a/c question

[sylvester]

hi.. i have a 1979 porsche 911 sc and my question is when its cool outside my a/c blows 40 degrees comming out the vent, but when its really hot outside like 90 degrees, it blows 60 degrees out of the vents, and i notice when i put the a/c on auto (thermostat) the switch in the console next to the blower switch, the compressor wont turn off and on like it does when its cold outside, will someone explain this to me i dont understand how the a/c works that well thank you for the help.

[Hugh R]

Hey, Do a search on air conditionig and you'll find all you ever wanted to know. That said, it has a thermostat and will cycle, don't know what you mean about putting it on auto, I have an 84. Sounds like you need an R-12 charge. Take it to a local a/c shop and pay $100/pound for R-12, your car takes about 2-1/2 pounds, or look into alternatives such as R-134a at about $5/lb, but doesn't cool as well. Some people advocate using some of the cheap propane-based refrigerants, but your looking at getting a potential fire/explosion if you go that route. Cheap, but dangerous when you get a fireball in your face. Do plenty of reading on your search, you'll find a lot of info. Do a search on "Jim Sims" and you will find info from the resident GOD on this subject.

[kuehl]

The reason your compressor is running constantly when it is hot outside is because the sensor tube, located in the evaporator, attached to the thermostat, is "seeing" that the evaporator is not cold enough.

Basically here is how the system operates:
1) Right switch, fan speed control, turns on the ac system and sets the evaporator fan speed.

2) Left switch, thermostat, sets how cold the evaporator can get. A sealed metal tube containing a "gas" runs from the thermostat to the evaporator. As the evaporator gets warm or cold it causes the refrigerant gas sealed in the sensor tube to expand or contract (do not get confused with "refrigerant" in the sensor tube vs. the refrigerant in your "ac system"; the mechanical thermostat needs a gas of some type which we will explain further).

3) As this refrigerant gas in this sensor tube expands, such as when warm air moving into the evaporator from the cockpit heats up the evaporator, it pushes on a bellow which moves a set of points or switch perse in the thermostat. The switch makes contact and turns on the compressor. To save some typing here we'll simply say when the compressor runs the evaporator gets cold and air coming out your vents gets cold.

3) As the evaporator gets colder, and depending upon where you set the LH thermostat knob, the gas in the sensor tube will contract (hot expands, cold contracts) and the bellow on the switch contracts and the points open which turns off the compressor. The time between when compressor runs and does not run is referred to as "cycling" (cycling on and off).

4) Since your evaporator fan is running, and pulling in warm air from the cockpit through it, the evaporator starts to warm up again and the refrigerant gas in the sensor tube located in the evaporator, starts to expand once again which will turn the compressor on again.

5) The thermostat and its sensor tube also have a design feature which prevents your evaporator from "icing". As the air from your cockpit (by the way your system is "recirculating" type system, meaning it does not draw in air from outside but only from the cockpit) moves through the evaporator the cooling process removes moisture from the air. The moisture becomes trapped on the evaporators cooling fins. Depending upon how much moisture is in the air and how cold the evaporator is running this moisture can collect rather quickly to form a dam which blocks air from moving through the evaporators cooling fins. When that happens you end up with a DYSFUNCTIONAL cooling system. The classic symptom of an iced evaporator can be experienced somewhere along the lines as follows: "I turned on the ac system and the vents were blowing nice and cold, then after a few minutes the air coming out of the vents started to get warmer and warmer even though the compressor was still running. When I shut the system off and waited 15 minutes or so it worked fine again, but soon thereafter it got warm again". What is happening here is the evaporator is working fine cooling the air. Moisture starts to accumulate on the cooling fins and the evaporator cannot absorb any more heat (or simple terms). When you turn the system off the ice melts and the cooling fins can once again do their job. A properly working thermostat and sensor will turn off the compressor when the evaporator reaches a preset cold setting.

You can read more about the full function of the thermostat here:
http://www.griffiths.com/achelp/achelp4.html
under the title:
The AC Temperature Switch - An example in detail with a typical 911 Porsche


As Hugh pointed out, there are some really interesting posts here about how the system works, what does work to improve it, and what does not work to improve it. You'll find some really nifty home made improvements and you can find some interesting ready to bolt on improvements.

Send a PM if you need help.

And stay ......hmmmm, KUEHL!