Porsche early A/C design tutorial...or NOT.

[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.

[wvicary]

The A/C compressor will turn off if the freon pressure is below a set point with an electrical switch located in the low pressure side of the system or through a mechanical switch that monitors if the evaporator freezes up. There is also a fuse for the clutch electrical circuit to prevent overloading and a mechanical pressure blow off valve on the high side circuit.

[wwest]

The A/C compressor will turn off if the freon pressure is below a set point with an electrical switch located in the low pressure side of the system or through a mechanical switch that monitors if the evaporator freezes up. There is also a fuse for the clutch electrical circuit to prevent overloading and a mechanical pressure blow off valve on the high side circuit.

The only control point I can find, discern, on my '88 Carrera is the capillary thermostatic switch that is used to control the temperature setpoint of the evaporator/evaporator airflow. If the capillary tube/bulb is not installed correctly, buried, within the evaporator cooling vane area the evaporator could freeze up and that would result in a failed compressor due to slugging. Since the thermostatic switch seems to be so highly prone failure, failure in a way that results in the compressor running continuously, what provisions, if any, did Porsche make to prevent this type of escalating failure...?

[wvicary]

If the evaporator freezes up and the control switch is working/installed correctly it will interrupt current to the compressor clutch.

[wwest]

If the evaporator freezes up and the control switch is working/installed correctly it will interrupt current to the compressor clutch.

Or...

The evaporator will NOT freeze up if the control switch is working/installed correctly because it will interrupt the current to the compressor clutch before, ~35F, the evaporator cooling vanes reach a freezing temperature..

[wvicary]

Sorry, I didn't read what I wrote correctly and you are correct. Yes it will stop the unit from freezing if all is working correctly.