Good enough for a Tech Note...?

Fous: 1988 Carrera Special Edition.

I have long been of the firm belief, and remain so, that refrigerant
leakage in our Porsche's is the result of a design flaw on the part of
Porsche.

Many naysayers will chime in and attempt to convince you that
the problem lies solely with the use of non-barrier hoses. But many
other marques of this era made use of these very same non-barrier
hoses and yet hold their refrigerant charge inatct for decade after
decade.

So what, then...?

Inadvertent over-pressurization, high side pressurization well beyond the 350-450 PSI that appears to be an acceptable high end pressure for A/C systems of this era.

Why, how....?

Many systems of this era used a HPRV (BOV), a vent to atmosphere
pressure relief valve (***1) should the high side pressure exceed the normal pressure range. In the alternative some systems of this era used a high side pressure sensing switch to disable the compressor clutch once system pressures reached 350 PSI.

Some even used both...

(***1.) Yes, R-12 VENT-TO-ATMOSPHERE....!!! Even in my 1992 Ford AeroStar...

Why make use of both provided the pressure switch prevents the
compressor from forcing the high side pressure from being
excessive...??

Because there is MORE to the equation.

Suppose the compressor has just driven the pressure to the limit, 350 PSI, but now you switch off the ignition...?? Now you have just disabled the airflow that was cooling the rear lid and front lip
condensers, keeping the high side pressure "in check", against the CONSTANT engine/exhaust radiant heating, as it were. Loss of cooling, now NOTHING to compensate, overcome engine/exhaust RADIANT heating of the rear lid condenser, the high side pressure will undoubtedly begin to rise.(***2)
More to the equation.....engine and exhaust manifold HEAT.

Even without the rear lid condenser being problematic Porsche engineers recognized the problem of engine compartment HEAT in the 964 and 993 series, and yes, even in our 996/997 cars.

With the 964 came a new design aspect that helped to prevent the engine compartment HEAT from rising too high in certain conditions. With extended idling, say iwith consistent, prolonged, rush hour stop and go driving, if the engine compartment temperature, convection + radiant, exceeded 70C the cabin heater blower would be powered on to help "wick" the exhaust manifold HEAT away from rising up through the engine cooling fins/vanes.

The new design aspect also included an "after-run" engine compartment cooling capability. If, as you switched off the ignition, the engine compartment temperature was above 30C then a 15 minute time delay circuit was "armed". Now, for the next 15 minutes, if the engine compartment temperature continued to rise and reached 70C the cabin heater blower/fan would be powered. Again, to wick away the exhaust manifold heat and discharge it into the rear wheelwells.(***2.)

For purposes of this dissertation lets make the following assumption, which is not exactlly a rare situation, circumstance. Assume that as we switch off the ignition the A/C evaporator core is already at ~33F, thus the TXV is mostly closed, the R/D is filled with seevral ounces of liquid refrigerant. And now with no air being circulated through the evaporator cooling fins/vanes it might take many minutes for the R/D's liquid refrigerant to be "exhausted" and then for the high side and low side to begin equalize. More than 20 minutes according to Charlie at Griffiths.

Sidebar: Google..

Bob Tindel cooling

Solution for leaking non-barrier hoses.

Tape and insulate a thermistor to the metal pipe as it enters the rear lid condenser. Use the thermistor value to discern if/when the high side pressure exceeds 300 PSI. Once 300 PSI is sensed then power the cabin heater blower/fan. Should the sensor indicate >350 PSI disable the compressor clutch circuit. Ignition on or off, POWER the cabin heater blower/fan as long as, whenever, the sensor value indicates >300 PSI.

This may not only prevent the loss of refrigerant on a 1 or 2 year cycle, it might well raise the overall A/C system performance to a more adequate level. With the engine idling at ~800 RPM I meaured ~100 Ft/m airflow at the center/top of the rear grille, with the cabin heater blower powered it went to ~200 Ft/m. Single point measurement, no attempt to calculate overall inlet airflow.

Thermistor alternative: The EPA requires the use of a high side pressure switch when converting an R-12 refrigerant A/C system to the use of R134a. This is because R-134a, absent some sort of control, will operate with pressures well above your system's design specifications. I recommend using the Red Dot trinary pressure switch. First, because it will limit the operating high side pressure within the limits of R12(***3).
Second, the 3rd function of the Red Dot trinary pressure switch can be used to control the cabin heater fan. And the front lip condenser fan if you wish.

***3. An R-134a trinary or binary pressure switch will allow the high side pressure to rise ABOVE the system design specifications whereas the Red Dot switch will not.