Earl Gillstrom

John,

Do not try to compare home AC systems to Automobile AC systems. Although they work on the same principles they are controlled differently. On a home system the compressor is controlled with the wall thermostat. On the 928, the compressor power is controlled by the AC button. Either on or off. The 928 has a very big system, needed to cool the car from a heat soak. After the car has been cooled the system is way too big for the load and is controlled by the anti-freeze switch. The anti-freeze switch is a normal control, NOT a safety. The temperature slider has NO control over the compressor. Yes, the pressure switch is in the same circuit and is a safety.
I work on dozens of 928 AC systems and ALL that are PROPERLY charged use the anti-freeze switch to control the system to reduce icing. At my location in PA the temperatures are in the 60 to 90 range in the Summer and the anti-freeze switch is the control that stops icing. The icing of the evaporator seems to cause no problem to the system, but the people get warm.
I have a lot of experience with the anti-freeze switch failures. Most appear to loose their internal fluid causing the adjustment to be very critical. You can sort of adjust them to work for some ambient temperatures but then they hang open, so no clutch or closed and ice up the evaporator. I have installed new ones that needed adjustment, but then work OK and the adjustment was not critical. It seems that normal adjustment results in temperatures in the 38 to 42 degree range and people do not notice the difference in temperature. Also, be careful working on adjustments with the car stationary and the hood open. The system seems to settle down with the hood closed and cruising at >60mph.

Earl Gillstrom

John,

Do not try to compare home AC systems to Automobile AC systems. Although they work on the same principles they are controlled differently. On a home system the compressor is controlled with the wall thermostat. On the 928, the compressor power is controlled by the AC button. Either on or off. The 928 has a very big system, needed to cool the car from a heat soak. After the car has been cooled the system is way too big for the load and is controlled by the anti-freeze switch. The anti-freeze switch is a normal control, NOT a safety. The temperature slider has NO control over the compressor. Yes, the pressure switch is in the same circuit and is a safety.
I work on dozens of 928 AC systems and ALL that are PROPERLY charged use the anti-freeze switch to control the system to reduce icing. At my location in PA the temperatures are in the 60 to 90 range in the Summer and the anti-freeze switch is the control that stops icing. The icing of the evaporator seems to cause no problem to the system, but the people get warm.
I have a lot of experience with the anti-freeze switch failures. Most appear to loose their internal fluid causing the adjustment to be very critical. You can sort of adjust them to work for some ambient temperatures but then they hang open, so no clutch or closed and ice up the evaporator. I have installed new ones that needed adjustment, but then work OK and the adjustment was not critical. It seems that normal adjustment results in temperatures in the 38 to 42 degree range and people do not notice the difference in temperature. Also, be careful working on adjustments with the car stationary and the hood open. The system seems to settle down with the hood closed and cruising at >60mph.

Dozman

Earl,

I was not saying they are the same type of system. Yes, I referred to a house system to try and explain evaporator saturation pressure/temp and how it relates to the temp of the evap and how ice forms on an evap. I will continually refer to the easiest system to try and explain things such as saturation temps etc. A few people tried saying a comp creates a vacum. Had to clarify that it does not create a vacum, it actually creates a pressure difference in essence. I noticed you did not correct a few misleading comments by others either...

Also compared the critical charge in small detail as this if undercharged WOULD create the scenerio being seen by a few on this thread.

Earl you said icing up of the evap does no harm? This is not true, icing up of the evap will cause low lubrication, since refrigerant has an affinity for oil and carries the oil to the compressor for lubrication. If low operating low side pressure happens, less than design lubrication will reach the compressor which means over time the compressor could fail due to lack of lubrication. Secondly if continually operating at lower than design low side pressure WILL damage the compressor. If icing up of the evap. does no harm, is the only reason for a freeze stat is to keep the evap from becoming a block of ice and thats it?

Seems a few people kept throwing out variables to talk about, explain and or to stump. I did not hear anyone talk about why there is a design pressures for high and low side, and why it's important to try and meet that, and if not what would the symptoms be. The person who understands this is Dan. It was and is apparent some do not understand the direct temperature/pressure relationship refrigerants have. From several posts, it's also apparent a few don't understand what actually the low side pressure means to the evap. I am sure also that not many know what the high side pressure means as well.

I am not trying to argue. But I think you should read through the other user/members post and explain to them about their inaccurate posts. TIA for the help trying to explain what could be the problem with their sharks in which this thread is about. redirecting you to the first few posts from users/members with their complaints.

Per the manual, the a/c switch is an on/off, the temp slider blends the temp desired by activating the HWSOV. The freeze stat is referred to in the wiring schematic as a protection device, not a control device. A protection device is a safety. Yes, a saftey without a manual reset will cycle the system depending on what scenerio the saftey is engineered for. One can argue all day and night what a freeze stat is, is it a control device or saftey that cycles the comp, due to too cold of evap.Porsche realized if the comp. for some reason due to maybe inaccurate engineering or knew that the 928 will operate outside the design paramters, they engineered a saftey to keep the compressor from damaging itself if the evap was to freeze up, or in other words the evap saturation pressure/temp too low..

But, taking into mind why Porsche on our cars put a freeze stat in as a protection device is to safely operate the compressor without damaging it. Cycling the system, due to the system operating either incorrectly or operating out of design spec for what ever reason.

I understand freeze stats on the 928 is not a well engineered stat, and could be improved on easily, and have been

I have worked on hundreds of cars system, not just dozens. I have around 10 or so sets of guages, one set for each type of refrigerant I work with, and a few for recoveries and or reclaim. The hoses I use for cars are either 10 or 12 feet, so that I can put the guages in the cabin and drive the car watch the pressures, I also have a few hand held flukes, and stand alone temp meters with up to 6 temp leads, as well as 6 pocket digital temp probes. This way I can see the temps and pressures of the system as I drive.

The problem still is resolved as a low charge or restriction in the system for the 2 users/members who are having problems in the begining of the thread.

Hopefully earl, you can explain to others why in the designing of our systems why there is a critical charge of refrigeration, the evap saturation pressure/temp, the condensor saturation pressure/temp, whhat the low side and high side pressures actually mean, not that they should be this or that. That 2/3rd's of the first ton of refrigeration is meant for dehumidification the last 1/3rd is used for temp drop. Why cars have a high tonnage compressor. Why the condensor is designed to be at least 125% larger than an evap in BTU rejection/absorption. Explain why R12 is actually a better cooling refrigerant for cars than R134a (not saying R12 is better for the atmosphere than R134a).

Because if some can not understand the a/c system in a car, I will continually compare the systems to try and explain the basics of a refrigerant system. Even an instructor at ferris State in Michigan agrees I did no wrong by comparing the 2 systems. ferris leads the State of Michigan in ratings for teaching refrigeration in accredited colleges.

Mrmerlin

One of the basic teaching skills ... simple to complex..... from known to unknown
Thanks to all for taking the time describe the AC operation

Earl Gillstrom

John,

I tried to nicely point out to others that your first reply was WRONG. Dan corrected your post but you continue with BS. John, you are WRONG.

Originally Posted by Dozman View Post
Even if replacing/adjusting the freeze stat is not going to correct the problem, if in fact the real culprit is a low charge of refrigerant. One should diagnois the actual problem why the evap is freezing up. A low charge or restriction.

If the system is low on refrigerant, a freeze stat is just goinng to keep the system cycling on/off, on low pressure.


I'm not sure I'm understanding your reasoning? If the refrigerant system has enough capacity to remove enough heat energy from the air being passed through the evaporator to cool it to under 32 deg F and cause ice to form on the evaporator fins, why would this mean that the charge is low? The charge is low when the system is unable to drop the temperature of the air flowing through the evaporator enough. Ice cold air is more than enough, and in fact that is why the freeze switch is there, so it doesn't get colder than freezing and form ice which will block air flow and hinder the heat exchaning efficiency of the evaporator.

While this car may be low on refrigerant, the fact that the evaporator is freezing up has nothing to do with that. A properly charged system will have the compressor cycling very frequently as it hits the limit of the freeze switch often.

Dan
'91 928GT S/C 475hp/460lb.ft

Dozman

Earl,
Reread the users complaints and symptoms of the problem they see. So your saying it's just a freeze stat problem. OK...So I guess the 2 users just correct your freeze stat, but you'll still have air colder than 32 degrees, and then warm, then cold then warm. Better yet call a refrigeration shop. They say charge the system by weight.

Please explain the evaporator saturation pressure/temp. What does this have to do with the refrigerant cycle. If you do, you would understand what a low side suction pressure represents to the evap and txv. Then what the pressures actually mean besides just make them this pressure.

Earl, please explain what are actually happens in a system if the system is under charged??? What are the symptoms if the system is over charged???

Sorry I call BS with a freeze stat is a control device to cycle the compressor. It is a safety even per the wiring schematic by Porsche.

I am not Wrong in my diagnois's You say I am wrong does not make me wrong, what so ever. Sorry if your feelings are hurt. But the facts still stand, there is a lower than design low side pressure while the 928's are running. The simple solution is a freeze stat? Well whats causing the freeze stat to cycle? Funny, after I worked on so many cars a/c, No one ever complained that the air is ice (32 degrees or colder) cold, then warm, then ice cold then warm. They say damn the a/c works great...If it's ice cold, below 32 degrees, something is wrong...

So whats wrong with diagnoising the problem? Just saying it's a freeze stat makes no sense. Why is the freeze stat tripping? Once again one must take pressure readings low and high side, One diagnoising must understand evap and cond,. saturation pressures.

The system capacity has enough capacity to remove heat while the evap sat pressure/temp is at 32 degrees, but has less capacity when the system evap sat. pressure/temp is at above freezing. If you can't understand the facts of refrigeration I suggest taking a class on automotive a/c systems. Not being mean, but what I am explaining is pure basics. If one can not understand evap sat. pressure/temp and what that has to do with capacity to remove BTU's, either in latent heat/humidity (which most of an a/c system removes) or sensible heat (measurebale heat via a thermometer).

I hate to LOL Earl, but evap saturation pressure at 32 degrees is what pressure on the low side? 28 psi, correct? Well inform me what pressure should the low side be? 30-35 psig, correact? If the pressure should be 30-35 then the evap saturation pressure/temp should also be 35-40 degrees, correct? If a refrigerant low side pressure goes down so does the evap sat. pressure, meaning in a car a/c system at 32 degrees, the system is loosing capacity. Once ice devlops on the evap, means the ice is sub cooling the evap making the evap sat pressure lower, making for less air flow which also lowers the evap sat pressure making for more ice, loosing more capacity and so on.

Your saying a freeze stat is a PORSCHE control device to cycle, they would of made it a better system, and not call it a protection device in wiring schematics.

Earl, you are wrong on a low charge. Please explain evap saturation pressure.

Earl you say my reasoning is wrong for a low side low pressure??? Lets make it even simpler after you answer my other questions, so I know you understand refrigeration.

Earl describe in a system what the low side pressure should be when a system is charged correctly by weight after a system is in a vacum say to be at least at 250 microns. Then explain a slightly low side pressure, as if slightly under charged, what happens to the low side pressure? Then explain in terms of evap saturation pressure/temp both situations. Then explain how many BTU's capacity each have. This will clarify things easily, if you explain it factually.

As you say ice cold air is enough, your wrong again. Ice cold air is too cold. A car's a/c system is not classified as a low temp a/c system. A car's a/c system is classified as a medium a/c system, which has a design temp above 32 degrees evap sat. pressure/temp. Ice cold air will make the freeze stat quicker and more often, very simple.

If Ice is forming on the evap, why would this happen Earl? One reason is low charge, and the major culprit. The others are an air restriction, as in dirt film on evap. A restriction in the system, a faulty TXV. If Ice starts to develop on the evap. THE SYSTEM HAS STARTED TO LOOSE CAPACITY, Hence ice hinders air flow and sub cools the evap... Thats why the freeze stat is there, because the freeze stat is saving the comp from damage.

The freeze stat is not engineered to cycle the compressor so one can under charge the system. If the a/c system is consistantly cycling, one should diagnois whats wrong. It was not put in place to cycle the compressor as a control device. The freeze stat is actually engineered to trip at what temp? If the evap is at that temp, what would the evap saturation pressure/temp be? what would this indicate, a PROBLEM with the refrigeration system.

Who wants to drive around with warm cold air, warm air, cold air warm air. Myself I would want to drive with cold air all the time, as the system was engineered to operate.

Earl, you said the fact that an evap is freezing up has nothing to do with a low charge??? Really. Then what are all the causes that can cause an evap to turn to a block of ice? Is one a lower than design refrigerant charge?

I disagree with you, that a properly operating system, and charged correctly to spec very frequently hits the freeze stat, and it cycles the compressor. I have never seen that after I work on a car. If the freeze stat keeps triping again the evap sat pressure/temp is too low. There must be something wrong. Please go back to a/c basics 101. Our cars are engineered for an evap saturation pressure/temp above 32 degrees not below. Plainly a system that puts out 32 degrees or less is operating incorrectly, or operating outside of design paramters. Can't explain it simplier than this.

Please do answer my questions I have asked.

Please do point out to others how they are wrong, unless this is just about trying to prove me wrong, and no one else. If thats the case np. At least the cars I have worked on, work properly and do not very frequently cycle on freeze stat.

May be low on refrigerant, the fact that the evaporator is freezing up has nothing to do with that. A properly charged system will have the compressor cycling very frequently as it hits the limit of the freeze switch often.

Now Earl, you have said an evap freezing up in fact has nothing to do with a low ref. charge. Hate to say this, but your WRONG, and I am trying to point this out to you.

dprantl

John, perhaps you can explain to me how the air will get colder than 32 deg F and then warmer again with a properly working freeze switch that is not allowing the evaporator to ice up? The freeze switch will let the compressor turn on again after it has warmed up only 4-5 degrees or so. The difference between 32 deg F and 38 deg F is hardly what I would call cold vs. warm.

Dan
'91 928GT S/C 475hp/460lb.ft

Dozman

Dan I asked a few minor easy things for people to explain, but no one wants to explain what I ask. please go back and answer my questions, and I will answer yours afterwards.

If you want to operate your system under charged, thats fine.

Dan your pretty good at putting words in my mouth, in which I never stated. The users/members said their air is cold then warm, not I. Please reread the users/members complaints/symptoms to clarify who said what.

The user member complaining is saying their air is cold then warm. No reason to explain your theoretical point, has nothing to do with the mebers/users complaint on this thread.

I can bring up many variables for you to explain. But I ask only basic questions, but you have choosen not to.

I am in no way upset frustarted or badgering.

If it is reccomended to charge a system by weight, ther's a reason.

dprantl

But that's just it, I *always* charge my systems by weight. I don't even check the high side pressure because of this because the factory specified a certain mass of refrigerant, so that's what I use. And right after I'm done, the compressor cycles off the freeze switch in short order.

John, I understand refrigeration, trust me. I know that the vent temps are directly related to the evaporator pressure, PV=nRT (refrigerants are not ideal gases, but close enough). Volume is constant, R is a constant and n is also constant, so really pressure = temperature. Very low pressure at the evaporator will cause very low vent temperatures. The actual pressure correlation with temperature depends on the type of refrigerant, they are all different.

Dan
'91 928GT S/C 475hp/460lb.ft

Dozman

I am glad Dan you do understand the ref. P/T chart. I never said you didn't. Once again, I agree to disagree with you, Earl and Jim. No big deal to me. I don't ever have a problem with my system very frequently cycling on freeze stat, I don't have my evap freeze up, I don't have more than normal water draining from the evap. BUT, the users/members here do in the begining of the thread.

Everyone has their opinions and they differ. One may think his/her explanation is right for this or that. If you'll think your right np, but I would like to see the ansers to the questions I have asked a few folks, since I answered most of everyone else questions.

The users members with their problems will believe who they want to.

Earl Gillstrom

John,

Jim, Dan, Dr Bob and myself all answered your questions correctly. Most if not all of your questions are not relevant to the thread so won't be answered. Part of your problem may be your 928. If you are judging your responses by the operation of an R12 928 charged with R134a, then I understand some of your problems. You mentioned changing it back to r12. That is a good idea. After you properly charge to R12 you will better understand the operation of the freeze switch. BTW, my '88 circuit diagram calls it "THERMO SWITCH EVAPORATOR FREEZING PROTECTION. That is an extremely descriptive translation from German.

Dozman

Earl,

Since you can't answer the questions I have asked, I take you do not fully understand how the refrigeration cycle works. You, Dan, and as you point out and try and drag Dr Bob into this dragged out thing did not answer the questions I asked. My questions were relevant to an a/c system, if one can not answer basic a/c 101 questions, then I can understand why there is confusion on some folks part. There fore no reason for me to try and explain the problem, since some folks can't answer the basic questions. Once again because you say something does not make it true. IE... Questions about evap sat pressure/temp at design and at low charge of sat pressure/temp of 32 degrees or colder and corresponding capacity of those systems. This would prove that the low charged unit has a lower capacity than a design charged system.

Your trying to put words in my mouth, If I am judging...Sorry earl, your twisting words and it appears to make an attempt at sarcasm. I do find your sarcasm some what enjoyable and funny.

I now comprehend why some folks have misconceptions on what effects the temperature and capacities of the system, since one does not understand evap saturation temperature and how it effects the system, as well as why things are engineered. Without understanding this I can understand why some folks don't fully understand the cars a/c system and it's proper operations.

I am glad you did find the "THERMO SWITCH EVAPORATOR FREEZING PROTECTION" on the wiring schematic. It may be very descriptive, but protection is still considered a safety, not a control device. I don't see the German very descriptive saying cycling device, I see "Protection".

Again I feel lucky you have taken such an interest in correcting myself only in something you think I am wrong on. But I think you missed a few other posters inaccurate postings. Did you forget to correct them too? If you'll don't, I understand the some folks go out of their way to correct me, or try to. Unfortunately not able to answer basic questions about a/c system just further proves why one thinks I am wrong.

Don't worry I agree to disagree with you as well on some points. My points of view and satements are verified by facts, not he said she said.

I know I am correct, no reason going back and forth like this, since basic questions can not be answered, but ignored I understand where some of your misunderstanding of the a/c system comes from.



But, I still stand correct the problem with the user/members problem is a low charge scenerio or restriction. I am still adressing the thread, not going out of my way to try and correct one individual. Have a great day Earl.

jcorenman

John,

One of the things I enjoy most about Rennlist is the opportunity to kick things around with smart folks who have a great deal of knowledge about cars in general and the 928 in particular. No one is right all the time, it just isn't possible. And there is nothing wrong with being wrong, it's all part of the learning curve.

You obviously know a lot about refrigeration systems. And in spite of getting pretty good grades in thermodynamics many decades ago I still get confused abut it. But when you present your points as "facts" and label others as "inaccurate" and "incorrect", and tell folks like Earl that they don't understand refrigeration, well that tends to stifle discussion.

And friendly discussion is what makes all this interesting, because without that we're just a bunch of guys bitching at each other. So please, take it a bit lighter and consider that other folks maybe aren't all dummies that you have to "agree to disagree" with.

My comments were all with respect to R12, that's what Porsche spec'ed and that's what we still use. I appreciate that R134a behaves differently, but if we're talking about what the designers intended then R12 is not irrelevant.

You made the point that the low-side pressure in an undercharged system was lower, therefore it was colder because of the evap temp/pressure relationship. My point was that if the charge was low enough so that the receiver/dryer ran out of liquid, then there may not be enough liquid to keep things saturated. If the freon is completely evaporated then the t/p relationship is no longer valid. I made a poor choice of words when I described the compressor as pulling a vacuum, but it will certainly pull negative pressure on its intake side, and if there is no liquid freon to evaporate and create gas then the pressure (but not the temperature) will be lower. It's a confusing situation and I agree with you that proper charge is important (and avoids that issue).

I don't think an iced evaporator is any risk to the compressor, gas still flows and the compressor still gets oil. It just doesn't work very well as a air-cooling device. Marine systems routinely freeze holding plates (blocks of ice) to 0F or below using automotive components.

OK, now I've got a question for the group, something that has stumped me for a while. I've attached a page from the WSM with a couple of graphs. The first shows low-side pressure vs. ambient air temp. At 27C/80F (where our evap will freeze with a dysfunctional freeze switch) the range of pressures is 1 to 1.5 bar, I assume that's gauge pressure? So that would be 15-22 psig, which corresponds to 12-22F for R12. Is that correct? (If it were 1-1.5 bar absolute pressure then that would be 0-7 psig or -30/-20F, so that's not right).

If that is correct then how can the evaporator NOT try to freeze with temps in the 80F range? That also matches my observation, above 90F freezing is not an issue (around 2 bar or 30psi on that graph, or around 30F R12 temperature).

But now look at the lower graph, temp at center-vent (#2 fan, max cool temp, 2000 rpm), at the same 27C outside temp the center-vent is 5-8C, 40-47F. Why the large (~30F) difference between the freon temp and the air temp? Just the efficiency of the evaporator? Or are they considering the effect of the freeze switch? I've been through all of the tech stuff on the Jim Morehouse CD's and can't find an answer.

OK, back to freeze switches: I mentioned a dysfunctional freeze switch. I ordered a new one a couple of weeks ago as a spare or replacement, and pulled the old one (our 90GT). It tested fine, opened quickly in icewater and closed when warmed. The new one (Porsche part from 928i) needed adjusting so I adjusted it and installed it, but the evaporator still froze. I pulled back the boot between the blower and evap housing, the nearside of the evap was clean but the far side (where the freeze sensor is) was caked with dust. Aha! I made a skinny vac attachment and cleaned it, but no help.

When peeking in there I noted that the guide tube was resting against the evaporator fins. Then I marked how far the capillary tube sensor went in, and pulled the guide-tube and checked how far the sensor was sticking out the other end. Nada, the length of the sensor was the same as the tube and it was resting against the fins. Ditto the old one. But the manuals all say that the end of the capillary tube goes between the fins. Hmmm...

So I took some 3/32" welding rod and filed a smooth bullet-nose on the end, and slid it down the guide-tube until it hit the fins, then wiggled and pushed gently and it pushed its way between the fins another 1/2" or so. I then stuck the sensor back in and it went the same distance 1/2" past the previous mark. Aha!

I haven't had a chance to test it yet, but I am thinking that maybe the reason my freeze-switch was dysfunctional was simply that the sensor-tip was never pushed between the fins. I don't know if maybe the PO messed it up, but our S4 freezes also and I know that its freeze-switch has never been touched, so that's next.

Anyway, I hope this is a useful contribution to the discussion.

dr bob

I spend quite a bit of my driving time under perfect isothermal conditions, at design speed on the compressor and design airflows through the condenser and evaporators and at design dew point. I'm pretty sure I do anyway. It's somewhere in the middle of 'recovering from a heat-soaked cabin' and 'making ice cubes in the evaporator'. For exactly the reson of all these variables, the refrigerant charge is defined by weight. Mass is the only thing that stays the same as pressure and temperature conditions vary in normal service. A pound is always a pound, big or small, right?

I made a calculation error today while working on a 600,000 pound per hour evaporator, disguised as a heat recovery steam generator. Discovered the error while verifying the heat-mass-energy balance from run data. I consider myself somewhat knowledgeable on thermodynamics, although I generally bow to the experts when it comes to moving heat around in process systems.

dr bob

Jim-

From those WSM graphs, one might assume from the wide lines that the factory actually expected driver to use the AC under a rather broad spectrum of conditions.

That should stimulate some discussions!

And thanks for the 'tip' on using the welding rod to get the cap tube inserted correctly. Cleaning the evap unit is on my to-do list. Don't want to make dirty ice by limiting heat transfer out of the evap unit.