

griffiths

[QUOTE=Mazinger;7851869]I suppose that taking a reading while the compressor is not in operation (and has not been in some time) is irrelevant to the am ount of gas charged?[/QUOTE

It could tell you that you have refrigerant in the system but not whether or not the system is properly evacuated or charged.

Mazinger

The pressure on the hi-side was about 240 psi, ambient is about 45C. using this chart would you consider it to be overly overcharged?

http://www2.dupont.com/Refrigerants/...placements.pdf

Thanks.

griffiths

Quote:

Originally Posted by Mazinger

The pressure on the hi-side was about 240 psi, ambient is about 45C. using this chart would you consider it to be overly overcharged?
Thanks.

Assuming "everything" was functioning perfectly, at idle, in a perfect world
with R12 you would be at 271 psi nominal or with R134a around 300 psi. However, depending upon the design of the system you could have lower pressures and still be in the game; lower is better for the life of the compressor.