Evaporator froze up again.

[kuehl]

At this hyperlink we provide the details of the thermostat,
read under the heading:


The AC Temperature Switch - An example in detail with a typical 911 Porsche

http://www.griffiths.com/achelp/achelp4.html

PM if you still have a problem after your tech works on it.

[bigchillcar]

nice work, bob.

[mthomas58]

Ok, bear with me b/c I really don't know what I'm talking about, but desperately want to learn.

System took one full can of R-12 today. Might of taken more but they only had on can of R-12 on hand (WTF)

1) Initial pressure readings showed more of an imbalance in high and low pressure readings with car and A/C off than the tech said he likes to see.

2) On start up low side reading was low at 10- 20 indicating low freon.

3) After one full can or coolant, low pressure raised to about 30 but low side pressure would drop as engine rpms were increased to 2,000+. Tech said that was a bad sign indicating a restriction (vacuum) somewhere in the system - most likely a faulty TXV (think mine is orig).

We agreed to monitor the system after the recharge. If freeze ups and performance does not improve, next likely step will be to replace the TXV.

Ambients around 80 today. Got vent temps in the high 30's to high 40's on the way to the shop and significantly improved temps after the recharge. On the commute home vent temps consistently maintained at low to high 30's.

We seemed to focus on low pressure readings. High side readings if I recall correctly were around 200 with rpms at 2000+.

Thoughts/suggestions appreciated.

Thanks!
Mark

[kuehl]

Low "low side" pressures are not always a sign of a undercharged system.

Low sides usually drop when you increase rpms.

If you really got 200 psi at 80F ambient you are a within normal high side ranges for R12. I'd be more concerned if your high side pressures, assuming the pump is good, were too high.

A freezing evaporator "problem" relates more to the thermostat not doing its job in seeing what the evaporator coil temp is and breaking the circuit when the low side temp drops too low. Prior to adjusting the position of the thermostatic sensor tube in the coil to resolve the icing issue, simply turn your thermostat CCW to the near off position and see if the compressor clutch disengages.

[rick-l]

Originally posted by kuehl
At this hyperlink we provide the details of the thermostat,
read under the heading:


The AC Temperature Switch - An example in detail with a typical 911 Porsche

http://www.griffiths.com/achelp/achelp4.html

Nice write up. Just for my future reference;

If I understand this right there is a switch with a mechanical hysteresis of about 7 degrees. With it set to low it turns on with the temperature at or above 33 degrees (32 psi of the r-12 in the tube) and off when it gets to 26 (25 psi)

So if you turn the setting to warmer it will hold the evaporator to some other temperature say 40 low 45 high.

So a car with a heater core relies on a minimum temperature bi metal switch and reheat to control temperature? This technique seems more efficient.

[scottb]

Jim Sims posted these awhile back. I hope it helps.

Typical high side pressures (without adjustment for altitude):

For R-12
80 F ambient 170 to 210 psi
85 F ambient 180 to 220 psi
90 F ambient 190 to 230 psi
95 F ambient 205 to 250 psi
100 F ambient 220 to 270 psi

For R-134a
70 to 80 F ambient 115 to 200 psi
80 to 90 F ambient 140 to 235 psi
90 to 100 F ambient 165 to 270 psi
100 to 110 F ambient 210 to 310 psi



Typical low side pressures (not adjusted for altitude):

For R-12
32 F Evaporator temp 30 psi
36 F Evaporator temp 35 psi
42 F Evaporator temp 40 psi
48 F Evaporator temp 45 psi
53 F Evaporator temp 50 psi

For 134a
33-50 F Evaporator temp 16 to 29 psi
33-60 F Evaporator temp 19 to 39 psi
40-65 F Evaporator temp 25 to 43 psi
48-65 F Evaporator temp 37 to 51 psi

[derkpitt]

Mark,
Wasn't your a/c system serviced less than a year ago?

Dude, you got a leak somewhere.

[mthomas58]

Originally posted by derkpitt
Mark,
Wasn't your a/c system serviced less than a year ago?

Dude, you got a leak somewhere.

Yep, last July/Aug...two new hoses, new drier and rear condenser. I have no idea how well he charged it though as I was not there to watch.

Not really what I want to hear Bruce! What do you suggest? Electronic sniffer? Got one?

[kuehl]

Originally posted by rick-l
Nice write up. Just for my future reference;

Thank you. It it is a good reference point to ponder.


If I understand this right there is a switch with a mechanical hysteresis of about 7 degrees.
I think you could say in reader terms that if you set the thermostat knob at a given point it usually will react or turn on and off the compressor within 2 degrees of the given point. However because of manufacturing tolerances (not hysteresis) not all thermostats will give you a given temperature if all are set at the same position.


With it set to low it turns on with the temperature at or above 33 degrees (32 psi of the r-12 in the tube) and off when it gets to 26 (25 psi)
Here I would say, for example, when you have your thermostat knob turned on maximum cold, which is almost fully clockwise CW (because at fully CCW you turn off the system, right?), then the compressor clutch should be engaged and it will stay engaged until such time that the thermostatic tube (capillary tube in the evaporator) senses that the evaporator core is near 26F (plus or minus 2 degrees or as low as 24F and as high as 28F) at which point the thermostat contacts should open (or break) and the compressor clutch should disengage. As the evaporator core temperature rises, since refrigerant is not flowing into it, and it reaches a temperature near 33F (plus or minus 2 degrees or as low as 29F or as high as 35F again because of manufacturing tolerances in our imperfect world) then the thermostat contacts will close (or make) and the compressor clutch should engage again.
At a given thermostat set point and with a given range of variables, this "on and off" is your cycling. I would not relate the evaporator core's temperature with an estimated pressure or psi. There are other variables in the total system that can affect pressures. And you have to wonder how the thermostat design was changed when the manufacturers switch from R12 to R134a gas in the capillary or thermostatic tube.

So if you turn the setting to warmer it will hold the evaporator to some other temperature say 40 low 45 high. Generally it should. But, not to confuse a reader, the wording is "evaporator" temperature as opposed to "vent" temperature. I throw that in here just in case someone gets the idea that they place a thermometer or temperature measuring device in the vent and expect that because the vent temperature shows to be 40F the evaporator core is 40F when actually it could be much much colder. And, when watching the point at which the evaporator core or vent temperature changes relative to the thermostat's setting, there could be a long delay, meaning for example: On a hot day you have your thermostat set at point that is giving you 40F out the vent and the compressor clutch thermostat contacts open and the compressor clutch disengages. The vent temperature quickly begins to rise to 45F, the thermostat makes contact again however there is a significant lag in time before the vent temperature drops back down to 40F. In that scenario there are several components and states that come into play besides the thermostat. Here you might consider the term hysteresis, however hysteresis in the physical sense not only applies to a delay but could mean that the set point never comes back to a given point, it is lost forever. So, I kinda like the words "tolerance" or "range".


So a car with a heater core relies on a minimum temperature bi metal switch and reheat to control temperature? This technique seems more efficient. I don't know what you mean by "heater".
My 87 cab has the standard P economy controls: pull up the red levers to "shake and bake".

[derkpitt]

Mark,
Find that leak before you dump anymore money.

Don't have a sniffer, sorry.

[bigchillcar]

keuhl, i think rick refers to water cooled vehicles in general that use a heater core, circulating heated coolant to provide a core for air to pass through (with a blower motor) before entering the ducts. forgive me if i've mistakenly pointed out something that you already understand far better than i.

[kuehl]

bigchill,

Thanks. No forgives needed. I need more coffee. I keep forgetting if I'm working on a 928 or 911.

Kuehl

[bigchillcar]

[scottb]

Any time Charlie ("Kuehl") Griffiths talks about a/c, I stop and read (even if he needs more coffee). Between him and Jim Sims, we on this BBS are very lucky to have such a depth of knowledge about the 911 a/c available to us as a resource!

[bigchillcar]

i didn't realize that keuhl was charles griffiths! sorry charlie.. d'ohhhh!

[kuehl]

Originally posted by scottb
Any time Charlie ("Kuehl") Griffiths talks about a/c, I stop and read (even if he needs more coffee). Between him and Jim Sims, we on this BBS are very lucky to have such a depth of knowledge about the 911 a/c available to us as a resource!

Shucks. Now I gotta change my user name again.

[scottb]

Either that, or change the name of your product line!!

[kuehl]

touche

[bigchillcar]

yeah, scott..don't be so touche, man!

[porschedurango]

First I have to thank everyone for their post. Just reading what people write takes the guesswork out of the job. 84 carrera, 3.2, nippondenso ac compressor, r-12 still working in it. Read the pressure tables in this post (thank you), just want to confirm before I overcharge.
95f day today, low pressure between 25-30, hi 200-250, trunk lid up,big fan on cooler, spray with water to keep it cool, engine temp normal and warmed up. low pressure goes to 20,,hi to 200. center duct inside stays at 60 degrees. nice cold sweating low side port connection (welcome to humid Florida).
system holds pressure, "real r-12", no freeze 12 or sealants. Do I want to add more r-12 to try to get the 60 degree cabin temp down, or is this normal standing still in the garage. Not street ready yet, cant take her out yet. Thank You in advance and keep up the great work.